Instrumentation of a Wind Tunnel
PERTH COLLEGE UHI
12
INSTRUMENTATION OF A WIND TUNNEL
PROJECT BEng(Hons)
Gaurab Ghosal
ABSTRACT
The wind tunnel being built in Scone airport is a closed loop wind tunnel. On completion this will provide a much needed facility for education purpose as it can be used by engineering students for aerodynamic evaluations. To conduct these evaluation proper evaluating tools needs to be setup. In this section the possible instruments that can be setup in this wind tunnel has been evaluated with Matlab based evaluation, AutoCAD drawings, morphological analysis and other analytical tools. The most viable and suitable instruments or equipments has been suggested under the given conditions. The constrains and the limitations of this project has been described with further suggestions for improvements has been recommended
CONTENTS
Introduction......................................................................................................4
Measurements and Analysis..........................................................................4 * Air speed...............................................................................................4 * Temperature..........................................................................................7 * Pressure................................................................................................8 * Reynolds Number...............................................................................11 * Turbulence..........................................................................................12 * Force....................................................................................................13 * Turn Table...........................................................................................19 * Fan speed control...............................................................................21 * Flow visualization...............................................................................23
Conclusion.....................................................................................................25
* Limitation and problems faced..........................................................25 * Further recommendations..................................................................25
References.....................................................................................................30
Acknoledgements.........................................................................................32
Appendix........................................................................................................33
* Matlab Program
Logbook
INSTRUMENTATION OF A WIND-TUNNEL
INTODUCTION:
A wind tunnel is aerodynamic research tool which simulates the condition of an object moving in air by blowing the air on it. It is used to study the effects of air moving around a solid object and the forces developed due to this flow. To study the effect the object is placed in a controlled environment and is monitored with sensitive instruments. Instrumentation is vital as it provides the essential experimental information. The data so collected can be used to understand the aerodynamic properties which is then analysed to improve the design or to solve problems related to the design. A wind tunnel without its instruments is like a ‘ship without a sail’. Instrumentation also allows easy recording of such data and preserve it for future use.
MEASUREMENTS AND ANALYSIS: 1. Air speed: The speed of air through the wind tunnel test section is vital information for aerodynamic research as the aerodynamic results depends on the relative airflow over the concerned object. Hence often the results so found are plotted with respect to the airspeed. Hence it is important to measure the airspeed correctly and to have a control on the speed of air flowing through the tunnel. For the purpose of measuring air speed the common system used is pitot probe but using a pitot probe in the test section is undesirable as if the probe is positioned behind the test model then the disturbance created by the model will affect the actual airspeed reaching the probe, whereas placing the probe ahead of the model can cause disturbed airflow over the model and therefore can induce inaccuracies in the research data. To overcome this problem a method known as ‘Centre line calibration’ can been used.
Fig 1.1 Centre line calibration Barlow.J.B(1999)
Centre Line Calibration is a method where the static pressure before and after the contraction is used to calibrate the velocity across the test section.
For this method the calibration a pitot probe is placed in the test section and the air speed is linked with the corresponding static pressure from the pressure tapings before and after contraction. Once this data base is created the pitot probe can be removed and the corresponding air speed can be displayed using the static pressure ratios. This eliminates the pitot probe and the errors that can be induced by it.
Mathematical calculations can be used to verify the results. According to Barlow.J.B “Low speed wind tunnel” (1999: 219): Assuming the flow to be incompressible the variation in speed is calculated using the following formula:
A1 V1 = A2 V2
Fig 1.2 Test Section dimensions as measured
Where A1 is the area before contraction and V1 is velocity before contraction. Whereas A2 is the area after contraction and V2 is the velocity entering the test section. From the dimension to the wind tunnel test section and the contraction ratio the two areas can be calculated. Knowing the initial fan velocity (V1) which in this case is known to be 1 to 30m/sec, the velocity at the test section can be easily calculated. For this calculation a Matlab program was designed and the results were as follows: 6.7000 40.2000 73.7000 107.2000 140.7000 174.2000 13.4000 46.9000 80.4000 113.9000 147.4000 180.9000 20.1000 53.6000 87.1000 120.6000 154.1000 187.6000 26.8000 60.3000 93.8000 127.3000 160.8000 194.3000 33.5000 67.0000 100.5000 134.0000 167.5000 201.0000
Another calculation for velocity can be done using dynamic pressure i.e.
Fig 1.3 Dynamic pressure Barlow.J.B(1999)
The results so obtained are: 6.7000 40.2000 73.7000 107.2000 140.7000 174.2000 13.4000 46.9000 80.4000 113.9000 147.4000 180.9000 20.1000 53.6000 87.1000 120.6000 154.1000 187.6000 26.8000 60.3000 93.8000 127.3000 160.8000 194.3000 33.5000 67.0000 100.5000 134.0000 167.5000 201.0000
This confirms the results. A graph was plotted to display the linear relation between the fan speed and the air speed through the tunnel.
2. Temperature:
As the speed of air flow through the tunnel increases the temperature also increases. This rise in temperature is denoted as dynamic temperature. Dynamic temperature can be calculated by the following formula:
Hence the
Total temperature = ambient air temperature + dynamic temperature
The ambient air temperature can be measured by using a thermometer. In this case for calculation purpose the ambient air temperature has been calculated using the elevation of scone airport.
Calculated rise in temperature (in degree C) with increase of airspeed (from 1m/sec to maximum):
0.0223 0.8043 2.7032 5.7192 9.8522 15.1022 0.0894 1.0947 3.2170 6.4564 10.8128 16.2863 0.2011 1.4298 3.7756 7.2383 11.8182 17.5150 0.3574 1.8096 4.3787 8.0649 12.8682 18.7884 0.5585 2.2341 5.0266 8.9362 13.9628 20.1065
Graphical representation using Matlab:
3. Pressure:
Pressure can be measured using Static pressure probes around the walls of the tunnel. Most important region being before and after the contraction.
Fig1.3 Static pressure probe Aero Instruments (2011)
Pressure probes can also be added to the test section itself as this can indicate any wall effect due to the model being tested. Accordingly corrections can be implemented to the final result. Mathematical calculations can be used to verify the pressure values. For this purpose as the wind tunnel is located in Scone airport the elevation of Scone airport was used to calculate the atmospheric pressure. Considering the atmospheric pressure as reference, the pressure inside the tunnel was calculated using.
P=rho*R*T
The value of density was calculated according to the atmospheric pressure and considering the flow to be incompressible, the value of rho was kept constant. R=287.05 and temperature inside the tunnel has been fed from the temperature calculation done above.
The pressure values so found were:
1.0e+05 *
1.0133 1.0161 1.0228 1.0334 1.0480 1.0665 1.0136 1.0171 1.0246 1.0360 1.0514 1.0707 1.0140 1.0183 1.0266 1.0388 1.0549 1.0750 1.0145 1.0196 1.0287 1.0417 1.0586 1.0795 1.0152 1.0211 1.0310 1.0448 1.0625 1.0841
Also co-efficient of pressure was calculated using:
Princeton.edu(n.d.)
The Cp was found to be 12.8257.
Graphical representation of pressure using Matlab:
Other kinds of pressure probes that can be included in future are: i. Stagnation probe: Such probes are used to find the stagnation pressure and at various positions in the tunnel. This helps in understanding the pressure gradient and the stagnation region of the tunnel. For centreline stagnation pressure measurement the experimental values can be verified using mathematical calculations:
According to princeton.edu “Bernoulli’s Equation” (N.D.):
Stagnation pressure=static pressure + dynamic pressure
1.0e+05 *
1.0133 1.0135 1.0140 1.0148 1.0160 1.0174 1.0133 1.0136 1.0141 1.0150 1.0162 1.0177 1.0133 1.0136 1.0143 1.0152 1.0165 1.0181 1.0133 1.0137 1.0145 1.0155 1.0168 1.0184 1.0134 1.0139 1.0146 1.0157 1.0171 1.0188
Graphical representation using matlab:
ii. Flow direction probe: An accurate knowledge of flow direction is very important to ensure correct aerodynamic analysis. A slight change in direction can make the air flow over the model angular. Such angular flow can adversely affect the aerodynamic data. To rectify such errors the flow direction needs to be measured. For this purpose flow direction probes are used. According to g.eng.cam.ac.uk “Pressure probes” (N.D.): The most common probes are ‘cobra’, ’wedge’, ‘five hole probe/pyramid probe’ and cylindrical probes. Flow direction is measured mainly by two methods: a) Null method: In this method the probe is moved until it reaches a point where the pressure from all sides becomes equal. b) Calibration method: In this case the probe is fixed. The static pressures measured are tallied with previous calibration data and flow data is deduced. Pyramid Cylindrical 4. Reynolds Number:
Aerodynamic forces depend on viscosity of the gas. Reynolds number is a dimensionless quantity which expresses the ratio between inertial forces and viscous forces. To determine and predict boundary layer, flow separation and other parameter Reynolds number is vital. According to Engineering tool box “Reynolds Number” (N.D.): Whenever the Reynolds number is less than 2300 the flow is laminar. If Reynolds number is greater than 2300 but less than 4000 then the flow is transient and if Reynolds number is greater than 4000 the flow is considered to be turbulent. One of the most commonly used methods of aerodynamic research is by keeping a constant Reynolds Number.
Reynolds number can be calculated using the following formula for pipe or duct:
Where v= kinematic viscosity of air (acquired from The Engineering Toolbox ), dh= hydraulic diameter and u=dynamic viscosity.
On calculation of Reynolds number using Matlab the following numbers were found:
RE =
1.0e+05 *
0.2365 1.4192 2.6019 3.7846 4.9673 6.1500 0.4731 1.6558 2.8385 4.0212 5.2039 6.3866 0.7096 1.8923 3.0750 4.2577 5.4404 6.6231 0.9462 2.1289 3.3115 4.4942 5.6769 6.8596 1.1827 2.3654 3.5481 4.7308 5.9135 7.0962
Graphical representation using Matlab:
5. Turbulence Sphere:
Turbulence sphere can be of different sizes and they help in measuring the turbulence factor over a range of tunnel speed. According to Barlow.J.B “Low speed wind tunnel” (1999: 226): The turbulence factor (TF) can also be calculated by comparing the tunnel’s Reynolds number to the free air Reynolds number.
TF=385000/RN
On calculating for the wind tunnel at Scone, the following results were found for varying air speed:
TF =
16.2764 2.7127 1.4797 1.0173 0.7751 0.6260 8.1382 2.3252 1.3564 0.9574 0.7398 0.6028 5.4255 2.0345 1.2520 0.9042 0.7077 0.5813 4.0691 1.8085 1.1626 0.8567 0.6782 0.5613 3.2553 1.6276 1.0851 0.8138 0.6511 0.5425
The use of turbulence sphere can verify these values and an average value of tunnel turbulence can be taken for further calculation. In this case the average of the above matrix was found to be= 2.1675
This does not give the magnitude of turbulence but with the use of turbulence sphere actual turbulence in either the axial direction or lateral direction can be found. Turbulence sphere may also be able to monitor any change in tunnel turbulence.
Fig 1.4 Turbulence Sphere Aerolab(N.D.)
6. Force measurement:
One of the critical aerodynamic data for a wind tunnel experiment is measurement of various forces developed due to the air flow. The basic forces measured are: Lift, Drag and Pitching moment. For this purpose a reference frame must be selected. According to Barlow.J.B “Low speed wind tunnel” (1999: 235): The two most common frames used are:
a) Wind Axis: Such axis is used for an external load measuring balance. For this the ‘x’ axis is considered to be facing the wind, ’z’ axis points down and ‘y’ across the test section. It should be noted that under this arrangement the lift force will act in the –ve ‘z’ direction, the drag will act in –ve ‘x’ direction but the side forces will act in the +ve ‘y’ direction. Due to irregularities or angular flow the actual wind axis or ‘x’ axis may be slightly offset to the test section. b) Body Axis: Such a axis is generally used for internal load measuring balances. As in this case the designed wind tunnel has a external balance, hence body axis has not been evaluated.
Different types of balance design are available for the either a three component or a six component balance. Due to budget constrains a 3 component balance will be suitable for this wind tunnel. Further such balances can be of two main types: 1) Internal and 2) external. Internal balances are those which are placed within the model itself where as external balances are common for a tunnel and are attached to the model externally. As most wind tunnel contains external balances and it is a cost efficient method of testing, setting up of external balance for this wind tunnel was proposed. The forces that can be measured by three component external balance are: Lift, Drag and Pitching moment. According to Barlow.J.B “Low speed wind tunnel” (1999: 251): The most common types of designs used are: a) Platform Balance: It uses three of four legs for support. It is a rugged design and naturally orthogonal. They are very easy to construct and align but such a design has the following disadvantages: i) Even for large forces moments appear as a small difference which is undesirable. ii) The resolving centre of the balance is not at the centre of the tunnel; hence the pitching moment must be transferred. iii) The drag side force puts the pitching and rolling moment on load ring which must be removed from the final data and calculation. Some of these can be eliminated by computational correction. With advanced software and programs such computation has become easy.
Fig 1.5 Platform balance Barlow.J.B (1999) b) Yoke Balance: This balance has a moment resolving centre near the centre of the tunnel and provides a better deflection for pitching and side force. The forces must be summed up: for drag three forces needs to be added, for lift two forces needs to be added. This type of balance overcomes most of the disadvantages of platform type balance and is still easy to construct.
Fig 1.6 Yoke balance Barlow.J.B (1999)
c) Pyramid balance: As such a design allows the balance to read the moments about the resolving centre the values can be directly used and no component needs to be added. Pyramid balance has many advantages over the other two designs but it is highly complicated and it requires six components.
According to the above analysis the Yoke type balance should be the most suitable design for this wind tunnel.
Maximum Force: Another important factor in deciding the components of the balance is the forces to be measured and the maximum force expected. This data is vital to acquire the correct components. Out of Lift, drag and pitching moment, lift is presumed to be the biggest. Hence for maximum load calculation, maximum lift value was calculated with a high coefficient of lift of 3.5 and aspect ratio of 2. According to Barlow.J.B “Low speed wind tunnel” (1999: 329): the maximum dimension of the model should not exceed ten percent of the test section dimension hence the wing span has been taken as 10% of 0.46m as 46cm is the width of the tunnel.
The following lift values (N) were found for the entire speed range. 0.1021 3.6754 12.3534 26.1361 45.0235 69.0156 0.4084 5.0026 14.7016 29.5052 49.4136 74.4266 0.9188 6.5340 17.2539 33.0785 54.0078 80.0418 1.6335 8.2696 20.0104 36.8560 58.8062 85.8612 2.5524 10.2094 22.9712 40.8377 63.8088 91.8847
Plotting a graph in matlab:
As shown in the graph above the lift force increases exponentially with increase in velocity the maximum force is experienced at the highest airspeed. Considering the Maximum force to measured as 92 N and giving a tolerance of 8N force sensing element requirement was set at a 100N.
Various for force sensing elements were evaluated for this purpose:
The three sensing elements analysed are: a) FlexiForce: Flexi force is based on a thin film containing printed ink which changes in electrical resistance as it is compressed. The total thickness of the sensor, ink and substrate combined is about 0.2 mm. One of the major drawback of this system is that it requires a tactile force between two solid objects to be applied, so that that there is any change to the sensor output. The other major disadvantage with such a thin and non-intrusive design is that it has low accuracy. According to Tekscan data sheet: The maximum accuracy of such system is around 3% which is quite low for wind tunnel application as it requires precise measurements and accurate data for analysis.
b) Strain Gauges: Strain gauges uses the strain induced in the component due to the load applied. The electrical conductance of a material depends on the conductor’s geometry and this principle is used for strain gauges. The variation in the conductance induces an error voltage in the circuit and this change in potential caused the current to flow which is then measured as an electrical signal. For precise measurement strain gauges are arranged in a Wheatstone circuit.
Fig1.7 Wheatstone bridge Wikipedia (N.D.)
When the gauge is under strain the conductivity or the resistance changes this creates an imbalance to the circuit which then initiates a flow from point B to D. This current can be measured and the load can be calculated. The accuracy of this system depends on the type of material used in the construction of the strain gauge and also on the sensitivity of the Wheatstone bridge circuit. On market research it was found that a strain gauge (Red mild steel strain gauge, 8mm type 11 from RS) that can measure more than 100N can be procured at £ 5.10
c) Load cells: Load cells are basically transducers that convert a force to electrical signal. It has two parts:
Fig 1.8 Load cell load cell online (2009)
i. Mechanical arrangement: The mechanical arrangement aligns the sensor gauges so that the forces can be sensed. ii. Force sensing gauge: This element deforms on application of load and generates a signal.
A sensing gauge generally consists of strain gauges in a Wheatstone bridge arrangement. Four strain gauge full bridge, two strain gauge or half bridge, and one strain gauge or quarter bridge are available. The most accurate being the full bridge arrangement. The electrical output is in order of mili-volts hence the signal is amplified and plugged into an algorithm to calculate the force. According to RS model 1042: a low profile aluminium load cell which can measure maximum of 100N force will cost around £ 96.39. The accuracy of this load cell is 0.15%. The other types of load cells with very high accuracy are the piezoelectric load cells and capacitive load cells. Due to cost constrains these types of load cells were not taken into consideration.
From the above analysis it was understood that the most suitable solution for the purpose of wind tunnel being built, should be load cells.
As the load cell balance is placed outside the test section, in this case on top of the test section, to transfer the loads on the model to the balance three struts are attached. The tail strut has the freedom to move up and down to provide pitching moment to the model. This pitching moment can be measured in terms of angle by using a rack and pinion arrangement. The tail strut can have grooves where a rotating gear can be meshed. As the strut moves the gear rotates. This rotation can be fed to a pointer on a dial calibrated in degrees. Thus precise pitching angle can be displayed. The pitching movement can also be electrically controlled by using a small motor. The operator enters the desired pitching angle and the signal so generated is passed to the motor after conversion and amplification. This signal drives the motor and the motor is coupled with the rotating gear. As the gear rotates the rack moves to the desired position and the dial indicates the pitch angle. 7. Turn Table:
Turn table is a mechanical set up that allows the rotational movement of the model. Small and precise angular deflection provided by the turn table allows yawing motion of the model and thereby helps in aerodynamic analysis during yawing. Such precise movement can be delivered by using a bevel gear mechanism. Bevelling the gear also allows bringing the control wheel to a desired height.
Fig 1.9 Bevel gear
Such a mechanism will not only provide better control over the rotary movement but also provide rigidity to the model by not allowing the aerodynamic forces acting on the model, during a test, to turn the turntable. The bevelled gear can be coupled with control wheel and indicator to display the angle of rotation.
Fig 1.10 Turn table
Fig.1.11 Turn table
In this design as shown in the figure above the turn table is mounted on top and rests on a rail containing roller bearings. As the bevelled gear rotate the turntable slide over the roller bearings. The turn table is so designed that the inner surface is flush with the top wall of the test section so as to minimise any disturbance to the air flow. Also the design is such that the turn table can easily be removed from top, to access the test section which is useful to set up a model of for maintenance purposes.
8. Fan speed Indication and control System:
As observed in the previous sections the temperature, pressure, Reynolds number, the turbulence, and the force acting etc all depends on the speed of air flow. Hence it is crucial to have a precise measurement and control system to control the speed of air. One of the simplest and common ways of controlling the speed of air through the tunnel is by adjusting the speed of the fan. To control the fan speed a speed indicator and control system was designed. This will ensure that the speed of the fan is set to the desired rpm and a feedback system will confirm the conformation with the input. The system is made of a control synchro, analogue to digital convertor, a computer or control panel, digital to analogue convertor, amplifier, a stepper motor and a potentiometer. The working of the design has been explained below.
Fig 1.12 Fan speed control
The fan in the wind tunnel provides initial velocity to the air and thus makes the air flow. To control and regulate the flow to a desired value the current speed of the fan must be known. For this purpose a control synchro can be used. Control synchro has a movable rotor arm in the transmitter side (Cx). This rotor is attached with the shaft of the fan and the small excitation charge passes through it. As the shaft rotates the rotor rotates. As the rotor rotates the magnetic lines of forces interact with the stator which induces an emf through mutual induction in the stator winding. This stator winding is then connected to the receiver side also known as the control transformer which is in the control room. This control transformer has a static rotor winding and as the stator carries the charge from the transmitter, the stator induces an emf. This emf will only be generated when there is a change in fan speed as only then the rate of change of magnetic field will change in the transmitter side. The emf so induced in the rotor of the receiver end can then be passed through an analogue to digital convertor so that the data is ready as a digital signal to be fed to a digital display or a computer. Every value of electricity can be assigned a corresponding speed value through calculation. Then the computer program can display the speed from its database. This speed can then be cross checked with the airspeed calculated through static pressure and centre line calibration. Any variation in the two results will also indicate the efficiency of the fan. Thus with time due to deposition of oil, dust, etc if the efficiency of the fan reduces this can be used as an indicator, hence corrective maintenance action can be taken.
Now changing the speed can be done in the following steps.
The operator can feed the desired air speed in the computer. The computer then calculates the corresponding fan speed required and accordingly can generate a signal. This signal is then passed through a digital to analogue convertor and then an amplifier. This amplified signal can then be fed to a stepper motor. A stepper motor has geared mechanism which provides precise rotational motion. The shaft of this motor can be connected to a variable potentiometer wiper. As the rotor rotates the wiper arm moves to change the potential. This potentiometer is connected to the fan motor. Hence a change in potential will change the motor speed and thereby change the fan speed. Air speed measured using centre line calibration can be used as a feedback system. If after changing the potential the actual air speed is does not conforms to the desired air speed then an error signal can be generated. The magnitude of the error signal will depend on the difference between the desired velocity and the pressure velocity. This error signal then gets converted to an analogue signal which is then amplified and fed to the stepper motor. Thus it will correct the velocity.
9. Flow visualization
One of the most important factors in a wind tunnel test is analysis of air flow in and around an object. As air is transparent it is difficult to directly observe the air movement. Instead some simple methods are used to conduct quantitative and qualitative analysis of the air flow. Such methods make the flow pattern visible. These methods can be categorized in mainly three types: i. Surface flow visualization: for example Turfts, oil etc. ii. Particle tracer method: Example smoke, fog, PIV etc. iii. Optical method: Shadowgraph, interferometry etc.
Some of the common methods used analysed for this wind tunnel are smoke, china clay, oil, turfts, PIV etc.
a) Turfts: One of the simplest methods of flow visualization is using turfts. In this method light and flexible turfts often made of yarn, cotton threads, fluorescent material monofilaments etc are used. These turfts needs to be attached to the surface of the model. Turfts shows where the flow is steady, region of flow separation, flow direction and buffering flow. One of the advantages of this method is that it is very cost effective. One of the biggest disadvantages of this system is that the effect of tuft itself over the air flow is very high. This can induce inaccuracies in the research work. Hence this method has not been used. b) Oil: In this method is used to show the surface flow. The oil is applied using a brush on the model surface. As the air flows over the surface the sheer stress acting on the surface oil causes the oil to flow and create a pattern. Studying this pattern turbulence, flow separation, flow pattern and flow direction can be determined. One of the major disadvantage of this system is that the tunnel requires extensive cleaning after using this method as oil deposited on the fan can reduce the fan efficiency. Another challenge is to create a contrast between the model and the oil used. Hence this method was not selected. c) China Clay: It is another surface visualization technique which is applied on the model surface. The tunnel is then started. The air flowing creates a pattern which can be studied to understand the flow. This method requires the fan to be started immediately after application of the paint else the paint dries quickly which provides in accurate indication. Also the section painted first will be drier than the section painted last. Such in-accuracies can have an adverse affect on the outcome. Hence this method was not selected. d) Smoke: In this method smoke is induced into the test section. As smoke is visible to naked eyes the flow pattern can be easily detected. This is one of the most common method used in low speed wind tunnels. The smoke required for this method can easily be created using burning oil, charcoal, tobacco, rotten wood or damp straw etc. Hence this method is very cost effective and easy. The slow can be recorded using high speed camera. These cameras can capture stop motion pictures which allows freezing a motion frame into a picture. Such frames can then be analysed and stored for future references. This was found to be the most suitable option for this wind tunnel e) PIV (Particle image Velocimetry): It is an optical method which is used to obtain instantaneous velocity and other properties in a fluid. In this method the particle concentration is so low that it is possible to identify individual particles. In this method the model is placed between two films and laser light is passed through them. It is widely used in low speed wind tunnel as it provides the best means to investigate flow fields. But these equipments for this method are very expensive and hence this method was discarded due to financial limitations.
Fig 1.13 PIV Wikipedia (n.d) f) Holography and Interferometry: Holographs can be made using laser with coherent lights. These holographs can then be used to measure density and flow visualization. The laser light is broken down into object beam and reference beam. The object beam passes through the film to combine with the reference beam and the object beam information is reconstructed to gather the flow data. As these methods are still being developed and has not matured as economically viable, it was discarded as an option.
Smoke visualization technique was found to be the most suitable one in terms of effective visual effects and cost effective solution. Hence this method can be used for the proposed wind tunnel. 10. Conclusion: * Limitations and Problems faced:
New Domain: As this is the first time I looked into an instrumentation side of a wind tunnel, it was an unknown domain. The instruments of a wind tunnel are very different and have different criteria’s than the types learnt for aircraft. So it was a new experience which required learning from scratch. E.g.: In aircraft to measure air speed a pitot probe is placed in the free air stream but to measure air speed static pressure, contraction ratios and centreline calibration is used. Systems like load cell balance, turn table mechanism and turbulence spheres were absolutely unknown. Learning and understanding these new concepts was a enjoyable challenge.
Load cells: Procuring load cells initially was found very expensive. All suppliers that were contacted was willing to cell the entire balance which was costing more than £ 3000. This was way higher than our budget due to which alternative solutions like Flexiforce and strain gauges were evaluated. As the accuracy requirement is quite high for wind tunnel application a morphological analysis was conducted to as to evaluate the options. On evaluation the most effective solution was found to be load cells. At such a later stage a breakthrough was achieved when load cell component supplier “RS” was found, but due to paucity of time the balance could not be designed.
* Further Recommendation i. Speed: the maximum speed of the fan motor is about 1400 rpm but as this is a low speed wind tunnel the speed of air flow cannot exceed the speed of sound. Currently a speed governor allows a maximum speed of 100 rpm which produces a maximum air speed of 201 m/sec which is about 0.59 mach. Most transport aircraft fly at a maximum speed of around 0.8 to 0.9 mach. So the speed through the governor can be increased so as to attain 0.9 mach. To do this first a matlab program was used to first calculate the corresponding speed of 0.9mach in metres/sec. To do this the temperature of scone airport calculated using airport elevation was used. The velocity so found was 305.8448 m/sec i.e. 306 m/sec approx. Now to attain this air velocity the fan speed needs to be increased from 30 m/sec to 45.6716 i.e. 46 m/sec approx. Allowing this increase of speed which just needs changing the speed governor will help in unleashing the full potential of this wind tunnel.
ii. Contoured wall: This technology is one of the most recent innovations in the low speed wind tunnel design. A model when placed in the tunnel causes a blockage to the air flow. Due to this blockage a bottle neck is created which then increases the effective airflow. This technique reduces the blockage caused to the air flow due to the model and there by reduces model error. It can also increase the effective cross section of the test section thereby allowing slightly larger model dimensions.
General wind tunnel
Fig 1.14 The Honda Tunnel (N.D.)
Contoured wall
Fig 1.15 The Honda Tunnel (N.D.)
iii. Ultraviolet light: Ultraviolet light illumination can be used for better flow visualization. When using smoke for flow visualization technique ultraviolet light can assist in better visual effect. For this system to be incorporated it is mandatory that the place around the wind tunnel is kept as dark as possible. Ultraviolet light can cause damage to naked eyes so proper protective glasses should be used. Other safety measures and precautions as instructed by the manufacturer should be followed. Glasgow University uses ultraviolet light systems provided by Yokohama.
iv. Wake survey rake: A wake survey rake can be added as it is highly recommended for drag calculations. It uses a multi tube manometer as shown in the figure below:
Fig1.16 Wake survey Wind tunnel operation (n.d.) v. Acoustic measurements: Acoustic measuring instruments can be included in the tunnel to measure the noise generated due to the turbulence in air flow around a surface. For this the test chamber needs to be modified so as to create a acoustically insulated environment. Sound sensors can be placed in and around the walls of the test section and this can help in acoustic mapping of the model during the test. The sound so collected can be analysed to improve the design and reduce noise and there by noise pollution.
Fig 1.17 Acoustic image pininfarina(n.d.)
Rolling Road: A rolling road is a device used to simulate rolling motion in a wind tunnel. Such a system is highly useful for aerodynamic analysis of automobile models. They are often used in designing of formula one car. As these cars have their wheels exposed to the airstream, a rolling road provides the opportunity to access the aerodynamic effect of the spinning wheels.
Fig 1.18 Rolling road
Fig 1.19 AutoCAD Rolling road
This project provided new exposure and provided with a better understanding of the working of a wind tunnel. This work has also taught new innovative methods that can be used. It gave a firsthand experience of practical projects and their challenges. It provided with the firsthand experience of procuring parts with a limited budget and the challenges so faced. It also provided an insight into designing and a better understanding of matlab modelling.
11. References
1. Jewel B, Barlow (1999) LOW SPEED WIND TUNNEL TESTING 2. Aero Instruments
Accessed from: www.aero-inst.com/Products/
Date: 18/05/2012 3. Princetron.edu
Accessed from: www.princeton.edu/~asmits/Bicycle_web/Bernoulli
Date: 08/04/2012 4. Pressure probes
Accessed from: www-g.eng.cam.ac.uk/whittle/current-research/hph/pressure-probes/pressure-probes
Date: 06/02/2012 5. Engineering tool box
Accessed from: www.engineeringtoolbox.com/reynolds-number-d_237
Accessed from: www.engineeringtoolbox.com/air-absolute-kinematic-viscosity-d_601
Accessed from: www.engineeringtoolbox.com/hydraulic-equivalent-diameter-d_458
Date: 09/02/2012 6. Aerolab
Accessed from: www.aerolab.com/Galleries
Date: 06/06/2012 7. Load cells online
Accessed from: www.loadcellonline.com/index.php/bending-beam-load-cell/load-cell
Date: 09/2/2012 8. RS
Accessed from: uk.rs-online.com/web/c/?sra=oss&searchTerm=load+cells&x=0&y=0
Date: 06/05/2012 9. Imperial college London (The Honda Wind tunnel)
Accessed from: www3.imperial.ac.uk/pls/portallive/docs/1/5039977.PDF
Date: 20/12/2011 10. Wind tunnel operation
Accessed from: maelabs.ucsd.edu/mae171/Winddocs/Old%20Procedures%20Wind%20Tunnel
Date: 05/06/2012 11. Pininfarina
Accessed from: www.pininfarina.com/en/services/wind_tunnel/facilities/computer_center/computer_center
Date: 07/06/2012
ACKNOWLEDGEMENTS:
My efforts to research and work on this project would not have been possible without the support of various individuals and organizations.
Firstly I would like to convey my deepest gratitude towards Mr. Andrew Ray without whom this work would not have been possible.
I would also like to thank Mr. Bassam Rakhshani and Mr. Daniel Olufisan for their valued assistance.
I would also like to convey special thanks to Mr. Peter Peffers and Mr. Robert Sutherland for their guidance.
My gratitude towards Anvesh Kadmi, Peter Bell and coleen Grieg for their coordination and effort.
I will take this opportunity to convey my gratitude to my parents, brother and friends for their support.
Last but not the least I would like to thank Perth College and all member of staff for giving me this opportunity for assisting me throughout my work.
12. Appendix
Matlab Program
* Logbook: * Glasgow Visit on 12th Oct 2011 * Jewel B, Barlow (1999) LOW SPEED WIND TUNNEL TESTING * Agard advisory group for aerospace research and development (1997) AERODYNAMICS OF WIND TUNNEL CIRCUITS AND THEIS COMPONENTS * No name (2010) NASA [online] available from * http://www.grc.nasa.gov/WWW/k-12/WindTunnel/build.html * Accessed on 4th Nov 2011
* VTI Instruments * www.vtiinstruments.com/Products.aspx * Accessed on 28th Jan 2012 * Strainstall * www.strainstallloadcells.co.uk/load_cells.html * accessed on 29the Jan 2012 * Kineoptics * www.kineoptics.com/WTB.html * accessed on 29th Jan 2012 * SENSOR * www.loadcells.com/?gclid=CL7ml6GS0K0CFasMtAodFW6Qng * Tekscan * www.tekscan.com/ * accessed on 30th Jan 2012 * National Instruments * www.ni.com/aerospace-defense/ * Accessed on 31st jan 2012 * HORIBA * * www.horiba.com/automotive-test-systems/products/mechatronic-systems/wind-tunnel-balances/ * Accessed on 31st jan 2012 * AEROLAB * www.aerolab.com/Display_Pages/Models_Probes.html * Accessed on 2nd Feb 2012 * NASA * http://windtunnels.arc.nasa.gov/balcallab/balances.htm * accessed on 4th Feb 2012 * John Sleath Race Cars * http://www.john-sleath.com/ * accessed on 9th Feb 2012 * Aero Instruments * Accessed from: www.aero-inst.com/Products/ * Date: 18/05/2012 * Princetron.edu * Accessed from: www.princeton.edu/~asmits/Bicycle_web/Bernoulli * Date: 08/04/2012 * Pressure probes * Accessed from: www-g.eng.cam.ac.uk/whittle/current-research/hph/pressure-probes/pressure-probes * Date: 06/02/2012 * Engineering tool box * Accessed from: www.engineeringtoolbox.com/reynolds-number-d_237 * Accessed from: www.engineeringtoolbox.com/air-absolute-kinematic-viscosity-d_601 * Accessed from: www.engineeringtoolbox.com/hydraulic-equivalent-diameter-d_458 * Date: 09/02/2012 * Aerolab * Accessed from: www.aerolab.com/Galleries * Date: 06/06/2012 * Load cells online * Accessed from: www.loadcellonline.com/index.php/bending-beam-load-cell/load-cell * Date: 09/2/2012 * RS * Accessed from: uk.rs-online.com/web/c/?sra=oss&searchTerm=load+cells&x=0&y=0 * Date: 06/05/2012 * Imperial college London (The Honda Wind tunnel) * Accessed from: www3.imperial.ac.uk/pls/portallive/docs/1/5039977.PDF * Date: 20/12/2011 * Wind tunnel operation * Accessed from: maelabs.ucsd.edu/mae171/Winddocs/Old%20Procedures%20Wind%20Tunnel * Date: 05/06/2012 * Pininfarina * Accessed from: www.pininfarina.com/en/services/wind_tunnel/facilities/computer_center/computer_center * Date: 07/06/2012
References: 1. Jewel B, Barlow (1999) LOW SPEED WIND TUNNEL TESTING 2 * Jewel B, Barlow (1999) LOW SPEED WIND TUNNEL TESTING * Agard advisory group for aerospace research and development (1997) AERODYNAMICS OF WIND TUNNEL CIRCUITS AND THEIS COMPONENTS * No name (2010) NASA [online] available from * http://www.grc.nasa.gov/WWW/k-12/WindTunnel/build.html
12
INSTRUMENTATION OF A WIND TUNNEL
PROJECT BEng(Hons)
Gaurab Ghosal
ABSTRACT
The wind tunnel being built in Scone airport is a closed loop wind tunnel. On completion this will provide a much needed facility for education purpose as it can be used by engineering students for aerodynamic evaluations. To conduct these evaluation proper evaluating tools needs to be setup. In this section the possible instruments that can be setup in this wind tunnel has been evaluated with Matlab based evaluation, AutoCAD drawings, morphological analysis and other analytical tools. The most viable and suitable instruments or equipments has been suggested under the given conditions. The constrains and the limitations of this project has been described with further suggestions for improvements has been recommended
CONTENTS
Introduction......................................................................................................4
Measurements and Analysis..........................................................................4 * Air speed...............................................................................................4 * Temperature..........................................................................................7 * Pressure................................................................................................8 * Reynolds Number...............................................................................11 * Turbulence..........................................................................................12 * Force....................................................................................................13 * Turn Table...........................................................................................19 * Fan speed control...............................................................................21 * Flow visualization...............................................................................23
Conclusion.....................................................................................................25
* Limitation and problems faced..........................................................25 * Further recommendations..................................................................25
References.....................................................................................................30
Acknoledgements.........................................................................................32
Appendix........................................................................................................33
* Matlab Program
Logbook
INSTRUMENTATION OF A WIND-TUNNEL
INTODUCTION:
A wind tunnel is aerodynamic research tool which simulates the condition of an object moving in air by blowing the air on it. It is used to study the effects of air moving around a solid object and the forces developed due to this flow. To study the effect the object is placed in a controlled environment and is monitored with sensitive instruments. Instrumentation is vital as it provides the essential experimental information. The data so collected can be used to understand the aerodynamic properties which is then analysed to improve the design or to solve problems related to the design. A wind tunnel without its instruments is like a ‘ship without a sail’. Instrumentation also allows easy recording of such data and preserve it for future use.
MEASUREMENTS AND ANALYSIS: 1. Air speed: The speed of air through the wind tunnel test section is vital information for aerodynamic research as the aerodynamic results depends on the relative airflow over the concerned object. Hence often the results so found are plotted with respect to the airspeed. Hence it is important to measure the airspeed correctly and to have a control on the speed of air flowing through the tunnel. For the purpose of measuring air speed the common system used is pitot probe but using a pitot probe in the test section is undesirable as if the probe is positioned behind the test model then the disturbance created by the model will affect the actual airspeed reaching the probe, whereas placing the probe ahead of the model can cause disturbed airflow over the model and therefore can induce inaccuracies in the research data. To overcome this problem a method known as ‘Centre line calibration’ can been used.
Fig 1.1 Centre line calibration Barlow.J.B(1999)
Centre Line Calibration is a method where the static pressure before and after the contraction is used to calibrate the velocity across the test section.
For this method the calibration a pitot probe is placed in the test section and the air speed is linked with the corresponding static pressure from the pressure tapings before and after contraction. Once this data base is created the pitot probe can be removed and the corresponding air speed can be displayed using the static pressure ratios. This eliminates the pitot probe and the errors that can be induced by it.
Mathematical calculations can be used to verify the results. According to Barlow.J.B “Low speed wind tunnel” (1999: 219): Assuming the flow to be incompressible the variation in speed is calculated using the following formula:
A1 V1 = A2 V2
Fig 1.2 Test Section dimensions as measured
Where A1 is the area before contraction and V1 is velocity before contraction. Whereas A2 is the area after contraction and V2 is the velocity entering the test section. From the dimension to the wind tunnel test section and the contraction ratio the two areas can be calculated. Knowing the initial fan velocity (V1) which in this case is known to be 1 to 30m/sec, the velocity at the test section can be easily calculated. For this calculation a Matlab program was designed and the results were as follows: 6.7000 40.2000 73.7000 107.2000 140.7000 174.2000 13.4000 46.9000 80.4000 113.9000 147.4000 180.9000 20.1000 53.6000 87.1000 120.6000 154.1000 187.6000 26.8000 60.3000 93.8000 127.3000 160.8000 194.3000 33.5000 67.0000 100.5000 134.0000 167.5000 201.0000
Another calculation for velocity can be done using dynamic pressure i.e.
Fig 1.3 Dynamic pressure Barlow.J.B(1999)
The results so obtained are: 6.7000 40.2000 73.7000 107.2000 140.7000 174.2000 13.4000 46.9000 80.4000 113.9000 147.4000 180.9000 20.1000 53.6000 87.1000 120.6000 154.1000 187.6000 26.8000 60.3000 93.8000 127.3000 160.8000 194.3000 33.5000 67.0000 100.5000 134.0000 167.5000 201.0000
This confirms the results. A graph was plotted to display the linear relation between the fan speed and the air speed through the tunnel.
2. Temperature:
As the speed of air flow through the tunnel increases the temperature also increases. This rise in temperature is denoted as dynamic temperature. Dynamic temperature can be calculated by the following formula:
Hence the
Total temperature = ambient air temperature + dynamic temperature
The ambient air temperature can be measured by using a thermometer. In this case for calculation purpose the ambient air temperature has been calculated using the elevation of scone airport.
Calculated rise in temperature (in degree C) with increase of airspeed (from 1m/sec to maximum):
0.0223 0.8043 2.7032 5.7192 9.8522 15.1022 0.0894 1.0947 3.2170 6.4564 10.8128 16.2863 0.2011 1.4298 3.7756 7.2383 11.8182 17.5150 0.3574 1.8096 4.3787 8.0649 12.8682 18.7884 0.5585 2.2341 5.0266 8.9362 13.9628 20.1065
Graphical representation using Matlab:
3. Pressure:
Pressure can be measured using Static pressure probes around the walls of the tunnel. Most important region being before and after the contraction.
Fig1.3 Static pressure probe Aero Instruments (2011)
Pressure probes can also be added to the test section itself as this can indicate any wall effect due to the model being tested. Accordingly corrections can be implemented to the final result. Mathematical calculations can be used to verify the pressure values. For this purpose as the wind tunnel is located in Scone airport the elevation of Scone airport was used to calculate the atmospheric pressure. Considering the atmospheric pressure as reference, the pressure inside the tunnel was calculated using.
P=rho*R*T
The value of density was calculated according to the atmospheric pressure and considering the flow to be incompressible, the value of rho was kept constant. R=287.05 and temperature inside the tunnel has been fed from the temperature calculation done above.
The pressure values so found were:
1.0e+05 *
1.0133 1.0161 1.0228 1.0334 1.0480 1.0665 1.0136 1.0171 1.0246 1.0360 1.0514 1.0707 1.0140 1.0183 1.0266 1.0388 1.0549 1.0750 1.0145 1.0196 1.0287 1.0417 1.0586 1.0795 1.0152 1.0211 1.0310 1.0448 1.0625 1.0841
Also co-efficient of pressure was calculated using:
Princeton.edu(n.d.)
The Cp was found to be 12.8257.
Graphical representation of pressure using Matlab:
Other kinds of pressure probes that can be included in future are: i. Stagnation probe: Such probes are used to find the stagnation pressure and at various positions in the tunnel. This helps in understanding the pressure gradient and the stagnation region of the tunnel. For centreline stagnation pressure measurement the experimental values can be verified using mathematical calculations:
According to princeton.edu “Bernoulli’s Equation” (N.D.):
Stagnation pressure=static pressure + dynamic pressure
1.0e+05 *
1.0133 1.0135 1.0140 1.0148 1.0160 1.0174 1.0133 1.0136 1.0141 1.0150 1.0162 1.0177 1.0133 1.0136 1.0143 1.0152 1.0165 1.0181 1.0133 1.0137 1.0145 1.0155 1.0168 1.0184 1.0134 1.0139 1.0146 1.0157 1.0171 1.0188
Graphical representation using matlab:
ii. Flow direction probe: An accurate knowledge of flow direction is very important to ensure correct aerodynamic analysis. A slight change in direction can make the air flow over the model angular. Such angular flow can adversely affect the aerodynamic data. To rectify such errors the flow direction needs to be measured. For this purpose flow direction probes are used. According to g.eng.cam.ac.uk “Pressure probes” (N.D.): The most common probes are ‘cobra’, ’wedge’, ‘five hole probe/pyramid probe’ and cylindrical probes. Flow direction is measured mainly by two methods: a) Null method: In this method the probe is moved until it reaches a point where the pressure from all sides becomes equal. b) Calibration method: In this case the probe is fixed. The static pressures measured are tallied with previous calibration data and flow data is deduced. Pyramid Cylindrical 4. Reynolds Number:
Aerodynamic forces depend on viscosity of the gas. Reynolds number is a dimensionless quantity which expresses the ratio between inertial forces and viscous forces. To determine and predict boundary layer, flow separation and other parameter Reynolds number is vital. According to Engineering tool box “Reynolds Number” (N.D.): Whenever the Reynolds number is less than 2300 the flow is laminar. If Reynolds number is greater than 2300 but less than 4000 then the flow is transient and if Reynolds number is greater than 4000 the flow is considered to be turbulent. One of the most commonly used methods of aerodynamic research is by keeping a constant Reynolds Number.
Reynolds number can be calculated using the following formula for pipe or duct:
Where v= kinematic viscosity of air (acquired from The Engineering Toolbox ), dh= hydraulic diameter and u=dynamic viscosity.
On calculation of Reynolds number using Matlab the following numbers were found:
RE =
1.0e+05 *
0.2365 1.4192 2.6019 3.7846 4.9673 6.1500 0.4731 1.6558 2.8385 4.0212 5.2039 6.3866 0.7096 1.8923 3.0750 4.2577 5.4404 6.6231 0.9462 2.1289 3.3115 4.4942 5.6769 6.8596 1.1827 2.3654 3.5481 4.7308 5.9135 7.0962
Graphical representation using Matlab:
5. Turbulence Sphere:
Turbulence sphere can be of different sizes and they help in measuring the turbulence factor over a range of tunnel speed. According to Barlow.J.B “Low speed wind tunnel” (1999: 226): The turbulence factor (TF) can also be calculated by comparing the tunnel’s Reynolds number to the free air Reynolds number.
TF=385000/RN
On calculating for the wind tunnel at Scone, the following results were found for varying air speed:
TF =
16.2764 2.7127 1.4797 1.0173 0.7751 0.6260 8.1382 2.3252 1.3564 0.9574 0.7398 0.6028 5.4255 2.0345 1.2520 0.9042 0.7077 0.5813 4.0691 1.8085 1.1626 0.8567 0.6782 0.5613 3.2553 1.6276 1.0851 0.8138 0.6511 0.5425
The use of turbulence sphere can verify these values and an average value of tunnel turbulence can be taken for further calculation. In this case the average of the above matrix was found to be= 2.1675
This does not give the magnitude of turbulence but with the use of turbulence sphere actual turbulence in either the axial direction or lateral direction can be found. Turbulence sphere may also be able to monitor any change in tunnel turbulence.
Fig 1.4 Turbulence Sphere Aerolab(N.D.)
6. Force measurement:
One of the critical aerodynamic data for a wind tunnel experiment is measurement of various forces developed due to the air flow. The basic forces measured are: Lift, Drag and Pitching moment. For this purpose a reference frame must be selected. According to Barlow.J.B “Low speed wind tunnel” (1999: 235): The two most common frames used are:
a) Wind Axis: Such axis is used for an external load measuring balance. For this the ‘x’ axis is considered to be facing the wind, ’z’ axis points down and ‘y’ across the test section. It should be noted that under this arrangement the lift force will act in the –ve ‘z’ direction, the drag will act in –ve ‘x’ direction but the side forces will act in the +ve ‘y’ direction. Due to irregularities or angular flow the actual wind axis or ‘x’ axis may be slightly offset to the test section. b) Body Axis: Such a axis is generally used for internal load measuring balances. As in this case the designed wind tunnel has a external balance, hence body axis has not been evaluated.
Different types of balance design are available for the either a three component or a six component balance. Due to budget constrains a 3 component balance will be suitable for this wind tunnel. Further such balances can be of two main types: 1) Internal and 2) external. Internal balances are those which are placed within the model itself where as external balances are common for a tunnel and are attached to the model externally. As most wind tunnel contains external balances and it is a cost efficient method of testing, setting up of external balance for this wind tunnel was proposed. The forces that can be measured by three component external balance are: Lift, Drag and Pitching moment. According to Barlow.J.B “Low speed wind tunnel” (1999: 251): The most common types of designs used are: a) Platform Balance: It uses three of four legs for support. It is a rugged design and naturally orthogonal. They are very easy to construct and align but such a design has the following disadvantages: i) Even for large forces moments appear as a small difference which is undesirable. ii) The resolving centre of the balance is not at the centre of the tunnel; hence the pitching moment must be transferred. iii) The drag side force puts the pitching and rolling moment on load ring which must be removed from the final data and calculation. Some of these can be eliminated by computational correction. With advanced software and programs such computation has become easy.
Fig 1.5 Platform balance Barlow.J.B (1999) b) Yoke Balance: This balance has a moment resolving centre near the centre of the tunnel and provides a better deflection for pitching and side force. The forces must be summed up: for drag three forces needs to be added, for lift two forces needs to be added. This type of balance overcomes most of the disadvantages of platform type balance and is still easy to construct.
Fig 1.6 Yoke balance Barlow.J.B (1999)
c) Pyramid balance: As such a design allows the balance to read the moments about the resolving centre the values can be directly used and no component needs to be added. Pyramid balance has many advantages over the other two designs but it is highly complicated and it requires six components.
According to the above analysis the Yoke type balance should be the most suitable design for this wind tunnel.
Maximum Force: Another important factor in deciding the components of the balance is the forces to be measured and the maximum force expected. This data is vital to acquire the correct components. Out of Lift, drag and pitching moment, lift is presumed to be the biggest. Hence for maximum load calculation, maximum lift value was calculated with a high coefficient of lift of 3.5 and aspect ratio of 2. According to Barlow.J.B “Low speed wind tunnel” (1999: 329): the maximum dimension of the model should not exceed ten percent of the test section dimension hence the wing span has been taken as 10% of 0.46m as 46cm is the width of the tunnel.
The following lift values (N) were found for the entire speed range. 0.1021 3.6754 12.3534 26.1361 45.0235 69.0156 0.4084 5.0026 14.7016 29.5052 49.4136 74.4266 0.9188 6.5340 17.2539 33.0785 54.0078 80.0418 1.6335 8.2696 20.0104 36.8560 58.8062 85.8612 2.5524 10.2094 22.9712 40.8377 63.8088 91.8847
Plotting a graph in matlab:
As shown in the graph above the lift force increases exponentially with increase in velocity the maximum force is experienced at the highest airspeed. Considering the Maximum force to measured as 92 N and giving a tolerance of 8N force sensing element requirement was set at a 100N.
Various for force sensing elements were evaluated for this purpose:
The three sensing elements analysed are: a) FlexiForce: Flexi force is based on a thin film containing printed ink which changes in electrical resistance as it is compressed. The total thickness of the sensor, ink and substrate combined is about 0.2 mm. One of the major drawback of this system is that it requires a tactile force between two solid objects to be applied, so that that there is any change to the sensor output. The other major disadvantage with such a thin and non-intrusive design is that it has low accuracy. According to Tekscan data sheet: The maximum accuracy of such system is around 3% which is quite low for wind tunnel application as it requires precise measurements and accurate data for analysis.
b) Strain Gauges: Strain gauges uses the strain induced in the component due to the load applied. The electrical conductance of a material depends on the conductor’s geometry and this principle is used for strain gauges. The variation in the conductance induces an error voltage in the circuit and this change in potential caused the current to flow which is then measured as an electrical signal. For precise measurement strain gauges are arranged in a Wheatstone circuit.
Fig1.7 Wheatstone bridge Wikipedia (N.D.)
When the gauge is under strain the conductivity or the resistance changes this creates an imbalance to the circuit which then initiates a flow from point B to D. This current can be measured and the load can be calculated. The accuracy of this system depends on the type of material used in the construction of the strain gauge and also on the sensitivity of the Wheatstone bridge circuit. On market research it was found that a strain gauge (Red mild steel strain gauge, 8mm type 11 from RS) that can measure more than 100N can be procured at £ 5.10
c) Load cells: Load cells are basically transducers that convert a force to electrical signal. It has two parts:
Fig 1.8 Load cell load cell online (2009)
i. Mechanical arrangement: The mechanical arrangement aligns the sensor gauges so that the forces can be sensed. ii. Force sensing gauge: This element deforms on application of load and generates a signal.
A sensing gauge generally consists of strain gauges in a Wheatstone bridge arrangement. Four strain gauge full bridge, two strain gauge or half bridge, and one strain gauge or quarter bridge are available. The most accurate being the full bridge arrangement. The electrical output is in order of mili-volts hence the signal is amplified and plugged into an algorithm to calculate the force. According to RS model 1042: a low profile aluminium load cell which can measure maximum of 100N force will cost around £ 96.39. The accuracy of this load cell is 0.15%. The other types of load cells with very high accuracy are the piezoelectric load cells and capacitive load cells. Due to cost constrains these types of load cells were not taken into consideration.
From the above analysis it was understood that the most suitable solution for the purpose of wind tunnel being built, should be load cells.
As the load cell balance is placed outside the test section, in this case on top of the test section, to transfer the loads on the model to the balance three struts are attached. The tail strut has the freedom to move up and down to provide pitching moment to the model. This pitching moment can be measured in terms of angle by using a rack and pinion arrangement. The tail strut can have grooves where a rotating gear can be meshed. As the strut moves the gear rotates. This rotation can be fed to a pointer on a dial calibrated in degrees. Thus precise pitching angle can be displayed. The pitching movement can also be electrically controlled by using a small motor. The operator enters the desired pitching angle and the signal so generated is passed to the motor after conversion and amplification. This signal drives the motor and the motor is coupled with the rotating gear. As the gear rotates the rack moves to the desired position and the dial indicates the pitch angle. 7. Turn Table:
Turn table is a mechanical set up that allows the rotational movement of the model. Small and precise angular deflection provided by the turn table allows yawing motion of the model and thereby helps in aerodynamic analysis during yawing. Such precise movement can be delivered by using a bevel gear mechanism. Bevelling the gear also allows bringing the control wheel to a desired height.
Fig 1.9 Bevel gear
Such a mechanism will not only provide better control over the rotary movement but also provide rigidity to the model by not allowing the aerodynamic forces acting on the model, during a test, to turn the turntable. The bevelled gear can be coupled with control wheel and indicator to display the angle of rotation.
Fig 1.10 Turn table
Fig.1.11 Turn table
In this design as shown in the figure above the turn table is mounted on top and rests on a rail containing roller bearings. As the bevelled gear rotate the turntable slide over the roller bearings. The turn table is so designed that the inner surface is flush with the top wall of the test section so as to minimise any disturbance to the air flow. Also the design is such that the turn table can easily be removed from top, to access the test section which is useful to set up a model of for maintenance purposes.
8. Fan speed Indication and control System:
As observed in the previous sections the temperature, pressure, Reynolds number, the turbulence, and the force acting etc all depends on the speed of air flow. Hence it is crucial to have a precise measurement and control system to control the speed of air. One of the simplest and common ways of controlling the speed of air through the tunnel is by adjusting the speed of the fan. To control the fan speed a speed indicator and control system was designed. This will ensure that the speed of the fan is set to the desired rpm and a feedback system will confirm the conformation with the input. The system is made of a control synchro, analogue to digital convertor, a computer or control panel, digital to analogue convertor, amplifier, a stepper motor and a potentiometer. The working of the design has been explained below.
Fig 1.12 Fan speed control
The fan in the wind tunnel provides initial velocity to the air and thus makes the air flow. To control and regulate the flow to a desired value the current speed of the fan must be known. For this purpose a control synchro can be used. Control synchro has a movable rotor arm in the transmitter side (Cx). This rotor is attached with the shaft of the fan and the small excitation charge passes through it. As the shaft rotates the rotor rotates. As the rotor rotates the magnetic lines of forces interact with the stator which induces an emf through mutual induction in the stator winding. This stator winding is then connected to the receiver side also known as the control transformer which is in the control room. This control transformer has a static rotor winding and as the stator carries the charge from the transmitter, the stator induces an emf. This emf will only be generated when there is a change in fan speed as only then the rate of change of magnetic field will change in the transmitter side. The emf so induced in the rotor of the receiver end can then be passed through an analogue to digital convertor so that the data is ready as a digital signal to be fed to a digital display or a computer. Every value of electricity can be assigned a corresponding speed value through calculation. Then the computer program can display the speed from its database. This speed can then be cross checked with the airspeed calculated through static pressure and centre line calibration. Any variation in the two results will also indicate the efficiency of the fan. Thus with time due to deposition of oil, dust, etc if the efficiency of the fan reduces this can be used as an indicator, hence corrective maintenance action can be taken.
Now changing the speed can be done in the following steps.
The operator can feed the desired air speed in the computer. The computer then calculates the corresponding fan speed required and accordingly can generate a signal. This signal is then passed through a digital to analogue convertor and then an amplifier. This amplified signal can then be fed to a stepper motor. A stepper motor has geared mechanism which provides precise rotational motion. The shaft of this motor can be connected to a variable potentiometer wiper. As the rotor rotates the wiper arm moves to change the potential. This potentiometer is connected to the fan motor. Hence a change in potential will change the motor speed and thereby change the fan speed. Air speed measured using centre line calibration can be used as a feedback system. If after changing the potential the actual air speed is does not conforms to the desired air speed then an error signal can be generated. The magnitude of the error signal will depend on the difference between the desired velocity and the pressure velocity. This error signal then gets converted to an analogue signal which is then amplified and fed to the stepper motor. Thus it will correct the velocity.
9. Flow visualization
One of the most important factors in a wind tunnel test is analysis of air flow in and around an object. As air is transparent it is difficult to directly observe the air movement. Instead some simple methods are used to conduct quantitative and qualitative analysis of the air flow. Such methods make the flow pattern visible. These methods can be categorized in mainly three types: i. Surface flow visualization: for example Turfts, oil etc. ii. Particle tracer method: Example smoke, fog, PIV etc. iii. Optical method: Shadowgraph, interferometry etc.
Some of the common methods used analysed for this wind tunnel are smoke, china clay, oil, turfts, PIV etc.
a) Turfts: One of the simplest methods of flow visualization is using turfts. In this method light and flexible turfts often made of yarn, cotton threads, fluorescent material monofilaments etc are used. These turfts needs to be attached to the surface of the model. Turfts shows where the flow is steady, region of flow separation, flow direction and buffering flow. One of the advantages of this method is that it is very cost effective. One of the biggest disadvantages of this system is that the effect of tuft itself over the air flow is very high. This can induce inaccuracies in the research work. Hence this method has not been used. b) Oil: In this method is used to show the surface flow. The oil is applied using a brush on the model surface. As the air flows over the surface the sheer stress acting on the surface oil causes the oil to flow and create a pattern. Studying this pattern turbulence, flow separation, flow pattern and flow direction can be determined. One of the major disadvantage of this system is that the tunnel requires extensive cleaning after using this method as oil deposited on the fan can reduce the fan efficiency. Another challenge is to create a contrast between the model and the oil used. Hence this method was not selected. c) China Clay: It is another surface visualization technique which is applied on the model surface. The tunnel is then started. The air flowing creates a pattern which can be studied to understand the flow. This method requires the fan to be started immediately after application of the paint else the paint dries quickly which provides in accurate indication. Also the section painted first will be drier than the section painted last. Such in-accuracies can have an adverse affect on the outcome. Hence this method was not selected. d) Smoke: In this method smoke is induced into the test section. As smoke is visible to naked eyes the flow pattern can be easily detected. This is one of the most common method used in low speed wind tunnels. The smoke required for this method can easily be created using burning oil, charcoal, tobacco, rotten wood or damp straw etc. Hence this method is very cost effective and easy. The slow can be recorded using high speed camera. These cameras can capture stop motion pictures which allows freezing a motion frame into a picture. Such frames can then be analysed and stored for future references. This was found to be the most suitable option for this wind tunnel e) PIV (Particle image Velocimetry): It is an optical method which is used to obtain instantaneous velocity and other properties in a fluid. In this method the particle concentration is so low that it is possible to identify individual particles. In this method the model is placed between two films and laser light is passed through them. It is widely used in low speed wind tunnel as it provides the best means to investigate flow fields. But these equipments for this method are very expensive and hence this method was discarded due to financial limitations.
Fig 1.13 PIV Wikipedia (n.d) f) Holography and Interferometry: Holographs can be made using laser with coherent lights. These holographs can then be used to measure density and flow visualization. The laser light is broken down into object beam and reference beam. The object beam passes through the film to combine with the reference beam and the object beam information is reconstructed to gather the flow data. As these methods are still being developed and has not matured as economically viable, it was discarded as an option.
Smoke visualization technique was found to be the most suitable one in terms of effective visual effects and cost effective solution. Hence this method can be used for the proposed wind tunnel. 10. Conclusion: * Limitations and Problems faced:
New Domain: As this is the first time I looked into an instrumentation side of a wind tunnel, it was an unknown domain. The instruments of a wind tunnel are very different and have different criteria’s than the types learnt for aircraft. So it was a new experience which required learning from scratch. E.g.: In aircraft to measure air speed a pitot probe is placed in the free air stream but to measure air speed static pressure, contraction ratios and centreline calibration is used. Systems like load cell balance, turn table mechanism and turbulence spheres were absolutely unknown. Learning and understanding these new concepts was a enjoyable challenge.
Load cells: Procuring load cells initially was found very expensive. All suppliers that were contacted was willing to cell the entire balance which was costing more than £ 3000. This was way higher than our budget due to which alternative solutions like Flexiforce and strain gauges were evaluated. As the accuracy requirement is quite high for wind tunnel application a morphological analysis was conducted to as to evaluate the options. On evaluation the most effective solution was found to be load cells. At such a later stage a breakthrough was achieved when load cell component supplier “RS” was found, but due to paucity of time the balance could not be designed.
* Further Recommendation i. Speed: the maximum speed of the fan motor is about 1400 rpm but as this is a low speed wind tunnel the speed of air flow cannot exceed the speed of sound. Currently a speed governor allows a maximum speed of 100 rpm which produces a maximum air speed of 201 m/sec which is about 0.59 mach. Most transport aircraft fly at a maximum speed of around 0.8 to 0.9 mach. So the speed through the governor can be increased so as to attain 0.9 mach. To do this first a matlab program was used to first calculate the corresponding speed of 0.9mach in metres/sec. To do this the temperature of scone airport calculated using airport elevation was used. The velocity so found was 305.8448 m/sec i.e. 306 m/sec approx. Now to attain this air velocity the fan speed needs to be increased from 30 m/sec to 45.6716 i.e. 46 m/sec approx. Allowing this increase of speed which just needs changing the speed governor will help in unleashing the full potential of this wind tunnel.
ii. Contoured wall: This technology is one of the most recent innovations in the low speed wind tunnel design. A model when placed in the tunnel causes a blockage to the air flow. Due to this blockage a bottle neck is created which then increases the effective airflow. This technique reduces the blockage caused to the air flow due to the model and there by reduces model error. It can also increase the effective cross section of the test section thereby allowing slightly larger model dimensions.
General wind tunnel
Fig 1.14 The Honda Tunnel (N.D.)
Contoured wall
Fig 1.15 The Honda Tunnel (N.D.)
iii. Ultraviolet light: Ultraviolet light illumination can be used for better flow visualization. When using smoke for flow visualization technique ultraviolet light can assist in better visual effect. For this system to be incorporated it is mandatory that the place around the wind tunnel is kept as dark as possible. Ultraviolet light can cause damage to naked eyes so proper protective glasses should be used. Other safety measures and precautions as instructed by the manufacturer should be followed. Glasgow University uses ultraviolet light systems provided by Yokohama.
iv. Wake survey rake: A wake survey rake can be added as it is highly recommended for drag calculations. It uses a multi tube manometer as shown in the figure below:
Fig1.16 Wake survey Wind tunnel operation (n.d.) v. Acoustic measurements: Acoustic measuring instruments can be included in the tunnel to measure the noise generated due to the turbulence in air flow around a surface. For this the test chamber needs to be modified so as to create a acoustically insulated environment. Sound sensors can be placed in and around the walls of the test section and this can help in acoustic mapping of the model during the test. The sound so collected can be analysed to improve the design and reduce noise and there by noise pollution.
Fig 1.17 Acoustic image pininfarina(n.d.)
Rolling Road: A rolling road is a device used to simulate rolling motion in a wind tunnel. Such a system is highly useful for aerodynamic analysis of automobile models. They are often used in designing of formula one car. As these cars have their wheels exposed to the airstream, a rolling road provides the opportunity to access the aerodynamic effect of the spinning wheels.
Fig 1.18 Rolling road
Fig 1.19 AutoCAD Rolling road
This project provided new exposure and provided with a better understanding of the working of a wind tunnel. This work has also taught new innovative methods that can be used. It gave a firsthand experience of practical projects and their challenges. It provided with the firsthand experience of procuring parts with a limited budget and the challenges so faced. It also provided an insight into designing and a better understanding of matlab modelling.
11. References
1. Jewel B, Barlow (1999) LOW SPEED WIND TUNNEL TESTING 2. Aero Instruments
Accessed from: www.aero-inst.com/Products/
Date: 18/05/2012 3. Princetron.edu
Accessed from: www.princeton.edu/~asmits/Bicycle_web/Bernoulli
Date: 08/04/2012 4. Pressure probes
Accessed from: www-g.eng.cam.ac.uk/whittle/current-research/hph/pressure-probes/pressure-probes
Date: 06/02/2012 5. Engineering tool box
Accessed from: www.engineeringtoolbox.com/reynolds-number-d_237
Accessed from: www.engineeringtoolbox.com/air-absolute-kinematic-viscosity-d_601
Accessed from: www.engineeringtoolbox.com/hydraulic-equivalent-diameter-d_458
Date: 09/02/2012 6. Aerolab
Accessed from: www.aerolab.com/Galleries
Date: 06/06/2012 7. Load cells online
Accessed from: www.loadcellonline.com/index.php/bending-beam-load-cell/load-cell
Date: 09/2/2012 8. RS
Accessed from: uk.rs-online.com/web/c/?sra=oss&searchTerm=load+cells&x=0&y=0
Date: 06/05/2012 9. Imperial college London (The Honda Wind tunnel)
Accessed from: www3.imperial.ac.uk/pls/portallive/docs/1/5039977.PDF
Date: 20/12/2011 10. Wind tunnel operation
Accessed from: maelabs.ucsd.edu/mae171/Winddocs/Old%20Procedures%20Wind%20Tunnel
Date: 05/06/2012 11. Pininfarina
Accessed from: www.pininfarina.com/en/services/wind_tunnel/facilities/computer_center/computer_center
Date: 07/06/2012
ACKNOWLEDGEMENTS:
My efforts to research and work on this project would not have been possible without the support of various individuals and organizations.
Firstly I would like to convey my deepest gratitude towards Mr. Andrew Ray without whom this work would not have been possible.
I would also like to thank Mr. Bassam Rakhshani and Mr. Daniel Olufisan for their valued assistance.
I would also like to convey special thanks to Mr. Peter Peffers and Mr. Robert Sutherland for their guidance.
My gratitude towards Anvesh Kadmi, Peter Bell and coleen Grieg for their coordination and effort.
I will take this opportunity to convey my gratitude to my parents, brother and friends for their support.
Last but not the least I would like to thank Perth College and all member of staff for giving me this opportunity for assisting me throughout my work.
12. Appendix
Matlab Program
* Logbook: * Glasgow Visit on 12th Oct 2011 * Jewel B, Barlow (1999) LOW SPEED WIND TUNNEL TESTING * Agard advisory group for aerospace research and development (1997) AERODYNAMICS OF WIND TUNNEL CIRCUITS AND THEIS COMPONENTS * No name (2010) NASA [online] available from * http://www.grc.nasa.gov/WWW/k-12/WindTunnel/build.html * Accessed on 4th Nov 2011
* VTI Instruments * www.vtiinstruments.com/Products.aspx * Accessed on 28th Jan 2012 * Strainstall * www.strainstallloadcells.co.uk/load_cells.html * accessed on 29the Jan 2012 * Kineoptics * www.kineoptics.com/WTB.html * accessed on 29th Jan 2012 * SENSOR * www.loadcells.com/?gclid=CL7ml6GS0K0CFasMtAodFW6Qng * Tekscan * www.tekscan.com/ * accessed on 30th Jan 2012 * National Instruments * www.ni.com/aerospace-defense/ * Accessed on 31st jan 2012 * HORIBA * * www.horiba.com/automotive-test-systems/products/mechatronic-systems/wind-tunnel-balances/ * Accessed on 31st jan 2012 * AEROLAB * www.aerolab.com/Display_Pages/Models_Probes.html * Accessed on 2nd Feb 2012 * NASA * http://windtunnels.arc.nasa.gov/balcallab/balances.htm * accessed on 4th Feb 2012 * John Sleath Race Cars * http://www.john-sleath.com/ * accessed on 9th Feb 2012 * Aero Instruments * Accessed from: www.aero-inst.com/Products/ * Date: 18/05/2012 * Princetron.edu * Accessed from: www.princeton.edu/~asmits/Bicycle_web/Bernoulli * Date: 08/04/2012 * Pressure probes * Accessed from: www-g.eng.cam.ac.uk/whittle/current-research/hph/pressure-probes/pressure-probes * Date: 06/02/2012 * Engineering tool box * Accessed from: www.engineeringtoolbox.com/reynolds-number-d_237 * Accessed from: www.engineeringtoolbox.com/air-absolute-kinematic-viscosity-d_601 * Accessed from: www.engineeringtoolbox.com/hydraulic-equivalent-diameter-d_458 * Date: 09/02/2012 * Aerolab * Accessed from: www.aerolab.com/Galleries * Date: 06/06/2012 * Load cells online * Accessed from: www.loadcellonline.com/index.php/bending-beam-load-cell/load-cell * Date: 09/2/2012 * RS * Accessed from: uk.rs-online.com/web/c/?sra=oss&searchTerm=load+cells&x=0&y=0 * Date: 06/05/2012 * Imperial college London (The Honda Wind tunnel) * Accessed from: www3.imperial.ac.uk/pls/portallive/docs/1/5039977.PDF * Date: 20/12/2011 * Wind tunnel operation * Accessed from: maelabs.ucsd.edu/mae171/Winddocs/Old%20Procedures%20Wind%20Tunnel * Date: 05/06/2012 * Pininfarina * Accessed from: www.pininfarina.com/en/services/wind_tunnel/facilities/computer_center/computer_center * Date: 07/06/2012
References: 1. Jewel B, Barlow (1999) LOW SPEED WIND TUNNEL TESTING 2 * Jewel B, Barlow (1999) LOW SPEED WIND TUNNEL TESTING * Agard advisory group for aerospace research and development (1997) AERODYNAMICS OF WIND TUNNEL CIRCUITS AND THEIS COMPONENTS * No name (2010) NASA [online] available from * http://www.grc.nasa.gov/WWW/k-12/WindTunnel/build.html
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