Wind Tunnel BULDING & TESTING

7/29/2019 Wind Tunnel BULDING & TESTING

1/14

Building and Testing a Wind Tunnel

Will Stark

1 Abstract

A wind tunnel has been built to measure drag and downorce o 1:18scale model cars. Te wind tunnel is composed o a contraction cone,test section, and diuser with the necessary instrumentation to measurethe drag and downorce acting on the vehicles. Wind speed can varyrom 0 MPH to 20.1 MPH with the use o a variable transormer. Sixvarious body styles have been tested or eectiveness. Te highest dragwas ound when testing a Hummer H2, 0.095 lb., while the lowest dragwas ound when testing the Ferrari F50 with .021 lb. o drag.

Figure 1: Te wind tunnel.

Tis paper was written for Dr. James Danns Applied ScienceResearch class in the spring of 2010.


2/14

2 Big Idea

A wind tunnel has been constructed in order to study drag orcesacting on dierent automotive designs. Te wind tunnel measuresaerodynamic orces acting on 1:18 scale model cars. Te authororiginally got the idea rom Dr. James Dann. Over the course o theproject, much has been learned about the properties o air. o produceconsistent results, the wind tunnel must produce a laminar stream oair. Once the wind tunnel was built, calibrations were done to measureair speeds with respect to the input voltages o the an. Finally, dierent

model cars have been placed in the wind tunnel or testing and analysis.3 Introduction

Airfow over a vehicle determines the drag orces, which in turnaects the cars perormance and eciency. I designed the testingequipment to measure both the vertical and horizontal componentso air resistance on a model car. Downorce, the vertical component, is

simply negative li. When a car goes aster and aster the weight o thecar changes due to air resistance. Depending on the direction (upwardor downward) this is called either li or downorce, respectively. I willalso measure the horizontal component, the drag o the model cars.Drag is the amount o orce that the air ispushingon the car at a certainspeed. In other words, it is the amount o resistance the air imposes onthe car in a horizontal direction. [1]

For a vehicle to maintain its speed it must overcome riction. Temost important source o riction at high speeds is air resistance. I airresistance can be minimized, the car will be more ecient because lessenergy is lost to riction. oday, with increasing energy costs, eciencyis nally receiving the attention it deserves. I a car can cut through theair with little resistance, it will get better gas mileage. Te automotiveindustry is accordingly devoting more attention and resources to theaerodynamics o their vehicles. For example, the 2010 Ferrari 458 Italia

has 309 lb. o downorce at 124 MPH. [7] I will hopeully be able toapply the skills that I learn building and testing the wind tunnel to mywork later in lie.

80 Will Stark


3/14

4 Design

Tere are three components that make up any open system windtunnel: the contraction cone, test section and diuser.

Te contraction cone is at the ront o the wind tunnel; it is responsibleor directing air into the test section. Te contraction cones conicalshape and screens help produce a laminar fow o air over the vehicle.Te test section is the next component o the system. Te clear acrylicconstruction allows the observer to see the vehicle. Te test section

also houses all o the testing equipment to measure the aerodynamicorces acting on the cars, notably a scale and orce probe.

Te diuser is the last piece o the wind tunnel; it houses the an. Tean must be placed at the end o the wind tunnel instead o the rontto produce a laminar fow o air through the test section. Fans producea turbulent fow o air just by their nature; the propellers createturbulence. Tereore, it is advantageous to place the an at the end o

the wind tunnel in order to create a vacuum. Pulling the air throughthe tunnel produces a laminar fow o air. Te diusers conical shapealso increases the wind speed through the test section. Because the anis 15 in diameter and the dimensions o the test section are only 8 by8, air is accelerated through the test section. With any liquid there is arelationship between cross-sectional area and velocity. Tis is evidentin a derived orm o Bernoullis Principle:

A1V1=A2V2

I can apply this equation to calculate the speeds at all three locationso the wind tunnel: at the diuser, through the test section, and out thean. See equations below:

ADiuser

VDiuser

= Aest Section

Vest Section

= AFan

VFan

(20 x 20)(VDiuser) = (8 x 8)(20.1 MPH) = (7.52

)(VFan)

VDiuser

= 3.2 MPH and VFan

= 7.2 MPH

THE MENLO ROUNDTABLE 81


4/14

All o the components o the wind tunnel can be seen in Figure 2 andFigure 3. For purposes o portability and storage I have decided to keep

the contraction cone, test section and diuser separate. However, whenput together the components must t together seamlessly. Tereore,I have used rubber gaskets along the contact points. [6] Figures 4-6illustrate the contraction cone, test section and diuser.

Figure 2: Te wind tunnel (to scale).

Figure 3: Te wind tunnel (to scale).

82 Will Stark


5/14

Figure 4: Te contraction cone is designed to control the air coming into thewind tunnel. For accurate results it is vital to have a laminar ow o air into

the tunnel. A laminar ow is attained via the shape o the contraction coneand a series o screens.



6/14

Figure 5: Te test section houses the model car in a transparent enclosure. It alsoserves as the platorm or all tests. A scale under the base o the car measures thevertical component o air resistance, downorce. Te horizontal component, drag,is measured with a orce probe located under the test section. Te orce probe isconnected to the ront o the car with a pulley and string.

84 Will Stark


7/14

Figure 6: Te difuser houses the an. Te smaller opening through the test section(8 x 8) makes the speed o the air much greater than it is through the an(20 x 20). It is desirable to have high speeds through the test section to havemore realistic wind speeds. See design section or more detail on Bernoullis Principle.

I designed and built the necessary circuitry to power the an and

measure the voltage going to the an. I used a variable transormer toadjust the voltage going to the an rom 0-150 V and then correlatedthe an voltage with wind speed. I did not use the wind speed probeduring testing because it would have produced turbulence. Tevoltage-adjustment dial on the transormer was very inaccurate, so Idecided to measure the voltage with a Vernier LabQuest (see Figure7). Te LabQuest maxes out at a mere 10 V, however. [2] I thereorehad to design a circuit that would step down the voltage with a series

o een 2,000 resistors (see Figure 8). I chose resistors with a highresistance to limit the current on this part o the circuit, allowing mosto the power to go directly to the an.



8/14

Variable ransformer Voltages

Transormer dial (V) Actual (V)0 0.5

10 13.1

20 26.6

30 38.7

40 50.7

50 62.0

60 74.1

70 84.9

80 96.5

90 109.4

100 120.9

110 131.3

120 144.0

130 150.0

Figure 7: Te voltage diference between what the dial on the transormershowed and the actual output. Notice the increasing spread between the twountil 150 V is reached.

Figure 8: Circuit Diagram. Tis circuit is used to power the an and measures thevoltage going to the an. Te an and the 15 resistors are connected in parallel toallow their currents to be diferent. Te an draws much more current than the

small 0.0005 A through the 15 resistors. See Ohms Law:V=IR

150V = I (2,000 15)

I = 0.005 A

86 Will Stark

r


9/14

5 Teory

Te main objective or wind tunnels is to obtain a laminar fow o airthrough the test section, that is, a parallel steady fow with uniormspeed throughout the section. [3] In the event, this did not provepossible. Tere will always be some background noise in the system nomatter the design.

In a wind tunnel it is desirable to have the highest possible speedwith the largest test section. Tis will reduce noise by giving the test

subject more room to bend the air. But high speeds and a large areaare competing demands. Tereore, I had to compromise with a testsection 8 x 8 and a an that can push 1,600 cubic eet per minute(CFM). I calculated the speed through the test section to be 51.1 MPHwith those parameters. However, with rictional losses the maximumspeed I achieved through the wind tunnel is 20.1 MPH.

Designing a wind tunnel or automotive purposes presented some

challenges, starting with blockage based on the rontal area. Te fowaround automobiles is characteristic o blu bodies as opposed tostreamlined bodies such as an aircra wing. Tese blu bodies resultin a sizable region o separated fow. [1] o achieve a smooth parallelfow these separated regions o air must close beore they exit thediuser. Otherwise there will be pressure problems inside the testsection. Tis pressure imbalance will cause noise and a large amounto drag aer the vehicle. [4] Tereore the test section has a large

empty area aer the car or the separated fow to join back together.Without this area behind the car, a large amount o turbulence wouldbe produced.



10/14

Figure 9: A ull-sized Porsche in a wind tunnel. In industry, colored air is usedto identiy areas o turbulence.

Controlling the stream o air is vital in assuring the accuracy o windtunnels. Any source o turbulence aer the test section will cause

distorted results. urbulences cause separation in the speed o theair. Tese dierent speeds then create pressure dierences. Te onlysource o turbulence in a wind tunnel should be the model car.

6 Calibration

Te wind tunnel needed to be calibrated to ensure its accuracy. o dothis I accounted or all error I could measure. Te rst procedure Iperormed was correlating the wind speed with the input voltages onthe an. I could then determine wind speed through the tunnel withoutthe need or an anemometer.

88 Will Stark


11/14

Figure 10: Voltage to windspeed. Shows the correlation between input voltage orthe an and wind speed through the test section.

I also had to calibrate the scale. Aerodynamic orces were acting on

the scale: as wind speed increases, so does the weight that the scale isreading. Tis is incorrect, so I adjusted all my data to show only thechanges in weight due to the aerodynamic orces acting on the cars.

Figure 11: Scale error. Shows how the scale got lighter as the wind speed wasincreased. I adjusted my weights to account or this source o error.



12/14

7 Results

I tested six dierent model cars in the wind tunnel. Te various bodystyles produced dierent results. I measured both the downorceand drag on each car. Dierent body styles gave dierent results. Forinstance, a blu body like the Hummer H2 experienced a drag o 0.095lb. while the sleeker Ferrari F50 only had 0.021 lb. o drag.

Figure 12: Te six diferent cars I decided to test. Varying designs produceddiferent results.

1963 Chevrolet Corvette

1995 Ferrari F50

2004 Porsche Carrera 2005 Hummer H2

2002 Dodge Viper

1964 Ford Mustang

90 Will Stark


13/14

Car Drag (lb.)

1963 Chevrolet Corvette 0.0341964 Ford Mustang 0.038

1995 Ferrari F50 0.021

2002 Dodge Viper 0.028

2004 Porsche Carerra G 0.023

2005 Hummer H2 0.095

Figure 13: Drag measurement or each car. Te highest was the Hummer H2with 0.095 lb.; the lowest was the Ferrari F50 with 0.021 lb.

Downorce/Lif (oz.)

At 6 MPH At 8 MPH At 20 MPH

1963 Chevrolet Corvette 0 0 -0.1

1964 Ford Mustang +0.2 +0.3 +1.9

1995 Ferrari F50 0 0 -0.7

2002 Dodge Viper -0.2 -0.2 -0.9

2004 Porsche Carerra G 0 0 -0.8

2005 Hummer H2 0 +0.4 +0.8

Figure 14: Downorce and li each vehicle experienced when exposed to 20.1MPH winds. Note that the Hummer and Mustang got heavier and the othercars became lighter.

Figure 15: Example o some o the data I took. Notice the delay between thean voltage and the ensuing drag rom the increased windspeed.



14/14

8 Citations

[1] Low-speed Wind unnel esting: by Jewel Barlow, William Rae,and Alan Pope

[2] Specications o Vernier LabQuest device:http://www.vernier.com/labquest/techspecs.html

[3] NASA: Build Your Own Wind unnelhttp://www.grc.nasa.gov/WWW/K-12/Windunnel/build.html

[4] Build a Wind unnel by Mike Fitzgeraldhttp://www.techdirections.com/BuildAWindunnel.pd

[5] echShophttp://techshop.ws/

[6] Science Buddies: How to Build a Subsonic Wind unnel

http://www.sciencebuddies.org/science-air-projects/wind-tunnel-toc.shtml

[7] 2010 Ferrari 458 Italia by Jon Yankahttp://www.caranddriver.com/reviews/car/09q3/2010_errari_458_italia-ocial_photos_and_ino

9 Acknowledgments

I would like to thank Dr. Dann or the idea o building a wind tunneland his continued support. I would also like to thank Jordan Jadallahor letting me borrow his model car collection in the name o science.

92 Will Stark

Wind Tunnel BULDING & TESTING

Documents

Transcript of Wind Tunnel BULDING & TESTING