A Full-Scale Wind Tunnel Test

400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760

SAE TECHNICALPAPER SERIES 2000-01-3548

A Full-Scale Wind Tunnel Test of aShort Track Race Car

Drew Landman and Eric KosterLangley Full Scale Tunnel

Reprinted From: Proceedings of the 2000 SAE MotorsportsEngineering Conference & Exposition

(P-361)

Motorsports Engineering Conference & ExpositionDearborn, Michigan

November 13-16, 2000

Downloaded from SAE International by Vellore Inst of Technology, Wednesday, June 25, 2014

The appearance of this ISSN code at the bottom of this page indicates SAE’s consent that copies of thepaper may be made for personal or internal use of specific clients. This consent is given on the condition,however, that the copier pay a $7.00 per article copy fee through the Copyright Clearance Center, Inc.Operations Center, 222 Rosewood Drive, Danvers, MA 01923 for copying beyond that permitted by Sec-tions 107 or 108 of the U.S. Copyright Law. This consent does not extend to other kinds of copying such ascopying for general distribution, for advertising or promotional purposes, for creating new collective works,or for resale.

SAE routinely stocks printed papers for a period of three years following date of publication. Direct yourorders to SAE Customer Sales and Satisfaction Department.

Quantity reprint rates can be obtained from the Customer Sales and Satisfaction Department.

To request permission to reprint a technical paper or permission to use copyrighted SAE publications inother works, contact the SAE Publications Group.

No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior writtenpermission of the publisher.

ISSN 0148-7191Copyright 2000 Society of Automotive Engineers, Inc.

Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solelyresponsible for the content of the paper. A process is available by which discussions will be printed with the paper if it is published inSAE Transactions. For permission to publish this paper in full or in part, contact the SAE Publications Group.

Persons wishing to submit papers to be considered for presentation or publication through SAE should send the manuscript or a 300word abstract of a proposed manuscript to: Secretary, Engineering Meetings Board, SAE.

Printed in USA

All SAE papers, standards, and selectedbooks are abstracted and indexed in theGlobal Mobility Database


2000-01-3548

A Full-Scale Wind Tunnel Test of a Short Track Race Car

Drew Landman and Eric Koster Langley Full Scale Tunnel

Copyright © 2000 Society of Automotive Engineers, Inc.

ABSTRACT

A full-scale investigative wind tunnel test was performed on a dirt track race car in the Langley Full Scale Tunnel (LFST). Lift and drag forces were measured and flow visualization studies performed for the purpose of quantifying the aerodynamic characteristics in order to assist designers and drivers of this class of vehicle. Results from the downforce measurements showed a rear axle biased aerodynamic balance. Flow visualization studies revealed large areas of separated flow on the forward portion of the side pods as well as over a large portion of the rear deck and spoiler behind the driver.

INTRODUCTION

Indigenous to the Northeastern United States, “dirt-modifieds” have traditionally exhibited cantankerous handling qualities, generally attributable to the combination of high center of gravity, in excess of 60% rear weight bias and primitive beam-axle suspensions. These mandated parameters coupled with the characteristically low traction of slick clay tracks make maintaining balanced suspension settings a challenge. Rear axle shaft torque in excess of 2500 ft-lbs. exacerbates the problem.

Historically, the primary focus for aerodynamic improvement of these cars has centered on the creation of increased downforce in order to alleviate corner exit power-induced wheelspin. Current rules prohibit the use of inverted wings however, airdams, deflectors and spoilers have been implemented and generally accepted as being successful in abating wheelspin. Designers believe that the large rear downforces generated are balanced by various configurations of front airdams and deflectors. Because no experimental force data has been available in the past, the growing problem of corner entry understeer has been countered through the evolutionary implementation of increased rear brake bias along with an increased differential in left to right rear tire circumference (known as stagger).

This paper provides results from experimental aerodynamic force testing as well as flow visualization techniques that should aid designers of dirt-modified style racecars. It is anticipated that the data will be used to design safer and more aerodynamically efficient vehicles.

FACILITY DESCRIPTION

Old Dominion University (ODU), working under a Memorandum of Agreement with NASA Langley Research Center, operates the Langley Full-Scale Tunnel.1,2 This facility is the second largest in the United States in terms of test section size and is the largest university-operated wind tunnel in the world.

The building which comprises the LFST measures 132 m long by 70 m wide by 30 m high. The open jet test section is semi-elliptical in cross section with a width of 18.29 m (60 ft) and a height of 9.14 m (30 ft). The ground board is 13 m (42.5 ft) wide by 16 m (52.3 ft) long and features a turntable with a diameter of 8.7 m (28.5 ft). The overall aerodynamic layout of the facility, showing the double return design, is given in references 1 and 2. Power is supplied by two 3 MW (4000 HP) electric motors driving two 11 m (36 ft) diameter four-bladed fans. The current maximum speed is limited by a fan speed of 210 RPM which is about 130 kph (~80 mph) in the test section. Vehicle drag and individual wheel downforce are measured using the current automobile balance which became fully operational in January of 1998.3

DESCRIPTION OF THE VEHICLE

The "dirt modified" race car that was tested is representative of the class and is shown in figures 1-3. Wheelbase and track measure 2.72 m (107 in) and 1.47 m (58 in). The entire chassis is constructed of tubular steel with body panels fabricated from aluminum sheet. The rear spoiler is translucent plastic, located between the side body panels, and extends across the full width of the car. The drivers compartment is located between two side pods which are also bounded by the exterior side body panels. This creates a duct-like passage on either side of the driver which air flows through. At the front end


of the car a 0.89 m (35 in) by 0.46 m (18 in) tall wide sloping concave flow deflector known commonly as a "snow plow" is bounded by short end plates. Tubular steel bumpers protrude through the snow plow and also through the side and rear body panels. The engine is contained within a rectangular body which tapers to a small cross section just aft of the snowplow. This compartment and a

raised wedge shaped airbox just forward of the driver are fed air through three "NACA ducts" visible in figure 2. The roof and roof pillars are sheet metal fabrication whereas the driver is protected by a tubular steel cage which is exposed to the air stream. A small windshield shields the driver from flying debris. All four tires are exposed with the rear tires protruding slightly under the sides. The front suspension shock absorber and springs, located between the front wheel and snow plow are exposed to the freestream flow. The central underbody is a sheeted smooth surface approximately 5 inches above the ground at race ride heights.

Average speeds for this vehicle vary between 36 m/s (80 mph) in the corners to 67 m/s (150 mph) on the straight portions of speedways. Since the driver slides the car through the turns, a yaw angle results that is larger than that typically found on hard surface track race cars.

EXPERIMENTAL DETAILS

The Langley Full-Scale Tunnel automotive force balance was used to measure the vehicle downforce resolved at the front and rear axles, as well as the vehicle drag. The design and operation of the balance is described in reference 3. The data presented was taken at a nominal test section velocity of 60 mph. This speed was chosen due to the drag force overload limits of the automobile balance. The uncertainty in the force measurements was not rigorously evaluated for this preliminary investigative test but for comparison purposes the 95% (2σ) uncertainty level for typical stock car racecars in lift and drag is no worse than ±0.0015 in drag coefficient and ±.0025 in lift coefficient. The car was tested for two configurations. The first was representative of the normal ride heights used during racing and the second representative of a lowered ride height (2 inches lower). The floor boundary layer control suction slot was not used for these initial tests.4,5 A tire boundary layer trip was used to simulate the correct separation point for a rotating wheel when evaluating the nearby downstream flow field. This device forces transition at the point of mounting rather than the natural point (for a non- rotating cylinder) located approximately 90 degrees clockwise, on the leeward side of the tire.6

FORCE MEASUREMENTS

Drag and individual wheel downforce data were obtained at two yaw angles:

1) with the car oriented parallel to the flow direction (yaw=0°)

2) with the nose of the car rotated eight degrees to the left (yaw=-8°).

The -8° yaw angle was chosen to represent a typical sliding turn to the left. Representative force data in

Figure 1 Dirt-Modified Race Car in Test Section

Figure 2 Dirt-Modified Race Car , Front View

Figure 3 Dirt-Modified Race Car , Rear View


coefficient form for the two configurations is given in table 1. The data in tables 2-5 summarize the expected loads at each axle for a range of typical operating speeds based on the assumption that the coefficients are invariant. Downforce is presented in pounds and drag is reported as the equivalent horsepower required at the given speed to overcome the force of aerodynamic drag. This convention was chosen based on preferences expressed by race team customers of the LFST. The most interesting aspect revealed by the force test may be the imbalance between the front and rear downforce. Unlike previous suppositions that blamed brake and suspension set-up, poor downforce distribution appears to be the chief reason for understeer.

FLOW VISUALIZATION

Flow visualization techniques were used to help identify problem areas. Tufts were placed over much of the body to map the surface streamlines and show separated regions. Colored oil was applied to body panels to identify regions of relatively low shear stress, useful when evaluating interference relationships, and identifying regions of recirculation and stagnation. Finally, a smoke wand helped visualize off body streamlines, stagnation points and underbody pressure trends.7,8,9

TUFTS

Tufts were applied over the entire body of the car and viewed using video and still cameras located in the test section. The overall flow pattern at zero yaw can be seen in figure 1 which shows the dirt car in the test section with the flow on. The external flow over the roof and side panels is nearly completely attached. The flow over the wedge shaped hood scoop is more problematic with separation evident in the fluttering tufts on the side. Perhaps the most interesting discovery was the separated region on the top surface of the inner body panels (side pods) thought to be due to the upstream interference of the front suspension components (spring towers). A recirculation region was indicated behind the driver’s compartment as expected. The trailing edge of the airdam

Table 1 Aerodynamic Force Coefficients Ride height yaw Clf Clr Cd

race 0 0.111 -0.647 0.694 race -8 0.094 -0.608 0.681 low 0 0.142 -0.65 0.657 low -8 0.113 -0.641 0.65

Table 2 Calculated Forces, Yaw=0, Race Height

Vehicle Velocity (mph)

Front Downforce

(lbs)

Rear Downforce

(lbs)

Drag (hp)

60 -36 209 36 80 -64 372 85 100 -100 581 166 120 -144 836 287 140 -196 1138 455 150 -225 1306 560

Table 3 Calculated Forces, Yaw=-8, Race Height


Front Downforce

(lbs)

Rear Downforce

(lbs)

Drag (hp)

60 -30 196 35 80 -53 348 83 100 -83 544 162 120 -120 784 280 140 -163 1067 445 150 -188 1225 548

Table 5 Calculated Forces, Yaw=-8, Low Height


Front Downforce

(lbs)

Rear Downforce

(lbs)

Drag (hp)

60 -36 207 33 80 -53 368 79 100 -83 575 155 120 -120 828 268 140 -163 1127 425 150 -188 1294 523

Table 4 Calculated Forces, Yaw=0, Low Height


Front Downforce

(lbs)

Rear Downforce

(lbs)

Drag (hp)

60 -36 207 33 80 -53 368 79 100 -83 575 155 120 -120 828 268 140 -163 1127 425 150 -188 1294 523


was also found to have a region of separation. Long tufts were attached to the aft corners of the body at the spoiler and roof. All of these tufts were seen to spin indicating trailing vortices in these areas. The tire trip of figure 4 was used to evaluate the sensitivity of the downstream flow field to changes in the front tire aft separation point. While the separation point was seen to change on the tire as predicted, the overall effect on forces was negligible.

OIL FLOW

Oil flow studies were conducted in regions that were thought to contain flow separation following the tuft study. A mixture of motor oil (SAE 30) and titanium dioxide was painted in a stripe over the rear deck just behind the driver. In areas where the airflow near the deck surface reached the spoiler, the oil stripe spread and can be seen in figures 5 and 6 as streaks moving toward the spoiler. Regions where the airflow is recirculatory or stagnant caused the oil to remain largely unchanged as can be

seen in the area immediately behind the driver’s seat. Application of a film of oil over the snow plow showed the wake of the bumper support struts propagating onto the top surface as shown in figure 7.

SMOKE

A smoke wand was first used to examine the overall flow field in the region surrounding the snow plow. Flow was seen to separate at the trailing edge of the snow plow perhaps aided by the interference from the bumper supports. Disturbed flow over the side pods indicated by the tuft study was proven to be caused by the upstream location of the spring towers. Similarly, the vortices found with the long tufts were confirmed using smoke. Flow under the car was seen to remain relatively smooth and may represent an area that can be exploited for creation of more downforce. As expected, the flow over and around the driver’s compartment dirtied the spoiler and

Figure 4 Front Tire Trip

Figure 5 Colored Oil Flow Over Rear Deck, Rear View

Figure 6 Colored Oil Flow on Rear Deck, Top View

Figure 7 Oil Flow on Snow Plow


rear deck where a suspected area of recirculation was confirmed.

DISCUSSION OF RESULTS

This simple test which involved approximately six hours of wind tunnel occupancy revealed several significant results which may help break the experience-based design evolution common to this class of vehicle.

The origin of understeer is now clearly understood; this car creates lift at the front axle and substantial rear axle downforce. The downforce imbalance is due in part to the spoiler and leverage created by the forward position of the rear axle. In addition, the snow plow appears to be relatively ineffective at creating downforce whereas the front suspension components cause significant interference on the side pod areas, degrading downforce production.

As regards drag, there are many areas for improvement including streamlining exposed tubing, reducing frontal areas of the passenger compartment and cleaning up the engine air intakes. Again, minimizing interferences will yield gains in drag as well as downforce.

CONCLUSIONS

Results from a wind tunnel study on a dirt-modified short track race car conducted at the Langley Full Scale Tunnel are felt to help designers understand the complex flow field around the vehicle. Unexpected levels of front lift point to a need for a new design with a more evenly distributed balance. Interior flow through the car over the side pods was shown to be adversely affected by the disturbed flow downstream of the front suspension components.

REFERENCES

1. Britcher, C. P. and Landman, D., "From the 30 by 60 to the Langley Full-Scale Tunnel," presented at the 36th AIAA Aerospace Sciences Meeting and Exhibit, January 1998

2. Britcher, C. P. and Landman, D., "Jurassic Tunnel: The Life, Death, and Resurrection of the Langley Full-Scale Tunnel," presented at the European Forum on Wind Tunnels and Wind Tunnel Test Techniques, April 1997

3. Landman, D. and Britcher, C. P., "Development of Race Car Testing at the Langley Full-Scale Tunnel," SAE 98MSV-21, 1998

4. Landman, D., “Road Simulation for NASCAR Vehicles at the Langley Full-Scale Tunnel," SAE 00MSV-31, November 2000

5. Landman, D., Britcher C.P., Martin, P., "A Study of Ground Simulation for Wind Tunnel Testing of Full-Scale NASCAR's," AIAA 2000-0153, January 2000

6. Katz, J., "New Directions in Race Car Aerodynamics, Designing for Speed," Robert Bentley Publishers, 1995

7. Hucho, W. H., "Aerodynamics of Road Vehicles," SAE, 1998

8. Barlow, J. B., Rae, W. H. and Pope, A., "Low Speed Wind Tunnel Testing," 3rd Ed., John Wiley and Sons, 1999

9. Barnard, R. H., "Road Vehicle Aerodynamic Design", Addison Wesley Longman Limited, 1996


Documents

A Full-Scale Wind Tunnel Test