1American Institute of Aeronautics and Astronautics

EXPERIMENTAL WAKE INVESTIGATION OF IDENTICAL CARS IN CLOSE PROXIMITYA. Albers*, J. Brys*, S. Illg*, J. Roop*, J. Ashworth#

* Amber Albers, Jason Brys, Sarah Illg, Joseph Roop: Student Members, Embry-Riddle Aeronautical University, Prescott, AZ# Dr. Jeffrey Ashworth: Member, Associate Professor, Department of Engineering, Embry-Riddle Aeronautical University, Prescott, AZ, project advisor

ABSTRACT

This investigation included design, fabrication, and wind tunnel testing of two identical cars in close proximity. The objective was to find the pressure drag and side force on each of the cars and determine how the wake of the front car affects the drag and side force on the aft car and at the same time how the front car is affected by having a car in its wake. The tests were performed in the 32-by-45 inch wind tunnel at Embry-Riddle Aeronautical University in Prescott, Arizona. The dimensions of the cars were 6-inches long by 3-inches wide and 2-1/10 inches tall. The flat plate used to simulate the ground had dimensions of 30-by-42 inches, and had slots drilled for movement of the cars. A total of 18 pressure taps were used in each car to find the pressure distribution about the car. The team found that the drag was most strongly affected when the cars were within one-and-a-half car lengths of each other in the direction of the flow. The front car’s wake produces a higher drag for the back car when the back car moves to the side. Flow visualization data was collected and compared with the pressure data. This experiment was run at Reynolds Numbers of 612,000, 514,000 and 364,000 based on the car length. A general trend of increase in wake size as the velocity increases was recorded during the flow visualization.

INTRODUCTION

This report documents an investigation on two model cars in the 32-by-45 inch wind tunnel at the Prescott campus of Embry-Riddle Aeronautical University. The two models were used to calculate the total drag change with a change in position, both horizontally (X) and laterally (Y). Eighteen pressure taps on each car were used to collect the data. Figure 1A below shows the 3-by- 6 inch model cars and Figure 1B shows the X-Y coordinate system attached to the plate.

Figure 1A: Model Cars

Figure 1B: X-Y Plane (Positions In Car Lengths)

The objective of this experiment was to find how the drag on a vehicle changes with a change in position of another vehicle. All vehicles are affected by the aerodynamics of other vehicles in the vicinity. Racecar teams in particular have benefited for many years from aerodynamic testing on cars. By utilizing the best position for the least drag, a racecar can save fuel and make fewer pit stops.

Measurements were taken in this experiment at a total of 25 car positions and three different free-stream dynamic pressures. The maximum dynamic pressure was set at 9-inches of water at a free stream velocity of 218 ft/s or 149 mph.

This experiment was conducted under the advisement of Dr. Jeff Ashworth. The equipment used in the experiment included a 30-by-42 inch flat plate constructed of Plexiglas and wood, two wooden model cars with dimensions of 6-by-3-by-2.1 inches, a 40-port-pressure-transducer, a vertical manometer, a temperature probe, an inclined manometer, and an ambient pressure gauge.

SYMBOLS

Ac/s = cross-sectional area of the front of the car in square feetc = length of the car in inchesCD = coefficient of drag

0.5 1.0 1.5 2.0

1.0

1.5

2.0

2.5

0.0

0.0

X

Y

V∞∞∞∞

3.0

21st Applied Aerodynamics Conference23-26 June 2003, Orlando, Florida

AIAA 2003-3817

Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

2American Institute of Aeronautics and Astronautics

D = total drag in poundsF = force in poundsL = length in inchesP = pressure in pounds per square footRe = Reynolds number V: = free-stream velocity inside the wind tunnel in feet per secondW = width in inchesX = position of the back car parallel to flow direction in car lengthsY = position of the front car perpendicular to flow direction in car lengths∆F = change in force in pounds ρ∞ = density in slugs per cubic footν = Kinematic viscosity in square feet per second

THEORY

The aerodynamics of a road vehicle is influenced by other vehicles traveling in close proximity. Racecars, tractor-trailers, and passenger cars are all affected by the wake of other vehicles on the road. This report will investigate the drag analysis of two identical cars and how their change in position affects the coefficient of drag on both cars.

Drafting is the process in which one vehicle follows closely behind another and causes an aerodynamic interaction between the vehicles. For car aerodynamics, “due to the wake of the leading car, the one in the back experiences less aerodynamic drag and can go faster with greater ease and with better fuel efficiency”.4

Racecar drivers utilize the drag benefits from the lead car.4 Drafting is illustrated in Figure 2. As vehicle 2 comes close to vehicle 1, the free-flow streamlines do not form a high-pressure stagnation point on the front of the drafting vehicle (vehicle 2). So, vehicle 2 has less drag.

Figure 2: Cars Drafting4

In a race, when a rear vehicle tries to pass the car it is following, the air flowing around both cars changes dramatically. Figure 3 below shows two vehicles and the aerodynamic interactions involved as vehicle 2 passes vehicle 1. As vehicle 2 passes 1, the lift and side force will increase on vehicle 2, and the lift will increase on vehicle 1.

Figure 3: Passing Cars (Flow from Right to Left)

Because vehicle 2 is moving at a faster speed than vehicle 1, it passes quickly, and the trends reverse when vehicle 2 is in front. The wake from the vehicle in front exerts a positive side force on the trailing vehicle that decreases with increasing distance. Separated flow regions on both vehicles cause a decrease in lift and drag.

An elevated ground plane, like the flat plate used in this experiment, is used to test vehicles in wind tunnels. A Plexiglas plate was mounted perpendicular to the airflow and placed above the thickness of the boundary layer created by the floor. The leading edge of the plate was beveled to shift the stagnation point to the top of the flat plate. By shifting the stagnation point, separation at the leading edge was reduced to a minimum. Plywood was bonded to the bottom of the Plexiglas to eliminate flutter that was encountered in the preliminary testing when the dynamic pressure was raised above 7 inches of water.

PROCEDURE

Pressure data was obtained by connecting the cars’ pressure taps to the 40-port-pressure-transducer and placing the cars in the wind tunnel on the flat plate. In order to compare data trends at three different dynamic pressures, the tunnel was run at dynamic pressures of 3, 6, and 9 inches of water. Reynolds numbers were calculated to be, 364,000, 514,000, and 612,000 respectively.

The Reynolds numbers were found by dividing the free-steam velocity and the length of the car by the viscosity.

Re=∞υ

cV (1)

2

1

3American Institute of Aeronautics and Astronautics

The coefficient of drag for each car was calculated from the pressure data. First, the cross-sectional areas were found for both the front and back windshields and front and back bumpers of each car. Car 1 and 2 had windshield and bumper dimensions that were symmetrical and identical. The equation for calculating the cross-sectional area is as follows:

scAWL /* = (2)

Once the cross-sectional area was found, the force was calculated. The pressure read by the 40-port-pressure-transducer was displayed in pounds per square foot. The force was calculated from the pressure data using the following equation:

FAP sc =/* (3)

The change in force was then calculated for the windshields. To calculate drag, the horizontal component of the force acting on the windshield was found. The back windshield was subtracted from the front windshield. The equation is as follows:

windshieldbackxfrontx FFF ∆=− −− (4)

The change in force was then calculated for the bumpers, the equation is as follows:

bumperbackfront FFF ∆=− (5)

Total drag was then calculated by adding the change in force from the windshield to the change in force from the bumper and the equation is as follows:

DFF bumperwindshield =∆+∆ (6)

The coefficient of drag was calculated from the above drag by dividing the drag by the dynamic pressure and area. This area is the cross-sectional area of the windshield and the bumper.

D

sc

CAV

D =∞∞ /

2

2

1 ρ(7)

Flow visualization was obtained by running the tunnel at a dynamic pressure of 0.5 inched of water and running a fog generator in the flow in front of the cars in various positions. Movement of the fog generator in the lateral (Y) direction helped illustrate the size and

shape of the car wakes along with side force affects on the cars.

RESULTS AND DISCUSSION

The purpose of this experiment was to gather flow visualization pictures and measure the total drag change with a change in car position, both horizontally and laterally, and then to gain side force data. By running the test at three different dynamic pressures, three different free-stream velocities were tested. Using the pressure data collected from the pressure taps on the cars, the drag coefficients for each car in each position were calculated. These coefficients of drag at dynamic pressures of 3 and 6 inches of water can be seen in Figures 4 through 7.

These graphs represent the coefficients of drag on the front and back cars. The Y-position of the front car of 0.0 is directly in front of the back car, and 1.0 is the X-position of the back car where the cars’ bumpers were touching. (See Figure 1B.)

As illustrated in Figures 4 and 5, there is a decline in drag on the back car when it is placed directly behind the front car (X=1 and Y=0), and as it moves back from the front car (X>1) the drag increases. As the front car moves out to the side of the back car, there is an increase in drag on the back car (Y=0.5). At this same Y position, the drag on the front car decreases initially then returns to the freestream value in the aft position of X=2.0. This increase in drag for the front car at Y=0.0 and decrease for Y=0.5 is due to the wake interference of the two cars. The same general trends were seen for all dynamic pressures tested.

Each of the graphs shows that the back car is most strongly affected when the front car is directly in front of it or at Y=0.5. The back car affects the drag on the front car when it is within one-and-a-half-car lengths and within one car length to the side of the front car.

4American Institute of Aeronautics and Astronautics

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.0 1.5 2.0 2.5 3.0 3.5

Position Back X-Direction

Cd

Y=0.0

Y=0.5

Y=1.0

Y=1.5

Y=2.0

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.0 1.5 2.0 2.5 3.0 3.5

Position Back X-Direction

Cd

Y=0.0

Y=0.5

Y=1.0

Y=1.5

Y=2.0

Figure 4: Front Car Drag at 6 in of Water

Figure 5: Rear Car Drag at 6 in of Water

5American Institute of Aeronautics and Astronautics

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.0 1.5 2.0 2.5 3.0 3.5

Positions Back X-Direction

Cd Y=0.0

Y=0.5

Y=1.0

Y=1.5

Y=2.0

Figure 6: Front Car Drag at 3 in of Water

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.0 1.5 2.0 2.5 3.0 3.5

Position Back X-Direction

Cd Y=0.0

Y=0.5

Y=1.0

Y=1.5

Y=2.0

Figure 7: Rear Car Drag at 3 in of Water

6American Institute of Aeronautics and Astronautics

Flow visualization made visible the shape and size of the wake from the front car, and clearly illustrated how it affects the wake of the back car. The smoke flow, in figures 8 through 10, illustrates the direction of the interactive flow between the cars as well as the stagnation (high pressure) points on the cars. All positions were tested using a smoke generation system at dynamic pressures of 0.5 inches of water. Figures 8 though 10 show the cars in various positions.

Figure 8: Flow Visualization (X-1.5, Y=0.5, top)

Figure 9: Flow Visualization (X=1.5, Y=1.0, top)

Figure 10: Flow Visualization (X=1.0, Y=0.5, top)

CONCLUSIONS

This experiment found that drafting by the back car causes less aerodynamic drag within one-and-a-half-car lengths behind the front car. The front car’s wake produces a higher drag for the back car when the back car moves to the side. The main location of the low-pressure wake effect tends to be directly behind the front car.

ACKNOWLEDGEMENTS

This work was supported, in part by Embry-Riddle Aeronautical University and the Ronald E. McNair Scholars Program, Prescott, AZ. The technical assistance of our project advisor Dr. Jeffrey Ashworth is greatly appreciated.

REFERENCES

1. Aeronautical Vestpocket Handbook, 23rd Ed., Pratt & Whitney, East Hartford, 1996.

2. Anderson, John D: Introduction to Flight, 4th Ed., McGraw-Hill, Boston, 2000.

3. Barlow, Jewel B., et al.: Low-Speed Wind Tunnel Testing, 3rd Ed., John Wiley & Sons, Inc., New York, 1999.

4. Katz, Joseph: Race Car Aerodynamics, Bentley Publishers, Cambridge, 1998V∞

V∞

V∞

V∞