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The study concluded that a 20 degree rear slant angle had the highest rating of crosswind sensitivity and a 0 degree model had the least.
Other things to consider:
Drag is generated by three different forces, and ultimately our goal is to reduce drag as much as possible.
Frontal pressure - effects of the car pushing through pre-existing air
Rear Vacuum - effects of the air not being able to fill the space that the car just took up
Friction against the outer part of the vehicle
Frontal pressure builds up, but just like any other high pressure area the air wants to flow back to atmospheric pressure - so our goal is to design an aerobody that does two things to counter this phenomenon. One, we need to have minimal pressure build-up at the front of the car, which we can do by limiting the amount of surface area that the hits the air head on. Two, we need to provide an easy way for the air to quickly flow away from the front of the car in a way that reduces turbulence as much as possible.
Another study suggested that a yaw angle of anything greater than 12 degrees while cornering is not realistic and they did all their research based on a maximum of 12 degrees. The study goes into depth on the effect that yaw angles have of the coefficient of drag and the coefficient of lift. This is the generic vehicle model used in this study for the simulations.
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Even though our vehicle won’t look like this, we can still see what parts are affected most by the increase of yaw angles and design our aerobody accordingly. As the yaw angle increases both aerodynamic coefficients increase. So to reduce drag and lift forces the best thing to do is to drive in a straight path wherever possible. The study compared the effects that a spoiler had on these coefficients at different yaw angles between 0-12 degrees. The spoiler greatly reduced the drag and lift at these different yaw angles.
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Where Cd is the coefficient of drag and Cl is the coefficient of lift. The following figures shows how the lift and drag coefficient change for each portion of the aerobody.
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In figure 7 the slant and base seem to be the components that have the most effect on drag as the yaw angle increases and the front of the vehicle has the opposite effect (probably because the front of the vehicle is pointing away from the direction of airflow at a higher yaw angle). In figure 8 the roof seems to be the component that contributes the most to the coefficient of lift and the underbody has the opposite effect. Below are some figures displaying how the surface pressure is distributed at different yaw angles.
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Front view:
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Downforce:
Downforce is a force that makes the car feel heavier. Downforce keeps the car from leaving the tracks and adds adding grip for better handling of the vehicle.
Sources:
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https://jeaconf.org/UploadedFiles/Document/d86c5fa8-2e77-4d47-b51d-5926832848ef.pdf
https://www.catchmentsandcreeks.com.au/docs/Race-Car-Aerodynamics-print.pdf
https://www.buildyourownracecar.com/race-car-aerodynamics-basics-and-design/4/
http://eprints.utm.my/id/eprint/84989/1/ShuhaimiMansor2019_YawAngleEffectontheAerodynamic.pdf