Aerodynamics: Summer project

Aerodynamics Project

I've been reading "Race Car Aerodynamics" by Prof. Joseph Katz. A thing about Dr. Katz; he is a professor at the San Diego State University. He was a professor while I was at the FSAE program also. Sadly he did not participate in the project. However, he is the authority on race car aerodynamics. Very detailed book, goes into how to test and collect data, how to do CFD simulations, wind tunnel testing, as well as calculating values needed. Of couse, it has a very comprehensive overview of aerodynamics in general and how it effects cars. And a basic section on how it has been applied in car technology. Alas this isn't a book review. I really recommend people to read books on whatever topics they are interested in. Speaking of which, please recommend me any low-priced books on composite fabrication.

This summer I am going to try and make a fiberglass air dam, ABS front undertray, front splitter, vortex generators, rear undertray and rear spoiler. I will probably get to only a couple of these projects. 

Explanation of Aerodynamic Concepts

Drag

     Let me explain what happens as you push the car through air, for those readers who never thought about aerodynamics. Its not all that much important when we are in the entry-level of this track hobby.  I will be using extreme laymen's terms so you techy readers, please forgive me.

     As air flows over a surface, it tends to stick to the surface. Air particles stuck on the surface pull other air with it, slowing them down. The layer of air that is slowed down in relation to the ambient air velocity is called the boundary layer. This friction contributes to total drag in the form of skin drag.

     Airflow likes to follow surfaces. Take any streamlined shape. Flow re-attaches in the end part of the shape, because airflow likes to follow surfaces. Wings are an example of such shape.

     Lets take a shape of the side mirrors. As airflow splits along the front half, the abrupt edge of the surface separates the airflow. As air tries to converge, it goes into a turbulent state and a slight "vacuum" state will exist as the airflow caves in to fill the void left by the mirror. This creates a negative pressure area, which pulls on the mirror that is trying to move forward. This area can be very long behind a moving vehicle. The airflow created by such vehicles are called wake flow. This is the most major contributor to drag. The drag created by this is called form drag. This sort of drag exists behind your canopy, mirrors, behind your car, doorknobs, and any other part of the car sticking out into the airstream.

Lift and Downforce

    Following Brunelli's principle, an increase in fluid velocity is accompanied by a decrease in pressure. Lets go back to the wing example. As the airflow's cross section gets "flattened" by the curvature of the airfoil, it is forced to go faster. The same reason water from a garden hose moves faster when you put your thumb on it. As this travels faster, it creates negative pressure on the top of the wing. This is what creates lift.

    The airflow over the car is generally faster because it is more streamlined. In contrast, airflow under the car is slow because it has to deal with all the junk hanging out of the bottom. Airflow is slowed down. There is less pressure above the car and more pressure below the car. Result is net positive lift. Your tires stick better when there are more forces pushing the ground and the tire together. All you have for this at this point is your cars weight. The positive lift will decrease the force of the car's weight, resulting in less traction.

Race-car applications

    My goal, and everyone else's goal is to reduce the positive lift in the car. This is achieved by reducing the pressure differential as much as possible. If you can accelerate the air under the car faster than the air over it, then you can even achieve net negative lift, also known as downforce. This is why you see race cars with smooth undertrays and side skirts. Not just for looks.

    A more effective way to reduce net lift and add downforce is to add an inverted wing in the areas it is needed. A wing can create much more downforce than what you can do under your car.

    My goals for this project is to reduce lift and drag. I will do this by accelerating airflow under my car and promoting smooth airflow over the car.

Upcoming projects

Fiberglass air dam

    The fiberglass air dam helps divert air upwards over the car, and less air under the car. This reduces the effects of the bottom of the car has on airflow. It also works in conjunction with an air splitter to create downforce, but for that explanation please read the section on air splitter.

    My design will be very simple. Flat, vertical air dam. The major design consideration is probably going to be bracing it properly, so it will not flex much and won't damage more than the air dam itself when you go off track.

Vortex Generator

Simple vortex generator to keep boundary layer attached to the rear roof and deck. The air's momentum is what separates the airflow. Vortex generators twirls the airflow into a vortice. This dissipates the momentum of the airflow, allowing it to stay attached longer. You can see vortice generators in airplane control surfaces, race car undertrays, etc. The Mitsubishi Evolution has them in the rear part of the canopy. My vortex generators will look like that.

As far as the design of the generator itself, I used published, public information to come up with a design. The design consideration will be figuring out how to optimize the positions of the generators and how far they need to be apart.

Splitter

    Whenever air hits your car, there is a pressure zone created. This can be any place on the car where air "piles up" The most major pressure zone is probably the nose of the car. When air hits the front of a car, there is a stagnation point where the air velocity reaches zero. This air pressure exerts forces in all directions. The splitter is necessary to utilize the downward force of the pressure zone, and also traps more air in the area. The air dam promotes this effect.

    Another function of the splitter is to "split" the airflow and create cleaner air going under your car.

    It will probably be made out of foam sandwich fiberglass. It will be adjustable slightly, but I will have to figure out how to properly bolt it in to the undertray without shattering it.

Front Undertray

    Its going to be a simple, mushroom shaped piece of plastic that bolts onto the bottom of the car. I just need to make sure its strong enough not to flutter on the track and break itself off, and make my splitter work with the undertray.

Final Note

As usual, my projects will be as low-cost as possible and easy to follow. Well, I hope they are usually easy to follow. Please correct me if anything I said here is wrong!

Photos

Porsche GT3 in wind tunnel - speedhunters.com

Marine Corps AV-8B Harrier vortex generator

Mitsubishi Evo vortex generator

Ford Fusion Sprint Cup Car front air dam and splitter - K&N Filters website
RE-Amemiya front splitter and canard assembly