After we got the hang of Fluent, we made 'batch files' which automated the set up of the simulations. The simulations were run as background processes on the math department's six-processor server. Achieving zero lift The results of the CFD analysis of the original design concept surprised everyone on the aerodynamics team with how much negative lift was generated. It was nearly 500 Newtons, which was like adding 50 kg to the weight of the car. On top of the higher rolling-resistance from the negative lift, the famous expression, 'lift is drag' was on our minds.
Lift always has a drag penalty.
The graphical outputs allowed us to see that ground effects were the culprit for the lift. The symmetrical airfoil basically split the air in half, and since the amount that went under the car had to pass through a much smaller space than the air that went over the top, it had to travel much faster. With the air moving faster under the airfoil, the car was functioning in a manner opposite to an airplane wing, with air pressure forcing the vehicle down toward the ground. We went back to FastSURF and made changes to the airfoil design hoping to reduce negative lift. Our approach was to force more of the air to flow over the top of the airfoil by varying the ratio of the thickness of the top of the car to the bottom.