Lab 1Lab 2Lab 3 |
The aerodynamics of a car is used to determine how well and how fast a car can move. Aerodynamics is the study of flow around and through a vehicle. In order to get a complete picture of the aerodynamics of a car, one has to study the lift and drag forces on the car as well as the pressure distribution. The "best" moving car will strive to have a low drag force, but a high negative lift force (also know as down force) on the vehicle. Drag is due primarily to the asymmetry in pressure caused by separation of the boundary layer and to a lesser degree to the friction in air caused by viscosity. A large down force is important because it increases the normal load on tires, allowing the car to corner faster and retain control. Specifically, we studied the aerodynamics of a General Lee Charger. The aerodynamics were studied using a 1/12 scale model of the General Lee Charger. The Charger was placed motionless in a wind tunnel. The flow of air over the Charger model was then analyzed. The pressure distribution, lift, and drag forces on the model were used to evaluate the aerodynamics of an actual size Charger. In our analysis, it was assumed that air was a Newtonian fluid and that the flow around the car was incompressible. By assuming air is Newtonian, we ignored any change in air viscosity around the car with respect to shear force. Since air really is close to Newtonian, this reduces the level of complexity of our aerodynamic analysis while not significantly limiting our results. Assuming the air flow around the car is incompressible meant that no changes in density were considered. For constant ρ, the air had to flow at a speed of < Mach 0.3. This is not much of a problem for a full-scale vehicle. However, in order to match dimensionless parameters, the 1/12 scale model would have to travel at a speed much greater than mach 0.3. Therefore, it impossible to measure the flow properties of a large car driving at full speed with our smaller model. Ideally, Reynold's number should be matched between the model and real car, but this would require the full-scale vehicle to be traveling at less than 4 mph. We can still gather information from the lower speeds, because for the high Reynold's number of our model, the coefficients of lift and drag should stay constant as Re gets even larger. Because of the Charger's sharp corners, the flow cannot go over the car smoothly. When the flow hits the front panel of the car, separation in the flow occurs. The separation causes turbulence of the car. The most turbulence builds up on the back windshield and the back tail of the charger after the flow has been separated by the front wind shield and top of car. This creates low pressure behind the car, and therefore a high drag force. Both a CFD diagram was constructed on solid works as well as an experimental PIV diagram which showed the velocity vectors over the car and further led us to our conclusions. |
A difference in pressure before and behind a vehicle causes pressure drag, the dominant force that opposes a car's motion. In this lab, we used pressure taps to find the distribution of pressure over the body of the car at speeds of 12 and 20 m/s. The two graphs were 'relatively' the same, that is pressures rose and fell in the same positions. The magnitudes of pressure were higher in the 20 m/s case, which is to be expected because there is more available energy when the air speed is higher. Since the pressures are specific to our small-scale model, we also calculated the pressure coefficient, Cp. This dimensionless quantity is independent of both air velocity and car size and is a function only of shape. For our two velocities, the values of Cp were the same to within the error bars for all points except those on the top of the car. Both Cp and P were highest on the front of the car, lowest on the hood and roof, and near zero on the back end.
The two main aerodynamic forces imposed on a vehicle are lift and drag. Drag is the force that acts opposite to the path of the vehicles motion. Lift is the force that acts on the vehicle normal to the surface which it is driving on. We measured these two force values for the General Lee (Dodge Charger) model in a wind tunnel. We then found the lift and drag coefficients at air velocities from 9 to 22 m/s. The drag coefficient converged at a value of -.6 while the lift coefficient converged at -.01. From the graphs we plotted of C_D vs. air velocity and Cl vs. air velocity we noticed that lift and drag change as the flow around the car turns from laminar to turbulent flow. Once the flow becomes fully turbulent these values each began to converge on a single value. The negative values of the coefficients meant that the drag acted in the direction of flow and the lift acted towards the ground.
PIV and CFD are great at coming really close to the solving the Navier-Stokes equation. Instead of using differential elements, they look at the flow in numerous very small pieces. We used CFD to get a prediction of what should be happening around the Charger, and PIV to see what's actually going on. The velocity distribution diagrams between the computer and lab coincided nicely with each other and the pressure distribution for CFD was similar to that given in Lab 1. Therefore, we are able to use the other outputs (like turbulent energy) from CFD with a degree of conviction.
