According to Bernouli's equation the velocity and pressure are inversely related. So in the 20 m/s case, when the velocity of the flow along the streamlines decreases (for instance when it hits the blunt front end of the car) the pressure on this region increases, and it increases more than in the 12 m/s case. The pressure is lowest on the horizontal surfaces and greatest on the highly angled surfaces. On the rear of the car the pressure rises slightly but not back to its initial value. The drag force causes this difference. If the flow was perfectly inviscid and there were no boundary layer, the pressure at tap 18 would be equal to the pressure at tap 1. However air, like all fluids at room temperature, has a nonzero viscosity. This creates high shear forces in the boundary layer immediately about the surface of the car. The friction in this region creates drag and is the reason for the asymmetry. Cp is a dimensionless parameter that does not depends on the size of the car or the velocity of the air. This relative value is constant for all incompressible flows in the tunnel, so we expected the 12 m/s and 20 m/s Cp values to be identical for corresponding position. Also the Cp value will be the same on the full size Charger at any speed. The majority of our numbers matched to within the error bars, but five of them did not. The Cp values will separate if the flow becomes turbulent or if it separates from the surface of the car. This is the case for tap 18, for the in the wake flow behind the car is very turbulent, and the flow has separated from the surface. The four taps on the roof of the car do not meet either of these requirements but Cp values for taps 9-12 are not the same. The uncertainty in the density of air was calculated and included in our analysis because a change in the air temperature can affect the air density. We estimated the temperature in the wind tunnel room to vary from 70ºF to 80ºF causing an uncertainty of ± .00969 kg/m³ in the air density. |