Wind tunnel force-balance and wake-traverse tests were made on .154-scale car models with various degrees of streamlining to determine the significance of ground treatment for increasing levels of aerodynamic cleanness. The wake-traverse analysis included investigations of spanwise distributions of vortex and viscous drag, which gave insight into the flow mechanisms involved.
It was found from these tests that thick, uncontrolled tunnel-floor boundary layers yielded wakes with viscous “side-lobes” at floor level, which were absent with a moving ground representation. For “bluff car designs, this was the only effect. For “slippery” designs, more typical of modern design practice, other more significant changes were noted. Measured changes in trailing-vortex strength and the associated vortex drag suggested that very low-drag designs experience an increase in effective angle-of-attack when the ground is fixed. This is qualitatively consistent with the upflow associated with the growth in boundary layer displacement thickness for an uncontrolled Fixed-ground. The use of tangential blowing upstream of the model was investigated as an alternative to a true moving ground. The results showed a number of similarities to those for a moving ground, notably in attenuating the side-lobe phenomena; but the moving-ground-induced changes in vortex structure for low-drag cars were not well reproduced. Nonetheless, drag correlations between wake integral and balance measurements showed that ground blowing alone provided the majority of the correlation improvement, which was brought about by the combined blowing and ground motion.
Finally, wake measurements for a configuration with rolling wheels showed further reductions in wake width, relative to a comparable stationary, flat-bottom-wheels case tested over a moving ground. However, this effect was less significant than the effect of ground motion itself.