Numerical analysis of periodic open-loop flow control on bluff bodies in ground proximity

The effectiveness of open-loop actuation at the rear of near-ground bluff bodies in achieving drag reduction is explored numerically. The investigations are conducted on an infinite-width two-dimensional flat-back Ahmed body of height H, placed at a height of 0.2H above ground, using large eddy simulations at a Reynolds number of 23,000. This case better mimics the upper and lower shear layers that might be seen behind a heavy vehicle than previous away-from-ground tests, which have shown significant drag reduction is achievable through the attenuation of von Kármán shedding in the near wake. In this study, this effect is shown to essentially vanish once the body is moved close to a moving ground-plane, because the regular formation of strong compact wake vortices in the near wake is prevented. This is also shown to be the case under natural conditions, where a lower drag coefficient than the away-from-ground case is observed. This is interpreted as the result of a mismatch in wake vorticity shed in the upper and lower shear layers, and the disrupting presence of an extra layer of vorticity at the ground, which act together to limit or even suppress vortex shedding into the wake. The periodic actuation strategy, which attempts to control the separating shear layers and, in turn, the formation and shedding of wake vortices, therefore loses applicability. It is postulated that the open-loop strategy may still be effective if employed on the side rear-edges of a heavy vehicle, even though the upper and lower rear-edges are unlikely to yield much success.