Low-frequency wake dynamics for a square-back vehicle with side trailing edge tapers

摘要

In this paper, the effects of side trailing edge tapering on the wake of a simplified square-back vehicle are investigated. The tapered surfaces are reported to trigger a switch from a laterally asymmetric bi-stable wake to a vertically asymmetric stable wake. The wake structure reported in the literature for lateral symmetry breaking states is seen to rotate by 90 degrees as the angle of the tapered surfaces phi(s), is increased. A 6% drag reduction over the simple square-back case is reported for 6 degrees phi(s) 12 degrees. This gain is found to be the result of the stretching of the circular vortex responsible for the suction zone visible in any symmetry breaking state. A downwash dominated wake is observed in these conditions. The sensitivity of such a wake to small variations of the model pitch angle (for phi(s) =12 degrees) is also assessed. As the pitch angle a is reduced from 0 degrees to - 2 degrees, the time averaged wake is reported to switch from a downwash dominated topology to an upwash dominated topology. A strengthening of the longtime instability is observed when the symmetry in the vertical direction is recovered and is accompanied with a 4.9% reduction in base drag over the same model tested at alpha = 0 degrees.