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

Abstract

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∘ as the angle of the tapered surfaces ϕs is increased. A 6% drag reduction over the simple square-back case is reported for 6∘<ϕs<12∘. 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 ϕs=12∘) is also assessed. As the pitch angle α is reduced from 0∘ to −2∘, the time averaged wake is reported to switch from a downwash dominated topology to an upwash dominated topology. A strengthening of the long-time 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 α=0∘.