Wakes of thin flat plates: Intermittency, cessation of vortex shedding and the emergence of an intermediate-wake instability

Abstract

The near and intermediate wake regions of thin flat plates with both sharp and circular trailing edges (TEs) are investigated with direct numerical simulations (DNSs). The TE is circular in two of the cases (IN & NS) and sharp in one of them (ST). The separating boundary layers are turbulent in all cases. The main objective here is to explore the effect of significantly reducing the Reynolds number (ReD, based on circular TE diameter, D) on the flow in the TE region, in particular the vortex shedding process (Cases IN and NS). Intermittent shedding is observed in Case IN. Case NS, with half the TE diameter of Case IN, is an essentially non-shedding case. The second objective is to understand the reasons underlying the findings of an earlier experimental wake investigation (sharp TE) where a broadband peak was observed in centerline cross-stream velocity (v) spectra. Case ST from the present study showed a wake instability resulting in spanwise vortices. The instability is intermittent and contributes to a broadband peak in the centerline v spectrum. Cases IN & NS also exhibit a similar wake instability in the intermediate wake and a corresponding spectral (v) broadband peak. The third objective is to study the distributions of the time-averaged velocity statistics in thin plate wakes. The turbulent stresses and, the budget terms for the streamwise intensity, obtained in Case IN, are included and discussed here. All the budget terms contribute appreciably to the overall budget in the transport equation for streamwise normal intensity.