Effects of forced frequency oscillations and unsteady wakes on the separation-induced transition in pressure gradient dominated flows

Abstract

The individual and cumulative effects of forced frequency oscillations and unsteady wakes, on the separation-induced transition caused by an adverse pressure gradient on a flat plate geometry, are investigated. The flat plate is subjected to a streamwise pressure gradient via an inviscid contour wall, which is representative of the pressure gradient on the suction surface of a low pressure turbine. Periodic Gaussian wakes are imposed at the inflow to mimic the effect of upstream stator wakes. The transitional and turbulent regimes of the flow are investigated using instantaneous and time-averaged flow fields, turbulent kinetic energy (TKE) budgets and a disturbance enstrophy-based nonlinear receptivity analysis based on direct numerical simulations. The impression of the wake in the boundary layer convects 50%–70% slower than the wake. The case with oscillations and wakes is most effective in triggering early transition and reducing the region of separation, specifically the secondary separation bubble is completely suppressed. Enhanced near-wall mixing is observed with a shift in the inflection point of the velocity profile toward the wall. The budget of the TKE shows the dominance of the production term for all cases considered. The disturbance enstrophy transport equation shows the role of the vortex stretching, enstrophy diffusion and dissipation for flows with wakes. Finally, a linear spatio-temporal receptivity analysis is performed for velocity profiles in the attached and separated boundary layer. This provides a description of the effects of pressure gradient and unsteady wakes in the transition to turbulence.