Effects of forced frequency oscillations and free stream turbulence on the separation-induced transition in pressure gradient dominated flows

Abstract

The individual and cumulative effects of forced frequency oscillations and free stream turbulence (FST), on the separation-induced transition caused by an adverse pressure gradient on a flat plate geometry, are numerically investigated by solving the Navier–Stokes equation. The flat plate is subjected to a streamwise pressure gradient via a contoured wall, representative of a low pressure turbine blade. The results are validated against existing numerical data, and insights into flow physics of the pre-transitional and turbulent regimes are gained using instantaneous, time-averaged, and phase-averaged flow fields, turbulent kinetic energy budgets, and a disturbance enstrophy-based nonlinear analysis based on direct numerical simulations. Oscillations are imposed at two values of reduced frequency (kosc). Higher reduced frequency oscillations are more effective in triggering early transition and reducing the separated region. This suppression of the separated region is more pronounced with FST. A reduction in the size of the separation bubble is noted along with enhanced near-wall mixing resulting in a shift in the inflection point of the velocity profile toward the wall, with FST and oscillations. The budget of the turbulent kinetic energy shows the dominance of the production term for the case with FST and forced oscillations. The disturbance enstrophy-based analysis provides insight into budget terms of the enstrophy equation, specifically on the role of vortex stretching. Finally, spatiotemporal linear receptivity of the case with oscillations near the separation bubble is reported. This provides a quantitative description of the instability triggered by the pressure gradient and oscillations.