Large-Eddy Simulation of Vortex-Induced Transition in a Separation Bubble

The transitional behaviour of laminar separation bubbles (LSBs) can be modulated significantly by periodically passing convective vortices. Such low Reynolds number aerodynamic features are commonly encountered in cascade flows in turbomachinery, flows over aerofoils, flow past surface mounted obstacles, dynamic stall, blade vortex interaction and impulsive motion of bluff bodies. In the present study, an LSB has been created on a flat plate by imposing a suction-blowing velocity profile in the free-stream resembling the pressure distribution on the suction side of a NACA 643-618 aerofoil. The Reynolds number is kept constant at Re=U_∞L_∞/ν=3200. Realistic free-stream turbulence, with a turbulence intensity Tu_∞=3.3%, has been imposed in the inlet to mimic grid turbulence. A high-fidelity LES has been performed using a dynamic subgrid-scale model to study the receptivity of the LSB to periodically passing vortices, instabilities induced by these vortices, transition mechanism and the associated flow structures as compared to an LSB without passing vortices. The passing frequency of the vortices (f_v) is varied as a multiple of the natural frequency of vortex shedding (f_n) from the LSB (without the presence of passing vortices) in the range (f_v/ f_n =1-10). The passing vortices with the same sign of vorticity as the separated shear layer lead to an earlier breakdown of the shear layer leading to a decrease in mean bubble length with decreasing frequency ratio (f_v/ f_n). The vortex breaks the bubble into multiple instantaneous separated regions, and the flow field relaxes for a significantly long time after a single vortex is passed. The time-averaged statistics show a disappearance of the mean bubble for f_v/ f_n ≤ 3. The phase-averaged statistics, calculated at the vortex passing frequency, depict a phase-dependent bubble structure dependent on the frequency ratio.