DNS of unsteady effects on the control of laminar separated boundary layers

The main factor that determines the efficiency of a high-lift low-pressure-turbine operating at low Reynolds numbers is the size of the laminar separation bubble that forms on the suction side of the turbine blade. In this study, the suction side of a low-pressure gas-turbine blade is modeled as a flat plate with a streamwise pressure distribution. The combined effects of discrete surface roughness and periodic large-scale wake forcing on separation bubble transition and control are investigated numerically by direct numerical simulation. The laminar separation bubble is controlled by positioning the roughness element upstream of the separation bubble and introducing a low-frequency large-scale wake forcing. The three-dimensional discrete roughness with varying roughness height in the spanwise direction is modeled using immersed boundary method. The wake effect is numerically modeled as the mean wake deficit profile created by a linear row of cylinders moving in a direction perpendicular to the flat plate. Results indicate that the location of transition and hence the shape and extent of the separation bubble, largely depend on which of the above-noted passive control mechanisms dominates the unsteady transition process. It is shown that considering the mean flow properties, the roughness effect is found to be negligible compared to the individual wake effect.