Optimisation of Trapped Vortex Cavity for Airfoil Separation Control

Abstract

The effects of a Trapped Vortex Cavity (TVC) on the aerodynamic performance of a NACA 0024 airfoil at a constant angle of attack (AoA) of 14 • were investigated in this study. It was observed that mass suction (MFR) was required to stabilise the vortex within the cavity segment. Lift to drag ratio (L/D) and MFR were chosen as performance objectives, along with a fully attached flow constraint (flow separation at X/c ≥ 95% ). Parametric analysis was carried on the baseline airfoil with and without suction and compared to the airfoil with TVC with and without suction. It was observed that L/D increases as MFR increases for a baseline airfoil, and flow separation is delayed at high suction values (MFR = 0.2 kg/s). The TVC modifies the pressure distribution on the baseline airfoil when MFR is applied to the cavity section and there is a significant increase in lift; thus, L/D increases and flow separation is delayed. A lower value of MFR = 0.08 kg/s is sufficient to stabilise the vortex and improve the efficiency of the TVC airfoil. The findings of these parametric studies were used to do a multi-objective optimisation using a genetic algorithm to attain the desired cavity shape while achieving the largest L/D and the lowest MFR (that is proportional to the power required for control) with a fully attached flow constraint. It was found that mass suction and cavity shape both had an equal influence on flow control. The Pareto optimal front yielded a series of optimum designs. One of them was subjected to an off-design analysis in order to validate its performance at other incidences. It was observed that it performs better than the baseline airfoil, with an improved L/D and an increase in stall angle from 10 • to 14 • .