Mechanism of drag reduction by transverse cavities in a hypersonic turbulent boundary layer using direct numerical simulation

A transverse cavity configuration is a promising passive approach for achieving drag reduction in a turbulent boundary layer, and its underlying physical mechanism remains incompletely understood. Presently, direct numerical simulation is employed to investigate the effects of different scale cavities on a hypersonic turbulent boundary layer, with the aim of unveiling the mechanism of drag reduction. The logarithmic relationship between drag and the dimensionless scale (defined as the ratio of cavity depth to the boundary layer thickness at the cavity leading edge) holds for the small-scale range studied here. This relationship enables the prediction of the critical point where drag reduction transitions to drag increase. The enhancement of drag reduction with decreasing cavity scale is primarily attributed to a reduction in pressure drag, which is associated with the reorganization of near-wall turbulent structures. By analyzing the instantaneous velocity field and two-point spatial auto-correlations, we observe that as the scale decreases, the streak breakdown is significantly weakened. This reorganization of the coherent structures modulates the turbulent intensity and turbulent transport. The turbulent statistics reveal that a reduction in the cavity scale enhances the stability of the vortex within the cavity and the shear layer, as it decreases the intensity of impingement on the rear wall of the cavity. The conclusion is further confirmed by the turbulent transport term of turbulent kinetic energy, which also shows the dominant role of turbulent transport in the energy transport process.