Mixing enhancement in a subsonic-supersonic shear layer with a cavity splitter plate

The development of more efficient ramjet engines has motivated the study of enhanced mixing in compressible shear layers. Cavity-mixer is a widely used passive flow control tool, but its mixing enhancement mechanisms in compressible subsonic-supersonic shear layers has yet to be comprehensively explored. In this work, a cavity-mixer is used to increase mixing in a subsonic-supersonic shear layer, and the underlying flow control mechanisms are examined. Parametric experiments were performed on a laboratory-scale blowdown wind tunnel and an extraction wind tunnel. Numerical simulations were performed in a large eddy simulation framework. Schlieren and nanoparticle-based planar laser scattering techniques were adopted for mixing flow visualization. It is found that the growth rate of the cavity (length-to-depth ratio of 5) disturbed shear layer is 31% higher than the benchmark configuration at convective Mach number of 0.49∼0.56. It is demonstrated that the compressibility and length-to-depth ratio affect the cavity-mixer performance. High convective Mach number suppresses the cavity disturbance and weakens its mixing enhancement efficiency. Cavity-mixer with a length-to-depth ratio of 3 shows the highest experimental and numerical growth rate, compared to 5 and 7 under a convective Mach number around 0.49. Large-scale coherent vortices are confirmed in the subsonic-supersonic mixing process. Cavity disturbance promotes the three-dimensional characteristics of the flow field, increases the vorticity of large coherent structures, and advances the K-H instability position of shear layer, which plays a key contribution to mixing enhancement.