Enhanced instability of supersonic boundary layer using passive acoustic feedback.

Tripping of boundary-layers from laminar to turbulent flow, which may be needed in specific practical applications, requires introducing high amplitude disturbances into the boundary layers without large drag penalties. As a possible improvement on fixed trip devices, a technique based on feedback instability is demonstrated in the present contribution for a Mach 1.5 boundary layer. The compressible Navier-Stokes equations are solved directly using a high-order (fifth-order in space and third-order in time) finite difference method. The geometries tested include wall-mounted rectangular blocks and off-surface cylinders. For a particular combination of an upstream cylinder and downstream block, a feedback loop is observed to provide more robust self-amplification of disturbances than a cylinder alone, whereas blocks by themselves only enhance a convective instability. A Fourier analysis and an impulse response simulation are used to confirm the presence of an acoustic feedback loop for the cylinder-block configuration.Aparametric study of the size of the cylinder, the height, and the location of the rectangular block is also provided. It is observed that the feedback strength is not monotonically related to the height and location of the rectangular block.