Gauging Centrifugal Instabilities in Compressible Free-Shear Layers Via Nonlinear Boundary Region Equations

Abstract: Curved free shear layers emerge in many engineering problems involving complex flow geometries, such as the flow over a backward facing step, flows with wall injection in a boundary layer, the flow inside side-dump combustors, or wakes generated by vertical axis wind turbines, among others. Previous studies involving centrifugal instabilities have mainly focused on wall-flows where Taylor instabilities between two rotating concentric cylinders or G\"{o}rtler vortices in boundary layers, resulting from the imbalance between the centrifugal forces and the radial pressure gradients, are generated. Curved free shear layer flows, however, have not received sufficient attention, especially in the nonlinear regime. The present work investigates the development of centrifugal instabilities evolving in a curved free shear layer flow in the nonlinear compressible regime. The compressible Navier-Stokes equations are reduced to the nonlinear boundary region equations (BRE) in a high Reynolds number asymptotic framework wherein the streamwise wavelengths of the disturbances are assumed to be much larger than the spanwise and wall-normal counterparts. We study the effect of the freestream Mach number $\boldsymbol{M_\infty}$, the shear layer thickness $\boldsymbol{\delta}$, the amplitude of the incoming disturbance $\boldsymbol{A}$, and the relative velocity difference across the shear layer $\boldsymbol{\Delta V}$ on the development of these centrifugal instabilities.