Free-stream coherent structures in parallel compressible boundary-layer flows at subsonic Mach numbers

TL;DR

This paper extends the theory of coherent structures in shear flows to compressible boundary layers at subsonic Mach numbers, revealing how Mach number and Prandtl number influence streak growth and suggesting similar transition mechanisms as in incompressible flows.

Contribution

It introduces a novel description of nonlinear equilibrium solutions in compressible boundary layers, extending incompressible theories to include thermal effects and Mach number influences.

Findings

Thermal streaks are enhanced with increasing Mach number.

Maximum amplitude of streaks occurs within the boundary layer.

Prandtl number shifts the location of maximum thermal streak amplitude.

Abstract

As a first step towards the asymptotic description of coherent structures in compressible shear flows, we present a description of nonlinear equilibrium solutions of the Navier--Stokes equations in the compressible asymptotic suction boundary layer (ASBL). The free-stream Mach number is assumed to be $< 0.8$ so that the flow is in the subsonic regime and we assume a perfect gas. We extend the large-Reynolds number free-stream coherent structure theory of \cite{deguchi_hall_2014a} for incompressible ASBL flow to describe a nonlinear interaction in a thin layer situated just below the free-stream which produces streaky disturbances to both the velocity and temperature fields, which can grow exponentially towards the wall. We complete the description of the growth of the velocity and thermal streaks throughout the flow by solving the compressible boundary-region equations numerically. We show that the velocity and thermal streaks obtain their maximum amplitude in the unperturbed boundary layer. Increasing the free-stream Mach number enhances the thermal streaks, whereas varying the Prandtl number changes the location of the maximum amplitude of the thermal streak relative to the velocity streak. Such nonlinear equilibrium states have been implicated in shear transition in incompressible flows; therefore, our results indicate that a similar mechanism may also be present in compressible flows.