Linear Stability Analysis of Compressible Channel Flow with Porous Walls

TL;DR

This study analyzes how permeable walls affect the stability of compressible channel flow, revealing the emergence of unstable modes at high permeability and their implications for flow control strategies.

Contribution

It introduces a linear stability analysis of compressible flow with porous walls modeled by acoustic impedance, identifying new unstable modes and their behavior.

Findings

High permeability induces two dominant unstable modes.

Both modes increase Reynolds shear stresses in the viscous sublayer.

Modes' structures are unaffected by permeability changes within studied range.

Abstract

We have investigated the effects of permeable walls, modeled by linear acoustic impedance with zero reactance, on compressible channel flow via linear stability analysis (LSA). Base flow profiles are taken from impermeable isothermal-wall laminar and turbulent channel flow simulations at bulk Reynolds number, $Re_b$= 6900 and Mach numbers, $M_b$ = 0.2, 0.5, 0.85. For a sufficiently high value of permeability, Two dominant modes are made unstable: a bulk pressure mode, causing symmetric expulsion and suction of mass from the porous walls (Mode 0); a standing-wave-like mode, with a pressure node at the centerline (Mode I). In the case of turbulent mean flow profiles, both modes generate additional Reynolds shear stresses augmenting the (base) turbulent ones, but concentrated in the viscous sublayer region; the trajectories of the two modes in the complex phase velocity space follow each other closely for values of wall permeability spanning two orders of magnitude, suggesting their coexistence. The transition from subcritical to supercritical permeability does not alter the structure of the two modes for the range of wavenumbers investigated, suggesting that wall permeability simply accentuates pre-existing otherwise stable modes. Results from the present investigation will inform the design of new compressible turbulent boundary layer control strategies via assigned wall-impedance.