Viscous effects on the acoustics and stability of a shear layer over an impedance wall

TL;DR

This paper investigates how viscosity and thermal conduction influence acoustics and stability in shear layers over impedance walls, revealing significant effects on wave damping, flow stability, and surface wave modes, with implications for aeroacoustic modeling.

Contribution

It introduces a combined viscothermal analysis using asymptotic and numerical methods to derive effective impedance boundary conditions that include shear and viscosity effects.

Findings

Viscothermal effects are as important as shear in acoustic damping.

Viscosity stabilizes short wavelength disturbances and alters instability characteristics.

Viscous flow supports more surface wave modes than inviscid flow.

Abstract

The effect of viscosity and thermal conduction on the acoustics in a shear layer above an impedance wall is investigated numerically and asymptotically by solving the compressible linearised Navier-Stokes equations. It is found that viscothermal effects can be as important as shear, and therefore including shear while neglecting viscothermal effects by solving the linearised Euler equations is questionable. In particular, the damping rate of upstream propagating waves is found to be dramatically under-predicted by the LEE in certain instances. The effects of viscosity on stability are also found to be important. Short wavelength disturbances are stabilised by viscosity, greatly altering the characteristic wavelength and maximum growth rate of instability. For the parameters typical of aeroacoustic simulations considered here, the Reynolds number below which the flow stabilizes ranges from $10^5$ to $10^7$. By assuming a thin but nonzero-thickness boundary layer, asymptotic analysis leads to a system of boundary layer governing equations for the acoustics. This system may be solved numerically to produce an effective impedance boundary condition, applicable at the wall of a uniform inviscid flow, that accounts for both the shear and viscosity within the boundary layer. An alternative asymptotic analysis in the high frequency limit yields a different set of equations with analytic solutions. The acoustic mode shapes and axial wavenumbers from both asymptotic analyses compare well with numerical solutions of the full LNSE. A closed-form effective impedance boundary condition is derived from the high-frequency asymptotics, suitable for application in frequency-domain numerical simulations. Finally, surface waves are considered, and it is shown that a viscous flow over an impedance lining supports a greater number of surface wave modes than an inviscid flow.