Non-Boussinesq stability analysis of natural-convection gaseous flow on inclined hot plates

TL;DR

This paper presents a non-Boussinesq stability analysis of natural convection gaseous flow over inclined hot plates, revealing how temperature variations affect flow instability modes and the critical inclination angle.

Contribution

It introduces a non-Boussinesq analysis accounting for temperature-dependent properties, extending previous Boussinesq-based stability studies for gaseous flows.

Findings

Spanwise traveling waves dominate for a wider range of angles when considering non-Boussinesq effects.

Critical Grashof number and wavelength depend on inclination angle and temperature ratio.

Crossover inclination angle varies with temperature ratio, affecting flow instability modes.

Abstract

The buoyancy-driven boundary-layer flow that develops over a semi-infinite inclined hot plate is known to become unstable at a finite distance from the leading edge, characterized by a critical value of the Grashof number Gr based on the local boundary-layer thickness. The nature of the resulting instability depends on the inclination angle $\phi$, measured from the vertical direction. For values of $\phi$ below a critical value $\phi_c$ the instability is characterized by the appearance of spanwise traveling waves, whereas for $\phi>\phi_c$ the bifurcated flow displays G\"ortler-like streamwise vortices. The Boussinesq approximation, employed in previous linear stability analyses, ceases to be valid for gaseous flow when the wall-to-ambient temperature ratio $\Theta_w$ is not close to unity. The corresponding non-Boussinesq analysis is presented here, accounting also for the variation with temperature of the different transport properties. A temporal stability analysis including nonparallel effects of the base flow is used to determine curves of neutral stability, which are then employed to delineate the dependences of the critical Grashof number and of its associated wave length on the inclination angle $\phi$ and on the temperature ratio $\Theta_w$ for the two instability modes, giving quantitative information of interest for configurations with $\Theta_w-1\sim 1$. The analysis provides in particular the predicted dependence of the crossover inclination angle $\phi_c$ on $\Theta_w$, indicating that for gaseous flow with $\Theta_w-1\sim 1$ spanwise traveling waves are predominant over a range of inclination angles $0 \le \phi \le \phi_c$ that is significantly wider than that predicted in the Boussinesq approximation.