Effects of Compressibility and Variable-Density on the Instability Growth within Buoyancy-Driven Shear Layers

Direct numerical simulations are performed to examine the effects of compressibility and compositional variable density (VD) on the instability growth within buoyancy-driven shear layers (BSL), which share characteristics with the canonical Rayleigh-Taylor (RT/RTI) and Kelvin-Helmholtz (KH/KHI) instabilities. The 2D simulations are performed in a periodic domain, with alternating columns of high and low density fluids and neutral background stratification. This study expands upon work previously done by Gat, Matheou, Chung, et al. in the incompressible regime. The Atwood number A, and the isothermal Mach number Ma, are varied to investigate the effects of variable density and compressibility on the BSL, respectively. The results suggest that the flow exhibits two distinct flow regimes. An early time regime is driven by shear instabilities and experiences minimal compressibility and variable density effects with the characteristics of KHI; the second regime follows and is driven by buoyancy while exhibiting certain similarities with RTI. It was found that as A increases, the BSL enters the second regime more quickly in non-dimensional time. In this regime, compressibility has a suppressive effect on the growth of the mixing layer and the heights of the bubble- and spike-like structures, even though the background stratification is neutral due to the homogeneity. Compressibility also enhances the heavy and light fluid mixing asymmetry induced by the variable density effect, a phenomenon observed in RTI. It also leads to a more mixed mixing layer, resulting in less large-scale mixing with shorter pure fluid penetrations across the initial interface. In addition, the reduced interfacial density gradients due to molecular mixing lead to a weakening of the baroclinic torque, the primary vorticity production term. This can lead to a concentration of vortical motions at dissipative scales, which eventually results in the cessation of mixing layer growth with sufficiently strong compressibility effects. Lastly, it is observed that the suppressive effect of compressibility appears to be enhanced by a greater A. Thus, greater A induces the flow regime transition more quickly, but also leads to greater instability suppression through compressibility at later times. In this way, compressibility and VD effects are coupled while also competing.

Grantor dc:publisher

University of Alabama Libraries

Year dc:date.issued

2025

Author dc:creator

• Staggs, Hutson

Advisors dc:contributor.advisor

• Hubner, Paul
• Aslangil, Denis

Contributors dc:contributor

• Wahidi, Redha

• Buoyancy-Driven
• Compressible
• Direct Numerical Simulation
• Mixing
• Shear Layer
• Variable-Density

Statement dc:rights

• All rights reserved by the author unless otherwise indicated.

Language dc:language.iso

en_US, English

Dc Identifier Other

1174552

OAI identifier oai:identifier

oai:ir.ua.edu:123456789/17050