Compressible turbulent mixing: Effects of Schmidt number

TL;DR

This study explores how the Schmidt number influences passive scalar transport in compressible turbulence, revealing distinct spectral behaviors, statistical properties, and mixing mechanisms across different Schmidt number regimes.

Contribution

It provides new insights into the spectral and statistical characteristics of scalar mixing in compressible turbulence, highlighting the effects of Schmidt number and compressibility.

Findings

Scalar spectrum follows k^{-5/3} in inertial range.

Different power laws observed for high and low Schmidt numbers.

Scalar dissipation increases with Schmidt number.

Abstract

We investigated the effects of Schmidt number on passive scalar transport in forced compressible turbulence. In the inertial-convective range the scalar spectrum followed the k^{-5/3} power law. For Sc >> 1, there appeared a k^{-1} power law in the viscous-convective range, while for Sc << 1, a k^{-17/3} power law was identified in the inertial-diffusive range. The scaling constant for the mixed third-order structure function of velocity-scalar increment grew over Sc, and the effect of compressibility made it smaller than the classical 4/3 value. At small amplitudes, the PDF of scalar fluctuations collapsed to the Gaussian distribution, whereas at large amplitudes it decayed more quickly than Gaussian. At large scales, the PDF of scalar increment behaved similarly to that of scalar fluctuation, while at small scales it resembled the PDF of scalar gradient. The scalar dissipation occurring at large magnitudes was found to grow with Sc. Due to low molecular diffusivity, for Sc >> 1, the scalar field rolled up and got mixed sufficiently. However, for Sc << 1, the scalar field lost the small-scale structures by high molecular diffusivity, and retained only the large-scale, cloudlike structures. The spectral densities of scalar advection and dissipation in both Sc >> 1 and Sc << 1 flows followed the k^{-5/3} scaling. This indicated that in compressible turbulence the processes of advection and dissipation except that of scalar-dilatation coupling might defer to the Kolmogorov picture. It then showed that at high wavenumbers, the magnitudes of spectral coherency in both Sc >> 1 and Sc << 1 flows decayed faster than the theoretical prediction of k^{-2/3} for incompressible flows. Finally, the comparison with incompressible results showed that the scalar in compressible turbulence with Sc=1 lacked a conspicuous bump structure in its spectrum, but was more intermittent in the dissipative range.