Effects of rotation on turbulent mixing: Nonpremixed passive scalars

Abstract

We study by direct numerical simulations the effects of uniform solid-body rotation on passive scalar mixing in turbulent flow, with a focus on the unsteady problem of nonpremixed scalars in forced rotating turbulence with isotropic initial conditions in the velocity field. The expectation of reduced mixing as a result of reduced spectral transfer is readily verified in several aspects, including slower decay rates for the scalar variance, increased scalar mixing times, and slower relaxation of the probability density function from its initially bimodal form to a near-Gaussian shape. Spectral transfer in the scalar field is shown to be dominated by very low-wavenumber velocity modes, and strongly suppressed at higher wavenumbers in the scalar field. Considerable departure from local isotropy is observed in the scalar gradient fluctuations, which are smaller in the direction along the axis of rotation where there is less mixing than in the orthogonal plane. A partial explanation is given in terms of the influence of a modified turbulence velocity structure on directional characteristics of spectral transfer, which leads to anisotropy in the scalar gradient spectra as well as one-dimensional spectra of the scalar field. The observed anisotropy is stronger than that for the velocity field, especially for high rotation rates, and is more pronounced at Schmidt number 1 than at 1/8. A reduction in intermittency compared with nonrotating turbulence is also observed.