Turbulence and Transport of Passive Scalar in Magnetohydrodynamic Channel Flow

Abstract

An imposed magnetic field influences the flow structure and transport characteristics of a moving electrically conducting fluid. Such magnetohydrodynamic (MHD) flows are ubiquitous in nature and technological applications, for example in casting of steel and aluminum and growth of semiconductor crystals. In many situations, the effect of the magnetic field is combined with that of mean shear and occurs in the presence of transport of heat and admixtures. In the performed doctoral research, extensive Direct Numerical Simulations (DNS) are conducted for the flows of an electrically conducting fluid in a channel with imposed magnetic field. The cases of wall-normal, spanwise and streamwise orientations of the magnetic field are considered. The strength of the magnetic field varies in such a way that the flow transitions from fully turbulent state to slightly below the laminarization threshold. The main goal of the investigation is to understand the flow transformation and the effect of the magnetic field on the characteristics of the transport of a passive scalar (e.g. temperature or admixture). It is found how the magnetic field affects the scalar distribution and the rate the turbulent transport across the channel. In the range of the magnetic field strengths considered, the effect is strong in the cases of the wall-normal and spanwise magnetic field, but weaker in the case of the streamwise field. A major outcome of the study is the establishment of a nearly linear dependency of the turbulent scalar flux of the magnetic interaction parameter (the Stuart number). One-dimensional models, of flow field and scalar distribution with approximations of eddy diffusivity and eddy viscosity are developed on the basis of the computational results. Scalar transport and perturbation dynamics are also investigated for the channel flow with spanwise magnetic field for the flow regime characterized by the large-scale intermittency characterized by long periods of nearly laminar, nearly two-dimensional behavior interrupted by brief turbulent bursts.