Modeling of magnetohydrodynamic turbulence

University dissertation from Stockholm : Mekanik

Abstract: Conventional one-point turbulence closures have beenextended with an additional transported scalar for modeling ofmagnetohydrodynamic (MHD) turbulence. The new scalar, ? ,captures the length scale anisotropy and tendency towardstwo-dimensionality, which is characteristic feature of MHDturbulence, and allows accurate modeling of the Jouledissipation of turbulence. The concept has been used for both afull Reynolds stress closure, and a three-equationK-? -?model. An exact transport equation for?was derived from the governing equations. All terms inthe equation require modeling, however. The proposed modeltransport equation for ? includes terms for magneticdissipation, nonlinear energy transfer, and effects of meanshear and strain. Modeling of the magnetic and strain-relatedterms was based on rapid distortion analysis of the linearizedequations, while modeling of nonlinear effects isphenomenological in nature. For homogeneous turbulence, themodel was compared with linear theory, direct numericalsimulations and experiments. For turbulence subjected to astrong magnetic field, the model reproduces the energy andlength scale evolution predicted by linear theory. Whennonlinear effects are of importance, it predicts energy decayand length scale evolution in agreement with experiments. Theeddy viscosity and Reynolds stress versions of the modelcoincide with the respective conventional models in the absenceof a magnetic field. The objective of this project has been todevelop efficient MHD turbulence models for engineeringapplications, especially for modeling of continuous steelcasting. The novel MHD turbulence models appear to benumerically robust, and they have been implemented in acommercial flow solver, together with electromagnetic equationsfor the Lorentz forces in the mean momentum equations.Keywords:Turbulence model, magnetohydrodynamics, MHD,magnetohydrodynamic turbulence, computational fluid dynamics,continuous casting, dimensionality, Reynolds stresses, eddyviscosity