Reorganization of turbulence structure in magnetic Rayleigh-Benard convection: a T-RANS study

Effects of a uniform, vertically oriented, magnetic field on the reorganiza tion of coherent structure in Rayleigh-Benard convection of electrically co nductive fluid were studied using a time-dependent Reynolds-average-Navier- Stokes (T-RAMS) approach. This method can be regarded as a very large eddy simulation (VLES) in which the unresolved random motion is modelled using a low-Re-number k-epsilon-theta (2) algebraic stress/flux single-point closu re model. The large-scale deterministic motion, which is the major mode of heat and momentum transfer in the bulk central region, is fully resolved by the time solution. In contrast to LESS, the contribution of both modes to the turbulent fluctuations are of the same order of magnitude. The approach was first assessed by comparison with the available direct numerical simul ations (DNSs) and experimental data for non-magnetic Rayleigh-Benard convec tion for Rayleigh (Ra) numbers 6.5 x 10(5) and 10(9), as well as with our o wn LES for Ra = 6.5 x 105, using several criteria: visual observation of th e large structure morphology, different structure identification criteria a nd long-term averaged mean flow and turbulence properties. A visible simila rity with large structures in DNSs was observed, confirming the suitability of the T-RAMS approach to reproduce the flows dominated by large coherent motions. Application of a uniform magnetic field oriented vertically, which generates the Lorentz force in the horizontal homogeneous direction, was s hown to suppress the horizontal motion and its fluctuations, aligning thus the velocity vector with the direction of magnetic field vector. Two cases were considered, both for Ra = 10(7), corresponding to two values of Hartma nn (Ha) number: 20 and 100. For the moderate magnetic field (Ha = 20), the effects are mild. For the strong magnetic field (Ha = 100), the vertical st ructure shows a strong two-dimensionality in the sense that the variation o f all flow properties in the vertical direction are significantly reduced. The total turbulence energy is very much suppressed, although it is still d etectable and strongly anisotropic, close to the one-component limit. The w all heat transfer is also strongly reduced as compared with the non-magneti c case for the same Rayleigh number.