Numerical investigation of the Lorentz force effect on two-point statistics in a turbulent channel flow

Abstract

Effects of a uniform and static magnetic field in the streamwise direction with different Hartmann numbers on the average structure of velocity and pressure fluctuations in a magneto-hydrodynamic turbulent channel flow are investigated. Direct numerical simulations (DNSs) are performed at low magnetic Reynolds numbers and under subcritical conditions with the bulk Reynolds number Reb = 5600, based on the channel height, using a very long domain to analyze two-point velocity and pressure fluctuations. Comparisons are made with DNS data of channel flow with a spanwise magnetic field and also without a magnetic field. Results indicate a substantial increase in the large-scale flow anisotropy, more alignment of flow structures with the mean flow and substantial elongation of flow structures in the streamwise direction in case of the streamwise magnetic field. On the contrary, the spanwise magnetic field did not have a comparable influence on the large scale flow anisotropy. Streak spacing increased linearly with increasing Hartmann number with the magnetic field in the streamwise direction. The magnetic field in the spanwise direction was more effective in increasing the streak spacing at a lower Hartmann number. The mean size of the quasi-streamwise vortex rolls was not affected with the introduction of a magnetic field. The effect of the Hartmann number on turbulent scales shows that scales grow largely with increasing magnetic field strength. The influence of the magnetic fields on pressure fluctuations was similar in the two cases and confined to large-scale weak correlations, where their length was substantially increased, especially in the spanwise direction.