Transportation and coherent structures in MHD turbulent channel flow subject to uniform streamwise and spanwise magnetic fields

This investigation concerns the response of wall turbulences in a channel flow to uniform streamwise and spanwise magnetic fields through direct numerical simulations. More than 20 flow cases with different Stuart numbers were considered. It is found that the spanwise magnetic field leads to flow relaminarization at Stuart numbers significantly smaller than the streamwise magnetic field. The explanation of this phenomenon is not straightforward, and a deep analysis of the fine turbulence structure is needed for a clear understanding. This is achieved in a first step by considering Reynolds shear stress transport equations. It is shown that there are source and destruction terms directly related to the magnetic field. The destruction term overcomes the source terms under the spanwise magnetic field, leading to significant drag reduction once the Stuart number exceeds a critical value. The source term is not negligible and retards the relaminarization under the streamwise magnetic field. Subsequently, the conditional averages of the fluctuating velocity field and the electric current educed from the near wall coherent quasistreamwise vortices are discussed in detail. The electric current field is decomposed into an electromotive and conductive part. Their conditional averages are analyzed separately, in order to shed light on the topological differences. It is further shown that the quasistreamwise vortex paradigm allows an easy way to analyze the results leading to pertinent interpretations.

Automated summary

This investigation uses direct numerical simulations to study the response of wall turbulences in channel flow to uniform streamwise and spanwise magnetic fields. It is found that spanwise magnetic field leads to flow relaminarization at lower Stuart numbers compared to the streamwise magnetic field. A detailed analysis of the turbulence structure is needed to fully understand this phenomenon.