Pumping effect in magnetohydrodynamic turbulent channel flow

In this study, direct numerical simulations of magnetohydrodynamic turbulent channel flow were performed to investigate the turbulent dynamo effects. A large-scale magnetic field was generated and sustained in the streamwise direction. The turbulence statistics were calculated, and the turbulent electromotive force was confirmed to represent turbulent dynamo effects from the viewpoint of energy transfer. The cross-helicity effect was identified as the dominant dynamo mechanism, which was consistent with previous large eddy simulations. The pumping effect was investigated in detail by examining the related production term in the transport equation for the turbulent electromotive force and was interpreted from the viewpoint of the pumping velocity. The pumping dynamo effect played the role of a dynamo mechanism in the core region. In the near-wall region, the behavior of the pumping effect was dependent on the sign of the mean magnetic field. When the mean magnetic field was positive, the pumping effect acted as a diffusion process, whereas when the mean magnetic field was negative, the pumping effect played the role of a dynamo mechanism. The energy transfer was also examined using transport equations for the turbulent kinetic and magnetic energies to clarify the generation mechanism of the pumping velocity.