DOI: 10.1615/TSFP5
TRANSITION IN MHD CHANNEL FLOW WITH SPANWISE MAGNETIC FIELD
RESUMO
The linear and nonlinear evolution of perturbations is studied in a magnetohydrodynamic channel flow with electrically insulating channel walls in which a strong magnetic field possesses an orientation orthogonal to the stream but parallel to the walls. The basic flow is unaffected by the magnetic field and retains the Poiseuille velocity profile. Linear optimal perturbation and their maximum amplifications over finite time intervals are computed using an iterative scheme based on the direct and adjoint governing equations for the subcritical Reynolds number Re = 5000. The presence of a magnetic field changes the spatial structure of optimal perturbations. As the Hartmann number increases, the optimal modes cease to be the classical streamwise rolls and become oblique rolls with axes at some angle to the direction of the flow. At sufficiently high Hartmann numbers, the optimal perturbations are the purely spanwise Orr modes. For Hartmann numbers in the range from 0 to 100 direct numerical simulations (DNS) are applied to investigate how the transition to turbulence is affected by the magnetic field. Simulations are conducted using the optimal modes as initial values with weak three-dimensional noise added at the maximum amplification time.