A STUDY OF OVER-REFLECTION IN HARTMANN FLOW

It is observed that Hartmann flow sustains wave propagation in its cen tre region for waves whose phase speed is less than the maximum flow s peed. Similar to the previous observations it is found that viscous bo undary layers around the critical level and at the wall replace the ex ponential regions and wave sinks required for over-reflection in the i nviscid flow. The uniform magnetic field stabilizes the flow for small -wave-number disturbances along the z-direction. Over-reflection is co nfined to a few ranges of phase speeds for which the two boundary laye rs are close together rather than widely separated. These ranges corre spond exactly to those for which unstable eigenmodes exist. Over-refle ction is associated with a wave phase tilt opposite in direction to th e shear.