Transition of a Wave Packet in a Rotating-Disk Flow

 

Abstrat: A laminar to turbulent transition process of a three-dimensional boundary layer in a rotating-disk flow initiated by a localized strong disturbance was experimentally investigated by a single hot-wire probe in a laboratory frame.  To generate the localized disturbance, a pulsejet was issued into the rotating-disk flow from a small hole, where the boundary layer is slightly subcritical to the cross-flow (C-F) instability.  The hot-wire analyses showed that the point disturbance develops into an isolated turbulent patch accompanied by different types of traveling waves.  The configuration in the plane view of the turbulent patch is a crescent different from an arrowhead-shaped turbulent spot in a Blasius boundary layer.  As the turbulent patch grows outward in radial direction, the convection velocity of its inward edge gradually decreases.  Comparison with the prediction by Itoh’s linear stability theory enabled us the identification of the traveling waves around the patch; the streamline-curvature (S-C) unstable waves propagating outward grow around outer edge of the patch while the cross-flow unstable waves developing in the upstream region of the trailing edge travel in the inward direction.  The S-C mode has about 5 to 7 times higher frequency than that of the C-F mode. Such characteristic features of these waves are in good agreement with the theoretical results.  Since the turbulence is produced in the center region of the wave packet, which mode mainly contributes to the breakdown into turbulence still remains unclear.