Harmonically forced enclosed swirling flow

The response of steady state vortex flows in an enclosed circular cylinder driven by the harmonic modulation of the rotating end wall is investigated experimentally and numerically. Three dynamic regimes have been identified, with a continuous variation in forcing frequency between them. For very low forcing frequency, the synchronous flow approaches quasistatic adjustment, and for very large forcing frequencies the oscillations in the synchronous flow are localized in the boundary layers on the various cylinder walls. These localized wall oscillations drive the synchronous flow in the cylinder interior to the underlying axisymmetric steady basic state. The third regime occurs for forcing frequencies in the range of the most dangerous axisymmetric Hopf eigenfrequencies, with the 1:1 resonances leading to greatly enhanced oscillation amplitudes localized in the axis region where the flow manifests vortex breakdown recirculation zones.