Successive Steps of 2D and 3D Transition in the Flow Past a Rotating Cylinder at Moderate Reynolds Numbers

The flow past a rotating circular cylinder, placed in a uniform stream, is investigated by means of 2D and 3D direct numerical simulations, using the finite-volume version of the code ICARE/IMFT. The flow transition is studied for Reynolds numbers from 40 to 500, and for rotation rates alpha (ratio of the angular and the free-stream velocities) up to 6. For a fixed Reynolds number, different flow patterns are observed as alpha increases: Von-Karman vortex shedding for low rotation rates, suppression of the vortex shedding at higher alpha, appearing of a second mode of instability for a high interval of alpha where only counter clockwise vortices are shedd, and steady state flow for very high rotation speeds where the rotation effects keep the vortex structure near the wall and inhibit detachment. Three dimensional computations are carried out showing that the secondary instability is attenuated under the rotation effect. The linear and non-linear growth of the 3D flow transition are quantified using the Ginzburg-Landau global oscillator model. The analysis of the coherent structures under the rotation effect is performed by the proper orthogonal decomposition, as well the pattern reconstruction using the first POD modes.