Dept. of Aerodynamics c-cfd Warsaw University of Technology ESPCI

Second regime

2nd regime top view
Visualisation of the flow behind the sphere at Re = 150

In second regime the flow is still stationary but two counter rotating vortices characterise the wake behind the sphere. At Reynolds number close to critical the vortices start to oscillate (see figure below). As found during our studies the oscillations are in fact the beginning of third regime, and are very closely coupled to hairpins vortices.

Re 270 Re 272 Re 279
Evolution of the oscillation between 2nd and 3rd regime.
Re = 268 Re = 275 Re = 290

To found out whether both phenomena are connected to one instability the frequency of oscillations and hairpins shedding was measured using PIV and visualization.

frequency of oscillations
Oscillations frequency in function of Reynolds number (280 - hairpins appearance)