Global mode of a sphere turbulent wake controlled by a small sphere

A small control sphere is displaced in a cross flow plane downstream of a main sphere of diameter D in a uniform flow at Re=33 000. The wake is studied by means of four fixed hot wire probes. It is shown that without the control sphere (reference case), vortex loops are shed from only one side of the sphere at a Strouhal number St=0.19. This reference wake structure has a planar symmetry defining an azimuthal phase θWθW that is observed to be random in time. The secondary smaller sphere is treated as a local disturbance of the reference wake. It is found that the shedding occurs closer to the symmetry axis for the control sphere placed at the center of the wake than for the natural case. When the control sphere is off-centered, a subharmonic at half the natural frequency appears and the azimuthal phase becomes imposed by the control sphere position. A pure subharmonic mode is observed when the control sphere reaches the separated shear, suggesting alternative vortex loops shedding from both sides of the wake.