Wake Transitions of Rolling and Sliding Cylinders and Spheres

Flow is predicted around the generic 2d and 3d bluff bodies, the cylinder and the sphere, for different forward or reverse rotation rates on a plane wall, using a high-order spectral element method. Dye visualisation is undertaken to validate the predictions. The Reynolds number ranges are [20, 500] and [100, 350] for the cylinder and sphere, respectively. The combined effects of the nearby wall and the imposed body motion have been analysed with respect to the dominant transitions in the wakes. For the cylinder, the forward rolling motion is shown to destabilise the wake flow, while reverse rotation delays the onset of unsteady flow. For the sphere, four primary wake modes, plus an additional unsteady mode, are identified. Good qualitative agreement is observed between the numerical and experimental results. Predictions of the wakes of cylinders in tandem rolling on a wall are presented, noting the interference effects as the rotation rate and separation distance are varied