Mean and fluctuating forces on a circular cylinder in cross-flow near a plane surface

An experimental investigation for the measurement of the mean and fluctuating forces acting on a circular cylinder in cross-flow, placed near a plane surface parallel to its axis, is described. Tests were carried out with three different types of boundary layer on the plane, having relative thickness delta/D from 0.1 to 1.1, at Reynolds numbers from 0.86 to 2.77 x 10^5, a range in which the flow around an isolated cylinder is characterized by a progressive disappearance of regular vortex shedding. The results showed that at subcritical Reynolds numbers periodic oscillating lift forces induced by regular vortex shedding are present at least down to relative distances from the plane, G/D, of 0.4, irrespective of the thickness of the boundary layer. The mean lift coefficient was found to rapidly decrease by increasing the gap size, whereas the mean drag coefficient showed non-monotonic trends with G/D, clearly dependent on the Reynolds number and significantly influenced by the thickness of the boundary layer on the plane.