Three-dimensional characteristics of the coherent structure behind a three-dimensional (hemisphere) or a two-dimensional (hemicylinder) obstacle on a flat plate were studied using flow visualization and image processing techniques. The three-dimensional time-averaged velocity field inside the separation zone behind a hemisphere and the turbulence structure of the vortices shed to the downstream were elucidated in detail. It is seen from the similarity and difference in the expects of the vortex shedding behind a hemicylinder and behind a hemisphere found experimentally, that a strong upward current occurs after the coalescence of shed vortices behind a three-dimensional obstacle and even behind a two-dimensional one. The difference in the generation mechanism of the upward current before and after the coalescence was explained in terms of the variation of the instantaneous Reynolds stress field or the fluctuation of the reattachment point.

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Theoretical study is made of the behaviour of a few spheres moving in low Reynolds number oscillatory flows under the assumptions that sphere Reynolds number is small and the distance between any two spheres is large compared with their radii. The method of reflection is applied once so that the effects of interaction between any spheres are incorporated. The resulting equations of motion is shown to be a nonlinear integro-differential equation since the velocity fields will be affected by the positions of spheres and the viscous term of Navier-Stokes equation represent a historical effect of spheres.
Two equal spheres oscillate obliquely relative to the flow direction and their orbit describes an ellipse when their center line has an angle of inclination to the flow. It must be emphasized that in case of the two spheres with a little different masses, their center of gravity moves with uniform average velocity. Furthermore, the numerical simulations of the simplified equations of motion of three spheres, which are obtained under the assumption that two equal larger spheres have no influences from a smaller sphere, will be presented. It will be remarked that a smaller sphere approaches to a larger one and then collides with it when the amplitude of a smaller sphere is small compared with the diameter of a larger one. Finally, it will be shown analytically that three equal spheres arranged with a cross-stream direction approach each other under oscillatory flows.

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