The paper deals with an experimental investigation of impacts of non-rotating spherical bodies on a flat, solid surface in water. The aim of the investigation was, using the PIV (Particle Image Velocimetry), method to analyze the velocity fields around the falling sphere which falls in an oblique direction. The experiments showed that the wake that forms behind the sphere is asymmetrical and that after the impact it continues its morion to the bottom, only along one side of the sphere, though. This non-symmetrical velocity field results in additional forces which push the sphere to the opposite direction than is the direction of the sphere motion just after the impact. and Obsahuje seznam literatury
We consider the homogeneous time-dependent Oseen system in the whole space R 3 . The initial data is assumed to behave as O(|x| −1−ε ), and its gradient as O(|x| −3/2−ε ), when |x| tends to infinity, where ε is a fixed positive number. Then we show that the velocity u decays according to the equation |u(x, t)| = O(|x| −1 ), and its spatial gradient ∇xu decreases with the rate |x| −3/2 , for |x| tending to infinity, uniformly with respect to the time variable t. Since these decay rates are optimal even in the stationary case, they should also be the best possible in the evolutionary case considered in this article. We also treat the case ε = 0. Then the preceding decay rates of u remain valid, but they are no longer uniform with respect to t.