Author(s)

M. Greco, S. Malavasi & D. Mirauda

Abstract

This paper analyses the vibrations and the flow field around a tethered sphere in a free surface flow in order to study the influence of the vortex shedding process on the dynamic response of the obstacle and on its oscillation frequency. The system is characterized by a low value of the mass ratio (ratio between the mass of the system and the added mass), a low value of damping and a low value of relative submergence (ratio between the depth of the current and the diameter of the sphere). The experiments are set up to measure the movements of the sphere in the main and transversal directions of the fluid flow with laser sensors and image analysis, whereas the description of the main wake structures of the flow downstream of the sphere are provided using the PIV technique. Keywords: dynamic response, sphere, flow field, vortex structures, resonance phenomenon. 1 Introduction Studies on the analysis of the two-dimensional and three-dimensional vibrating structures in bounded and free surface flows, have highlighted the existence of a strong dependence of the maximum transverse amplitude on some dimensionless groups, as shown below: ( ) G S U A A *; * * max = (1) where A*max=y0/D is the ratio between the maximum transverse amplitude (y0) and the characteristic dimension of the body (D), U*=U0/fnD, called reduced velocity, is the ratio between the mean velocity of the flow (U0) and the product

Keywords

dynamic response, sphere, flow field, vortex structures, resonance phenomenon.