Hydrodynamic Forces on a Circular Cylinder Placed in Wave-Current Co-existing Fields

Abstract

The in-line hydrodynamic force acting on a circular cylinder in a wave-current co-existing field and its generating mechanism are discussed. This study focuses on the asymmetries of both the water particle movement and the resultant vortex property with respect to the cylinder as being the causes of an inherent generating mechanism of the hydrodynamic force in the wave-current co-existing field. First of all, the vortex property around a circular cylinder in the wave-adverse current co-existing field is examined by flow visualization tests. It is found that the vortex property depends on the flow characteristics around the trough phase when the wave-current composite velocity becomes maximum and can be represented with a newly proposed K.C. number for the co-existing field. Next, the characteristics of the in-line force are made clear by evaluating the drag coefficient and the mass coefficient in the expanded Morison's equation for the co-existing field. These coefficients can be well arranged by (K.C.)₂, which is one of the newly proposed K.C. numbers, and their characteristics coincide with the existing results in the wave-only field. The in-line hydrodynamic force in the co-existing field can be explained sufficiently by considering the vortex property in the same manner as clarified in the wave-only field.