Experimental Investigation on Hydrodynamic Forces of Semi-Submerged Cylinder in Combined Steady and Oscillatory Flow

Abstract

Abstract The hydrodynamic forces on the semi-submerged cylinder under the combination of steady and oscillatory flow are experimentally investigated. The semi-submerged cylinder is forced to move with different periods and amplitudes as towing in the still water to simulate the equivalent combined flow. Compared with the steady flow and oscillatory flow, the feasibility of Morison equation, the hydrodynamic coefficients including drag and added mass coefficients are respectively studied. The results show that the hydrodynamic forces on semi-submerged cylinder in these three types of flows under non-overtopping and overtopping have very different features. The Morison equation is further verified that it can well describe the hydrodynamic force on the semi-submerged cylinder in steady flow and oscillatory flow, while its limitation under combined steady and oscillatory flow when the overtopping occurs is clearly witnessed. The modified Morison equation by changing the of flow velocity squared term to the higher order index term is then proposed. The flow velocity indexes in the modified equation under the non-overtopping and overtopping conditions are respectively determined. The magnification of flow velocity indexes can be clearly found when the overtopping starts to happen. The maximum index can reach 4.4. Moreover, the distributions of drag and added mass coefficients with Froude number and KC number are also observed. This work can provide a useful reference for the design of related marine structures and more valuable guidance for future research.