Numerical analysis of the effect of oscillation and current on hydrodynamic coefficients of simple geometries

Abstract

During the launch operation of subsea equipment in the offshore industry, several factors are important for a safe and economical installation within an operating window. One of these factors is the hydrodynamic coefficients of the equipment to be installed. The coefficients can be obtained, experimentally or numerically, by applying an oscillatory motion that represents the effects of sea conditions and vessel motion, but these applications simplify conditions during a real operation and disregard important effects such as ocean currents. This paper investigates numerically the effect of current together with forced oscillation on the hydrodynamic coefficients of disks by analyzing two influence parameters, the Keulegan-Carpenter number (KC) and the reduced current velocity (Vr). A numerical model validation process was carried out in stages to overcome the lack of references on the subject. The model was compared with Direct Numerical Simulation (DNS) results for low Reynolds number (Re), up to 600, in qualitative and quantitative analysis, and then applied to broader conditions found in real operations, with KC up to 3, Vr up to 0.3, and for high Re, up to 240,000. On the model scale, higher Vr generates an increase in the drag coefficient (Cd) and a decrease in the additional mass coefficient (Ca). Under real scale conditions, the results are similar, with variations of Cd up to 300% and of Ca up to 36% for low KC. Analysis of the flow topology indicates that KC and Vr influenced vortex shedding and convection. Correlations for the hydrodynamic coefficients as functions of KC and Vr are proposed, with a R2 higher than 93%.