Drag and Inertia Coefficient Estimation for Wave-Current Numerical Simulations
Ivandito Herdayanditya, Ajie Brama Krishna Pribadi, Gael Verao Fernandez,
Evert Lataire, Pieter Rauwoens (Ghent University)
The Morison Equation is commonly used as a base for many numerical tools to assess the hydrodynamic force on a cylindrical structure subjected to an oscillatory flow. The force is split into a drag and an inertia component where two empirical coefficients are required: i) the drag coefficient, 𝐶D; and ii) the inertia coefficient, 𝐶M. The coefficients are typically obtained from laboratory experiments where either the cylinder is moved alternately, or the cylinder is hold steady and encountered by an oscillatory flow. It is known that the coefficients are dependent on the Reynolds Number and the Keulegan-Carpenter Number which are calculated from the oscillatory flow properties. Dependence of 𝐶D and 𝐶M on current speed is further investigated in this study by simulating a cylinder under a wave-current condition. It is found that the force on the cylinder, with the same relative oscillation kinematics and oscillation properties, cannot be reproduced accurately by Morison equation using the same 𝐶D and 𝐶M of the oscillation properties of wave only characteristics. Therefore, it is needed to estimate the coefficient values for wave-current flow condition, so the coefficient value is different between wave only and wave-current condition (e.g. Ghadirian, et al., 2021). In this study, the coefficients for a monopile in wave-current condition are derived with a Reynold-Averaged-Navier-Stokes (RANS) numerical simulation. Firstly, the wave-current generation in the RANS solver is validated against an experimental study in wave-current coplanar condition (Umeyama, 2011). Secondly, a verification of run-up with large cylinders with wave-current coplanar condition (Kim, 2022) is applied to show the wave-current-monopile simulation. Lastly, an initial study of finding the two coefficients in a wave-monopile combination are provided as a preliminary case for wave-current-monopile simulation.
References:
Ghadirian, A., Vested, M. H., Carstensen, S., Christiensen, E. D., & Bredmose, H. (2021). Wave-current interaction effects on waves and their loads on a vertical cylinder. Coastal Engineering, 165, 103832.
Kim, S. J., & Kim, M. (2022). The nonlinear wave and current effects on fixed and floating bodies by a three-dimensional fully-nonlinear numerical wave tank. Ocean Engineering, 245, 110458.
Umeyama, M. (2011). Coupled PIV and PTV Measurements of Particle Velocities and Trajectories for Surface Waves Following a Steady Current. Journal of Waterway, Port, Coastal, and Ocean Engineering, 137(2), 85–94. https://doi.org/10.1061/(asce)ww.1943-5460.0000067
