Kazeminezhad, M.H., 2013. Numerical simulation of oscillatory flow around submarine pipelines

Submarine pipelines, which often installed over seabed without any protection, are widely used to transport products from one location to another. In shallow water regions, the pipelines are affected severely by the wave-induced oscillatory motion. In this study a numerical model is presented to simulate the flow pattern around pipeline exposed to the oscillatory flow. The flow hydrodynamics is described via the solution of the Reynolds-Averaged-Navier-Stokes equations with a k-ϵ turbulence closure model. The numerical model is employed to describe the flow pattern, vortex shedding, gap flow prevailed below the pipe and hydrodynamic forces in oscillatory flow condition. Then, the flow velocity in the gap beneath pipe is discussed for different gap to diameter ratios in both unidirectional and oscillatory flow conditions. Results indicated that the k-ϵ turbulence closure model is capable of simulating the vortex shedding behind the pipe in oscillatory flow condition. It is also indicated that for the oscillatory flow case (KC=7, Re=9500), the maximum velocity of the gap flow increases with decreasing the gap between pipe and bed surfaces. By contrast, in the steady current case (Re=9500), the maximum velocity of the gap flow slightly decreases with decreasing the gap. In oscillatory flow conditions, a phase-lag between the gap flow and free-stream flow was found, especially for the small gap ratios. The relative maximum flow velocity between pipe and bed surfaces (u_{max gap}/u_m) decreases as the Keulegan-Carpenter number increases. According to the obtained results, the maximum velocity of the gap flow in oscillatory flow conditions is higher than that of the steady currents.