Hydrodynamic impact of internal solitary waves on a cylinder close to free surface

Internal solitary waves (ISWs) can induce strong and rapidly varying currents near the ocean surface. These currents are not easily detected and can harm surface platforms and offshore structures. In this work, we perform direct numerical simulations (DNS) using the volume of fluid method to understand the effect of transient flow induced by an ISW on a submerged cylinder close to the free surface. Simulations are first performed for steady inlet flow using a constant inlet velocity for various submergence depths and Froude numbers to identify the effect on flow characteristics and hydrodynamics. The time-varying surface current associated with the ISW is computed by solving the Dubreil-Jacotin-Long (DJL) equation, and the effect of the background density profile on the surface current is analyzed. In DNS of flow over the submerged cylinder, the surface current is adapted as a transient inlet flow to the computational domain. From the DNS results, a hysteresis effect in the wake velocity is observed, manifesting two distinct flow states corresponding to the same transient inlet velocity induced by the ISW. The highly transient velocity in the cylinder wake is decomposed using empirical mode decomposition (EMD) into intrinsic mode functions (IMF). Subsequently, Hilbert transform is performed on these IMFs to obtain the local frequency, which shows delays in vortex shedding. Our results also show a notable difference in hydrodynamic forces between transient inlet cases and their corresponding steady inlet flow cases.