Hydrodynamic characteristics of internal waves induced by a heaving body in a two-layer fluid

Abstract

The stratification of fluid caused by the change of water densities can generate two water waves on each fluid boundary. One is the surface wave on the free surface, and the other is the internal wave on the interface between the fluid layers. Two wave modes exist in the two-layer fluid: barotropic mode and baroclinic mode. This study develops a numerical model to simulate the surface and internal waves in the time domain. The time histories of the water waves on both surface boundaries are simulated using the numerical wave tank (NWT) technique by an oscillating circular body located on the free surface or underwater in the lower fluid domain. The NWT technique is based on the boundary element method with properly defined boundary conditions, including interface boundaries. The leapfrog method is used for the time integration of the water surface boundary conditions. The time-varying generated waves are disassembled through the Fast Fourier Transform (FFT). Each wave component is compared with analytic solutions. The relative magnitudes of the generated waves on both fluid layers are examined according to each wave mode. The dominant wave mode varies according to the oscillating body frequencies. The amplitude ratios of the two waves are compared for various density ratios and fluid depths.