Jets and Waves Generated by an Obstacle in Stratified or Homogeneous Fluids

Jets and waves, whose origin is in gravity force, are often observed in fluids. When the fluid has a vertical density stratification, both can be generated due to the buoyancy force. The jet appears when an obstacle descends vertically in a stratified fluid. The generation is supported by the molecular diffusion of the stratifying scalar such as salt. If there is no scalar/density diffusion, the density must be conserved along the fluid motion, and the originally horizontal isopycnal surfaces are simply deformed as they are dragged down by the obstacle, so that the dragged fluid will move downward indefinitely and will never go back to its original height. If the diffusion exists, the fluid can change its original density, and move away from the isopycnal surface to which the fluid originally belonged. In this study, we demonstrate the generation process of the jet, along with its interaction with the internal gravity waves. As another example of waves in fluids, water waves with capillary effects in a homogeneous fluid are numerically simulated, showing the excitation of short capillary waves by a long solitary wave. The phenomenon has been predicted by a weakly nonlinear theory, but has not yet been observed.