Impact of ship motion-induced single disturbance on gravity currents using particle image velocimetry and planar laser-induced fluorescence techniques

Saltwater intrusion in artificial canals is commonly caused by gravity currents; however, the influence of ship motion on gravity currents remains unclear. This study investigates the behavior of gravity currents influenced by ship motion through laboratory experiments, focusing on fluid dynamics and the mixing process between dense and light fluids under a single-ship passage disturbance. A lock-exchange setup with a ship-model control system was used, where ship velocity was linked to propeller rotation via the apparent advance coefficient. Particle image velocimetry and planar laser-induced fluorescence imaging were used to capture the synchronized data of the velocity and density fields. Three cases with different ship speeds were conducted: 2×, 3×, and 6× the current frontal velocities (uf). Three main stages of flow behavior were observed (approach, compression, and mixing), with the mixing stage further divided into three sub-phases: mixing enhancement, decay, and stratification. The findings revealed that higher ship speeds amplified density oscillations and sustained fluctuation periods, with the ship's wake generating significant turbulence and fluid mixing, particularly in the mixing stage. An exponential power-law decay model was applied to the turbulence intensity, which highlighted an increased stratification over time, ultimately reducing turbulent kinetic energy production. During mixing enhancement, the density change rate and turbulence intensity exhibited a linear relationship, which transitioned to a quadratic function in the decay phase, highlighting the dynamics between mixing and turbulence within the fluid. This study enhances our understanding of the effects of a single disturbance induced by ship motion on gravity currents.