Variable time step (VTS) based PTV-LIF measurements of stratified jet transitions: from plunging flows and dense bottom jets to internal hydraulic jumps

Stratified flows driven by density differences exhibit diverse dynamic regimes, yet transitions between plunging flows, internal hydraulic jumps, and gravity plumes remain poorly characterized, particularly in confined geometries. This study addresses this gap by introducing a Variable Time Step (VTS) enhanced Particle Tracking Velocimetry-Laser Induced Fluorescence (PTV-LIF) methodology to investigate saline bottom jets in a narrow sloping channel. Isopropanol was employed as a refractive index matching agent to eliminate optical distortions, enabling simultaneous measurement of velocity and density fields using a single high-speed camera. The VTS method effectively reduced quantization errors inherent in traditional fixed-time-step algorithms, significantly improving measurement accuracy of flow and density fields. By systematically varying ambient water depths, clear transitions from plunging flows through submerged jumps and internal hydraulic jumps to gravity plumes were observed and classified for the first time. Results confirmed that flow regimes varied significantly with ambient fluid depth. Both subaerial and subaqueous hydraulic jump heights were well described by existing theoretical frameworks, though significant differences in jump lengths were observed, likely due to turbulent shear at the stratified interface. These findings extend existing regime maps and provide new experimental evidence elucidating the critical role of confinement and buoyancy in controlling stratified flow transitions, with important implications for understanding dense discharge behavior in submarine outfalls, sediment-laden river plumes, and ocean current dynamics.