Experimental Study On Gravity Currents Down A Ramp In Linearly Stratified Environments

Gravity currents play an important role in many aspects of natural processes and engineering practice. However, there is few research major in hydrodynamics of gravity currents down a ramp in linearly stratified environments. This study presents a systematic lock-exchange experimental study to investigate the influence of linear ambient stratification and ramp slope on hydrodynamics of gravity currents. A high-speed camcorder and Particle Image Velocimetry (PIV) are applied to analyze the macro-and micro-structures of gravity currents.Lock-exchange gravity currents are unsteady flows. If the relative stratification parameter S>1, the current would separate from the ramp and then horizontally intrude into the ambient where the current is neutrally buoyant. This development process can be divided into three stages, namely, acceleration stage, deceleration stage and separation stage. It is observed that flow fields can divide into two layers both in the stratified and unstratified environments. The positive vortexes appearing at the upper boundary are mainly due to the Kelvin-Helmholtz instability and the negative vortexes in the lower boundary arose from the viscous forces in the boundary layer. The vorticity fields of the gravity currents are quantitatively investigated to show that the weaker stratified ambient can lead to a stronger vorticity field.Based on the thermal theory, a set of improved formulas is developed to determine the front velocity at the acceleration and deceleration stage by considering the ambient density variation at depth and the slope, the results show the improved formulas give a good description of the flow in linear ambient stratification. An improved formula taking into account of the ramp slope, ambient stratification and inflow buoyancy flux is also proposed to predict the separation depth where the horizontal intrusion happens. Our results indicate that in the slope angle between 6°≤0≤12°,the front location and velocity are not very sensitive to the variation of slope angle. The front velocity of the gravity current increases firstly then decreases for both stratified and unstratified environments in the same slope angle. Yet the density difference that drives the current decreases more quickly in stratified environments as the current descends the ramp, causing more intense deceleration of the front velocity, which indicates the suppression of ambient stratification on gravity current.Finally, the impact of an obstacle on the movement of gravity currents is preliminarily analyzed. The results show the maximum damping of the front velocity increases as the height of an obstacle increases in the stratified environment. Meanwhile, the separation depth under an obstacle is smaller than that in situation without obstacle. This study results can be applied to explain the phenomena such as salinity intrusion of estuary and submarine turbidity currents, etc.