| Mass transport across a stratified interface is an essential aspect of many geophysical processes. Such transport can be performed by gravity currents. These currents could be generated by a density difference of only a few percent. In this experimental study, gravity currents are created either by a continuous supply of dense fluid or by the release of a finite volume of dense fluid along an inclined ramp. The surrounding fluid is composed of a two-layer stably stratified environment. A chemical reaction (phenolphthalein) technique and Planar Laser Induced Fluorescence (PLIF) are used to measure the entrainment rate of the gravity current. The change of volume in the head of the gravity current quantifies the entrainment and mixing. The results for both cases (constant flux and finite volume gravity currents) are similar for the mixing mechanisms and the entrainment/mixing rate, but the amount of entrained fluid is decreasing as the slope angle is decreasing. To further understand the entrainment process and to quantify the effect of the impingement on the internal structure of the gravity current, Particle Image Velocimetry (PIV) measurements are performed. Both instantaneous velocity and vorticity fields are quantified. Vertical profiles of velocity and vorticity are computed at two different locations, upstream and downstream of the impingement region. The vortical structure of the gravity current is clearly affected by the presence of the stratified interface. The level of restructuring depends on the Richardson number. The Richardson number is based on the gravity current size and velocity, and the density difference across the interface. At low Ri, both vorticity and velocity increase following the impingement, whereas at higher Ri, a decrease in vorticity and velocity is observed as well as a significant change in the head structure of the gravity current. The slope angle is varied to study the effect on entrainment and mixing; and on the internal vortical structure of an impinging gravity current. The effect of the stratified interface on the vorticity and velocity fields of the head is similar for both slopes angles tested, 3° and 6°. |
