  Gravity currents modify their flow characteristics by entraining ambient
fluid, which depends on a variety of governing parameters such as the initial
density, $\Delta \rho$, the total initial height of the fluid, $H$, and the
slope of the terrain, $\alpha$, from where it is released. Depending on these
parameters, the gravity current may be designated as sub-critical, critical, or
super-critical. It is imperative to study the entrainment dynamics of a gravity
current in order to have a clear understanding of mixing transitions that
govern the flow physics, the shear layer thickness, $\delta_{u}$, and the
mixing layer thickness, $\delta_{\rho}$. Experiments were conducted in a
lock-exchange facility in which the dense fluid was separated from the ambient
lighter fluid using a gate. As the gate is released instantaneously, an energy
conserving gravity current is formed, for which the only governing parameter is
the Reynolds number defined as $Re=\frac{Uh}{\nu}$, where $U$ is the front
velocity of the gravity current, and $h$ is the height of the current. In our
study, the bulk Richardson number, $Ri_{b}$=$\frac{g^{'}H}{U_{b}^{2}}$=1, takes
a constant value for all the experiments, with $U_{b}$ being the bulk velocity
of the layer defined as $U_{b}$=$\sqrt{g^{'}H}$. Simultaneous Particle Image
Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) measurement
techniques are employed to get the velocity and density statistics. A
flux-based method is used to calculate the entrainment coefficient, E$_{F}$,
for a Reynolds number range of $Re\approx$400-13000 used in our experiments.
The result shows a mixing transition at $Re\approx$2700 that is attributed to
the flow transitioning from weak Holmboe waves to Kelvin-Helmholtz type
instabilities.
