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ItemInvestigation of mixing in gravity currents using high-resolution molecular tagging techniques( 2020)Gravity currents are horizontal flows of fluid of a higher density into an ambient fluid of slightly lower density. They occur frequently in the atmosphere as sea-breeze fronts, thunderstorm outflows, katabatic flows etc., and are also encountered in industrial applications. The initial density difference between the two fluids can either be due to the presence of a salt or a temperature difference. While a majority of the studies employ a salinity based stratification, this work focuses on the flow dynamics of a gravity current generated as a result of an initial temperature difference. In the laboratory environment, a gravity current can be produced using a lock-exchange experiment in which the two fluids, initially at rest, are separated by a vertical barrier (or lock gate). At time $t$ = 0, a rapid removal of the lock gate results in the formation of a gravity current. The present gravity currents were produced in a Perspex tank of 2.0 m x 0.2 m x 0.2 m where the lock was located mid-way. The present flows were first visualized by mixing a dye in the heavier (cold) side to evaluate the bulk properties of the flow e.g. Froude number, $Fr$. Subsequently, simultaneous measurements of streamwise velocity and temperature field were conducted using the single-component molecular tagging velocimetry (1c-MTV) and molecular tagging thermometry (MTT) respectively. These experiments were focused at the interface between the hot and cold fluid to estimate the resultant mixing across the interface. The measurements were acquired using a 1024 x 1024 pixel Princeton Instruments PI: MAX4 camera and were shown to resolve the Kolmogorov (velocity) and Batchelor (scalar) length scales. To the author's knowledge, to date no previous experimental study has documented lock-exchange mixing at this level of resolution. The obtained density (temperature) distribution allows an estimation of the background potential energy of the flow which was used to quantify the diapycnal mixing. Specifically, mixing is attributed to the irreversible changes in fluid properties associated with fluid motions [1] and therefore differentiated from buoyancy induced reversible stirring. These measurements yield a mixing efficiency of 0.13 for the Reynolds number range considered ($Re \leq \mathcal{O}(10^4)$). Flow analysis revealed that the locally high values of mixing efficiency occur \textit{after} the occurrence of certain dissipative stirring events in the flow. These events, largely associated with vortical overturns, are commonly observed at the interface between the two fluids and are shown to lead the locally efficient mixing.