A numerical study of intrusive compositional gravity currents

The formation, evolution, and structure of intrusive Boussinesq gravity currents propagating into a two-layer fluid with a sharp interface are investigated using highly resolved large eddy simulation at Reynolds numbers defined with the front velocity in the range of Re-f=3000-5000. Two cases are studied in which the density of the lock fluid is equal to the depth-weighted mean of the densities inside the two layers of ambient fluid. In the first one (case SC) the depths of the two layers of ambient fluid are equal. In the second one (case NSC) the depths of the two layers are different. The propagation speed in the slumping phase and the evolution of the currents are found to be close to both experiments and theory. Experimental observations showed the front continued to propagate for some time after the bore has caught the head region with the same constant velocity observed during the slumping phase. The present simulations show the reason is that unmixed lock fluid is still left inside the head region at the end of the slumping phase. This is in contrast to the case of gravity currents propagating over no-slip surfaces. It is also confirmed that even after the end of the slumping phase the local dissipation rate inside the region of unmixed fluid present inside the head remains very small compared to typical values observed in the interfacial layers behind the head. However, the size of the head and the region of unmixed fluid inside the head and dissipative-wake continue to decrease after the end of the slumping phase. The growth of three-dimensional (3D) instabilities at the front of the intrusion currents is observed to generate large-scale structures similar to the lobes observed in the case of gravity currents propagating over a no-slip surface. The dominant wavelength present in the initial growth stages of the 3D front instability in case SC is about half the one predicted by stability theory for a current propagating over a no-slip surface at the same Grashof number. The length scales of the front structures during the slumping phase are found to be smaller than the ones observed for currents propagating over no-slip surfaces at similar Grashof numbers. (c) 2007 American Institute of Physics.