Density-driven exchange flow propagating over an array of densified obstacles

Abstract

The evolution of bottom-propagating gravity currents with the presence of an array of densified obstacles submerged in a channel is investigated using large-eddy simulations. Our attention is particularly focused on the flow transition of gravity currents over rough surfaces with extra resistance that provokes significant dissipative processes. Two geometric parameters of the roughness elements, namely, the submergence ratio of the obstacle D/H and the gap-spacing ratio λ/D between obstacles, govern their kinematic and dynamic effects on the propagation of gravity currents. Physically, D/H plays a significant role in the control of the current diversion, and λ/D regulates the flow pathway of gravity current propagation. The integrated measures show that two distinct flow morphologies are identified. For a low submergence ratio (D/H<0.15), an overtopping flow is formed in which the gravity current travels on the top of the array and undergoes an inconspicuous loss of buoyancy, subject to minimal vertical convective instability interacting with the underlying ambient fluid within the gap regions. For a sufficiently high submergence ratio (D/H≥0.15) and a certain gap spacing (2≤λ/D<4), an overrunning flow is formed in which the current rapidly decelerates to a buoyancy–inertia state and then transitions to a drag-dominated state with a gain in excessive drag, in which the front velocity is proportional to t−0.5. However, the simulation results show a turning point toward an increase in the gap spacing as λ/D≥4, that the maximum drag acting on the gravity current is measured when it impinges on the second obstacle of an array, and that the drag coefficient goes up by 10%–40%, depending on D/H. The propagation of the gravity current does not show a higher sensitivity to the retarding effect instead. Meanwhile, the promotion of energy conversion occurs because of the gravity current encountering the continuous climbing and plunging flow behavior between two adjacent obstacles in regular motions.