Gravity Currents Flowing Over an Array of Obstacles

We experimentally investigate the density and velocity structure of a gravity current as it interacts with, and flows over, an array of closely spaced obstacles. Measurements of the fluid density and velocity are used to understand the transient response of the current as it interacts with the array of obstacles, with heights ranging from 18 - 50% of the fluid depth. As the current flows over the array, convective instabilities at the base of the current lead to mixing, in addition to that due to shear instabilities on the upper edge of the current. The additional mixing causes the front velocity of the current to reduce, as well as the density in the head of the current. Unlike when the obstacle array is sparsely packed, the current in maintains the head and tail structure of a smooth bed gravity current, despite the decreased density within the head. The velocity profiles are dependent on the obstacle height, which determines both the position of the current in relation to the obstacle array, and the strength of the return flow at the base of the tank. The experimental results provide a detailed understanding of the transient interaction of a gravity current with a closely spaced obstacle array, including the current structure, and the dominant mixing processes.