Mixing in gravity currents over an array of cylindrical obstacles

Gravity currents, driven by density variations caused by gradients in temperature, salinity, or sediment concentration, arise due to hydrostatic imbalances between adjacent fluids. These flows play a pivotal role in a wide range of geophysical and engineering applications, shaping atmospheric, terrestrial, and subaqueous environments. In natural settings, the propagation of gravity currents often encounters uneven topographies, where the dynamics of the dense flow are significantly influenced by topographic features. Recent research has increasingly focused on understanding gravity currents moving through channels obstructed by finite-size patches of obstacles, which adds complexity to their behavior and mixing processes. This experimental study investigates the interaction mechanisms between gravity currents and such obstructions, providing insights into their dynamics and mixing implications through a non-intrusive image analysis technique based on light reflection to evaluate instantaneous density fields.

Laboratory experiments were conducted in a Perspex tank with dimensions of 3 m in length, 0.3 m in height, and 0.2 m in width. An array of rigid plastic cylinders, each with a diameter of 2.5 cm, was placed at the bottom of the tank spanning its entire width. The gravity current was reproduced using the lock-release technique with a density difference ∆ρ=6 kg/m³. A total of 15 full-depth lock-exchange experiments were performed to analyze the submergence ratio, i.e. the ratio between the initial current depth and the obstacle height, and the gap-spacing ratio, i.e. the ratio between the spacing of the bottom obstacles and the obstacle height.

The analysis of instantaneous density fields provides valuable insights into the complex dynamics of gravity currents. During the initial slumping phase, the front of the dense current advances at a constant velocity. However, upon reaching the obstacles, the gravity current slows down, leading to the emergence of distinct flow regimes. High-resolution density measurements reveal that the submergence ratio plays a critical role in controlling current diversion, while obstacle spacing governs the flow pathway. An increase in the submergence ratio enhances the interactions between the current and the roughness elements, resulting in marked fluctuations in potential energy and mixing intensity that significantly affect the current evolution. Although bottom roughness generally reduces the front velocity and alters entrainment behavior, the effect of obstacle spacing is less important, particularly for low submergence ratio. For large submergence ratio, the current exhibits a shift in mixing dynamics, deviating from the near-linear growth of background potential energy observed in smoother cases.