Flow morphology in bottom-propagating gravity currents over immersed obstacles

The interaction of bottom-propagating gravity currents with immersed obstacles in the path of the lock-exchange configuration was numerically investigated based on large eddy simulations. The three-dimensional Navier–Stokes solver was quantitatively employed to resolve the flow structure of gravity currents and their dynamics during impact. The integral measure of analysis comprises the front condition, the energy budget, the turbulent mixing, and the force response. Some flow parameters involved in momentum and energy fluctuations are the fractional depth of volume release, relative density difference, and obstacle dimensions. A particular focus in this study was on the scale effect of obstacles (W/D, the aspect ratio of a cross-sectional obstacle with side length W to height D) that affect the propagation of gravity currents. Depending on integral measures of the simulation, the flow morphology could be demarcated for the condition W/D = 2 with different flow regimes in accordance with the reattachment of the current as the plunged front overflows and separates from the obstacle. For W/D > 2, as the current impinges on the obstacle, the plunged current front overtops and travels on the obstacle surface and consequently causes the entrainment without intense mixing to form a circulation zone at the downstream of the obstacle. Accordingly, the predicted drag forces acting on the downstream surface are reduced by ∼25% for W/D ≥ 4 comparing to the case of W/D = 0.5, which is beneficial to the structural stability of the barrier in practical aspect. Notably, the integrated analysis of gravity currents provides insights into physical mechanisms by identifying distinct propagation stages during transitions, including the impact stage, transient stage, and quasi-steady stage.