estebandgj - The impact of a gravity current on a submerged structure

A sudden and unexpected propagation of a gravity current could constitute a very serious hazard to engineering installations mounted on or above the floor of oceans, lakes, or rivers. Some examples of these installations are pipelines, cables, dams, dikes, and screens. The design of these structures requires a proper estimation of the unsteady forces that could act upon them. While forces from currents and waves are usually taken into account, very little is known about the forces involved in a potential impact from a gravity current. Hence, we have begun a high-accuracy numerical investigation in order to gain an unprecedented level of insight into the mechanisms by which gravity currents interact with these submarine structures.

We consider the flow configuration shown below. Initially, two quiescent miscible fluids of different densities are separated by a vertical gate. The gate is then removed and, after an initial transient, a gravity current flow with constant velocity V is established. An obstacle is placed downstream of the gate. We start with the study of moderate Reynolds number gravity currents using two-dimensional direct numerical simulations.

Our objectives are to answer the following questions: How do the drag and lift on the obstacle vary with time? How is this variation affected by the different flow structures that develop? What are the physical mechanisms behind the formation of such structures and the drag and lift variation with time?

Schematic of the flow configuration.

Impact of a partial-depth (h/H=0.2) low Reynolds number (Re=2,000) gravity current on a square ridge (G/h=0, D/h=0.15). The concentration field is shown on the top frame, the vorticity field on the middle frame, and the drag and lift variation with time on the bottom frame. Instantaneous streamlines are superimposed on the vorticity field in the middle frame. Notice the formation of a recirculation region upstream of the ridge. This vortical structure is seen to have a noticeable effect on the forces variation with time. Click here to play movie

Impact of a partial-depth (h/H=0.2) moderate Reynolds number (Re=10,000) gravity current on a square ridge (G/h=0, D/h=0.15). The forces fluctuations are seen to be a result of the "impact of a train of vortices" on the obstacle. train_of_vortices_lowq.avi

Impact of a partial-depth (h/H=0.2) low Reynolds number (Re=2000) gravity current on a large square ridge (G/h=0, D/h=1.1). Notice the initial splash when the gravity current meets the obstacle. The height of this splash is an important quantity in the design of containment barriers and has not been studied in depth. The dense fluid flux blocked by the obstacle is predicted well by past theoretical work (rottman-85a, laneserff-95). splash_lowq.avi

Impact of a partial-depth (h/H=0.2) low Reynolds number (Re=2,000) gravity current on two identical square ridges (G/h=0, D/h=0.15). The drag forces on each of the obstacles is shown in the bottom frame. tandem_lowq.avi

Impact of a full-depth (h/H=1.0) moderate Reynolds number (Re=6,000) gravity current on a circular cylinder (G/h=0.15, D/h=0.15). circular_re6k_lowq.avi