Effect of scour depth on flow around circular cylinder in gravity current

• Effect of scour depth on the gravity flow past a cylinder is investigated.
• Various physical quantities with regard to scour effect are examined.
• Scour significantly affects hydrodynamic forces and flow features around a cylinder.

We investigated the effect of scour depth on the flow around a circular cylinder in a gravity current. In order to simulate the gravity current flow past a circular cylinder placed above a scour, we solved the incompressible Navier–Stokes and concentration transport equations based on the finite volume method. Vorticity fields, hydrodynamic forces, and pressure distributions on the cylinder and streamlines with regard to scour depth are examined to investigate the effect of scour depth on the flow over the cylinder. As the scour depth increases, the first maximum at the impact stage and mean drag during the quasi-steady state stage subsequently decrease. In particular, the first maximum drag at the impact stage is almost 2.5 times greater than the mean drag during the quasi-steady state stage, regardless of the scour depth. For a smaller scour depth, a root mean square (RMS) lift value of approximately zero reveals that no periodic vortex shedding occurs, indicating that the scour effect on vortex shedding is significant. However, as the scour depth increases, the RMS lift increases, resulting in an increase in the strength of the vortex shedding. For a larger scour depth, Kármán vortex shedding occurs near the cylinder. However, due to the existence of the scour, only negative vortices separated from the top side of the cylinder move farther downstream, resulting in a single vortex row on the smooth bed.