Detached Eddy Simulation of the Nonaerated Skimming Flow over a Stepped Spillway

Abstract

The unsteady three-dimensional (3D), nonaerated, skimming flow over a stepped spillway is investigated by means of a detached eddy simulation (DES). Special emphasis is placed on the description of the instantaneous velocity and vorticity fields on the step cavities, as well as on the 3D, long-term, average flow structure at a Reynolds number of approximately 105. Mean-flow velocities obtained from simulations on both relatively-coarse and fine meshes at different locations in the spillway are in very good agreement with particle image velocimetry (PIV) data published elsewhere. Nondimensional profiles of turbulence intensity and Reynolds stresses are also close to data, although numerical values are larger than the experimental counterparts. Snapshots of velocity vectors reveal that the flow near the pseudo-bottom is continuously flapping, and that it intermittently interacts with ejections of positive vorticity emanating from the cavities. As a result of the interaction between the shear flow and the 3D cavity flow, patches of positive and negative vorticity are carried away downstream, eventually interacting with the free surface. It is also found that the ratios of redistribution among components (terms) of the turbulent kinetic energy (TKE) at the step edges are very similar to those universal values for the intermediate region of open-channel flows. It is additionally shown that large-scale structures extracted from isosurfaces of the Q-criterion correspond primarily to well-organized, randomly-distributed thin tubes of vorticity. The instantaneous flow field revealed in this paper is considered crucial for modeling the even more complex aerated region.