The structure of vortical flow over a rounded broad-crested weir

Abstract

Turbulent flow over a rounded broad-crested weir is investigated by means of detached eddy simulation (DES) and large eddy simulation (LES) with special emphasis on the interaction of coherent vortex structures with free-surface. In order to set up and validate the computational model, experimental studies were conducted in a laboratory flume using a moderately rounded broad-crested weir with a rounding ratio of R/P=0.15, where R is the radius of the upstream nose and P is the height of the weir. The simulated mean velocity, Reynolds stresses and free-surface profiles show good agreement with the experimental measurements. Spatial variation of the boundary layer on the crest is well captured using a dimensionless form of the Lamb vector divergence. Boundary layer shape factor calculated over the weir was found to be lie between 0.76 and 0.92. A horseshoe vortex system emanating from the bottom of the channel interacts with the free-surface at the entrance of the crest causing undulation on the free-surface. Unsteady characteristics of the flow are examined in terms of the power spectral density (PSD) of vortex-induced forces acting on the weir. It is found that a free-surface boundary layer develops from the undulation to the wall boundary layer on the crest. It was revealed from the simulations for various Reynolds numbers that the installation of an artificial pool upstream of the weir significantly modified vortex structures and reduced undulation effects by 86% according to a proposed undulation index.