TORNADO VORTICES IN SETTLING TANKS

Settling tanks are hydraulically designed on a settling velocity. Observations have indicated that out-of-the-basin sediment transport may be initiated by comer vortices, which are able to scour a sediment layer and significantly reduce the efficiency of sediment deposition. Using four different materials - with densities from close to water up to sand, and diameters from 0.55 to 3.0 mm - the various forms of vortex scour were experimentally determined. The densimetric particle Froude number could be identified as the scaling quantity that governs these processes. The scour process can be reduced by inserting so-called antivortex elements, by which the 3D comer flow characteristics are influenced to result in a more 2D flow over a wall. The location and length of these elements were found to be similar to elements recently recommended for standard gates.