Effects of Froude Scaling on Velocity-Induced Vortices in Physical Models

Velocity-induced vortices are common occurrences at hydraulic intakes and are detrimental to the operation and efficiency of hydraulic structures. Velocity-induced vortices also form in physical hydraulic models and although Froude scaling principles account for initial and gravity forces, the scale effects associated with vortices in Froude are less certain. This work examines scale effects on vortex formation in physical models through the use of four identical experiments built at differing Froude scales with the largest of the four being defined as a prototype. Each experiment created velocityinduced vortices at the intake to a sluice gate. The unique approach conditions caused surface swirl, downward velocity towards the sluice gate opening, and vortex flow. This study contributes to the existing literature by providing a set of both qualitative and quantitative observations made from images in multiple perspectives and measured data. It was found that vortex behavior scaled as expected using Froude scaling principles with relatively small length ratios (~1:2 to ~1:3). As model size decreased, scaling accuracy decreased due to low approach and vortex tangential velocities. For example, identical conditions in an experimental model at a scale 8 times smaller than the prototype experienced vortex formation of vastly decreased strength and size. Instead of an air core vortex, only a weak vortex having a shallow surface depression and swirl was formed at the same flow condition. Other model sizes showed a clear trend of decreasing vortex size and strength as the model size decreased. Decrease in strength is due largely to a decrease of approach and tangential velocity in the smaller models. Results from this study are also presented visually as a series of photographs and overlaid outline profiles for comparison. Additionally, other quantifiable results including dimensionless parameters are reported