The Use of LSPIV to Measure Large Streamwise Vortices

The impact of large streamwise vortices or macroturbulence upon suspended sediment transport and mobility of aquatic species is well recognized. In order to advance the understanding of large streamwise vortices, it is increasingly important to map the turbulent structures on a large scale domain such as for a total river reach. Large Scale Particle Image Velocimetry (LSPIV) serves as a promising experimental tool to map large streamwise vortices due to its extrusive nature and the ability to capture the dynamics of free-surface flows over large areas, potentially from airplanes, helicopters, and satellites. This work explored the ability of LSPIV to capture the characteristics of large streamwise vortices in a controlled experimental setting where a gravel-bed river was modeled. Visualization and statistical techniques were used to measure the largest coherent turbulent scales from LSPIV data obtained for unobstructed flow in a laboratory flume. Results revealed that LSPIV has the ability to capture free surface motion including sweeping and upwelling or ejections which indicate the upstream and downstream extent of the large streamwise vortices. Length scales and frequencies agree with published field and laboratory data for large streamwise vortices. The large streamwise vortices show quasi-stable behavior for the conditions tested.