Particle Image Velocity Measurements of Undular and Hydraulic Jumps

Measurements of the mean and turbulent flow fields in undular and hydraulic jumps have been acquired with single-camera particle image velocimetry (PIV). Three Froude numbers, ranging from 1.4 to 3.0, were studied, and in each case data were collected at numerous streamwise locations. The data from these streamwise locations were subsequently compiled into spatially dense (∼80,000 grid points) “mosaic” images encompassing both the supercritical and subcritical portions of the flow. The measured mean and turbulent velocity fields provide more detailed views inside undular and hydraulic jumps than were previously available from studies using pointwise measurement techniques. The two-dimensional spatial density of the measurements provides for the determination of gradient-based quantities such as vorticity. The potential for determining boundary shear stress from the velocity data is evaluated with several methodologies. The results are found to be consistent with recent measurements made using Preston tubes. Discussion of the technical aspects of and difficulties involved with applying PIV to hydraulic jumps is provided. These challenges included the identification and tracking of the free surface through image analysis and the scattering of laser light by entrained air bubbles in the roller region.