Sand Transport Under Oscillatory Flows With Phase-Separated Particle Image Velocimetry

Standard Particle Image Velocimetry techniques severely limit measurements at the fluid-sediment interface and do not allow for the observation of separate phases in multi-phase flow (e.g. sand grains in water). We have implemented phase-separated Particle Image Velocimetry (PIV) by adding fluorescent tracer particles to the fluid in order to observe fluid flow and sediment transport simultaneously under oscillatory flows. Optical filters were installed on two high-speed cameras to capture only the light emitted by the fluorescent tracer particles used to determine fluid velocities. A third high-speed camera was used to capture the light scattered by the sediment allowing for sediment particle tracking. Together, these overlapping, simultaneously recorded images gave sediment particle and fluid velocities at high temporal and spatial resolution. A Nortek Vectrino Profiler was used in conjunction with the PIV system to continuously measure three-components of fluid velocity in a one-dimensional profile. Additionally, a Bed LAser Surface Tracking (BLAST) system was used to obtain time-dependent bed profiles to independently determine the onset of sediment motion. Measurements were made under a wide range of symmetric and asymmetric oscillatory flows over flat and rippled beds. The set of observations allow for the investigation of the relative importance of pressure gradients and shear stresses on incipient motion and other regimes of sediment transport. Previous work has shown that the importance of pressure gradients may be comparable to shear stresses in driving sediment motion under moderate wave forcing.