Exploratory Study of Oscillatory Flow over a Movable Sediment Bed with Particle-Image-Velocimetry (PIV)

Sand ripples develop spontaneously in an oscillatory flow over a movable sediment bed. The mechanics of sediment suspension in the presence of ripples is complex and its direct observation is complicated unless a technique can be used to capture the flow field and particle motion above and around the ripples. This work presents the results of an exploratory study conducted in a U-tube. The goal of the study was to use Particle-Image-Velocimetry (PIV) to capture the interaction between turbulent flow structures, suspended particles, and ripples and other bed instabilities. The PIV technique was chosen because of its potential to simultaneously determine the fluid velocity and the solid-particle velocity fields. An oscillating piston with a maximum stroke of about 23 cm was used to generate ripples in the test section of the U-tube. The test section has a length of 149 cm, a width of 21 cm and a height of 23 cm. The bed is 30 cm deep and is made up of sand with a mean diameter of 0.58 mm. The period of oscillation used in the experiments was 2.5 s. In order to differentiate between the solid and fluid phases, fluorescent particles were used as tracers. For the PIV measurements, a one million pixel digital video camera with a frame rate of 30 Hz was fitted with a filter to isolate the fluorescent seeding particles. Exploratory measurements indicate that the mechanisms responsible for the resuspension of bed particles in the presence of ripples are more complicated than anticipated. As water flows past each ripple, separation takes place at the crest of the ripple. Vortical structures generated downstream of the crest entrain sediment particles into the flow. When the flow reverses direction, each structure, along with the sediment it has suspended, is carried away from the ripples and into the outer flow field. As the structure dissipates, the sediment suspended in it is released. The PIV techniques implemented in this study provide information about velocity, vorticity, and turbulence distributions, helping to explain this complex flow field.