January 3, 2007, 3:00 - 4:00 p.m., MSB 114
An Experimental Study of Surf and Swash Zone Hydrodynamics Using a Particle Image Velocimetry Technique and the Other Applications of Imaging-based Measurement Technique
Most beach erosion takes place within the surf and swash zones. About 70% of the beaches in the United States are being eroded. Despite the extensive studies in the surf zone in the past decades, the internal flow and turbulence structures within this region are still not well understood due to the difficulty in making field measurements in the highly aerated flow, particularly in the thin swash flow. Laboratory data on detailed internal flow and turbulence structures not only can validate numerical models of wave runup, but also provide spatially dense information that could be used to drive sediment transport models to estimate beach erosion rates. A particle image velocimetry (PIV) measurement technique is developed in the laboratory-modeled surf and swash zones. Fluorescent seeding particles and an optical filter are used to avoid the bubbles to be imaged. Unlike the commonly used laser Doppler velocimetry and acoustic Doppler velocimetry employed in the laboratory and field studies, the PIV technique used in this study provides full two-dimensional phase-resolved instantaneous flow fields. A detailed spatial spectral analysis is possible without invoking the Taylor frozen hypothesis. In addition, the turbulent kinetic energy production and dissipation rates can be evaluated directly with the instantaneous spatial gradients. The flow characteristics will be presented. Discussion will be focused on the interaction between the uprush bore front and drawdown flow as well as the flow reversal. The turbulence characteristics will be discussed using spatial spectral analysis. Finally, the turbulent kinetic energy budget will be presented.
The PIV technique used in this study can be applied to the study of flows in porous media, (e.g., surf and swash flows on sandy beach) and sediment transport. A preliminary study of coupling PIV and laser-induced fluorescence (PIV-LIF) technique to study a turbulent jet under a train of progressive waves (e.g., mixing of upwelled deep ocean water) will be presented.