Experimental Study of Coherent Structures in Breaking Waves

South Dakota State University, Brookings SD

Investigators

Abstract

It is proposed to study coherent structures in breaking waves in the surf zone. The primary objective is to measure the flow properties of the intermittent, coherent turbulence produced by wave breaking on a plane beach. A wide range of breaking wave conditions including solitary waves, and regular and irregular waves will be studied. Laboratory experiments will be carried out in a two-dimensional wave flume at South Dakota State University (SDSU). The flume has Plexiglas sidewalls and bottom extending its full length and it can be tilted from horizontal to 3% slope. In the first part of this project, a solitary wave will be generated by a programmable wave generator installed at one end and caused to break at a predetermined location along the flume. Wave elevations across the surf zone will be measured to define regions of interest for detailed velocity measurements. At three separated locations corresponding to the outer, middle and inner surf zone, instantaneous three-dimensional velocity fields in the horizontal longitudinal, vertical longitudinal and vertical transverse planes in the breaking wave flow field will be measured using a stereoscopic particle image velocimetry (PIV) system. The measured fluid velocity fields will be decomposed into ensemble average and coherent and incoherent turbulence components by conditional averaging. The results will be used to understand the evolution of the large eddies and their kinematical and dynamical flow properties. One spilling breaker and two plunging breakers will be studied. The experimental study on solitary waves would allow us to understand the essential physics behind the evolution of large eddies under breaking waves without the influence of wave-induced currents and residual turbulence. In the second part of this project, coherent structures under regular and irregular waves will be investigated. PIV measurements of instantaneous fluid velocity fields will be conducted inside the surf zone for spilling and plunging regular waves, irregular waves with predominantly spilling breakers, and irregular waves with predominantly plunging breakers. The experimental investigation will focus on the near-bed region to determine the effects of wave-induced currents, residual turbulence and irregular wave breaking on the occurrence frequency, timing, and properties of coherent structures impinging on the bed from the core of the flow field. The impact of the proposed research stems from the importance of wave breaking in many coastal processes such as beach erosion, sediment transport, coastal inundation, and wave impingement and impact on shore-sited structures. In the surf zone, the wave breaking process has been and remains an area in which our ability to predict the physical phenomena is poor. Improved understanding of the large eddies in breaking waves is relevant to sediment suspension and would feed back to improved predictive models for surf zone hydrodynamics and sediment transport. The proposed research would also provide a high quality data set to test and improve detailed models of surf zone wave breaking process, including both present day models and models that are under long-range development for use in the future.