Velocity, vorticity and turbulence measurements in the surf zone.

Abstract

The coastal regions of the world play a significant role with regards to social and economic development, and with the ever increasing human activities along the coast, it has become necessary to understand and protect this vast resource. This requires a thorough understanding of the hydrodynamic processes that occur in the surf zone as a result of wave breaking. Laboratory investigations fonn an essential part of unravelling any physical process. It is the aim of this thesis, therefore, to investigate via laboratory measurements, the hydrodynamics of plunging and spilling waves in the surf-zone with the view to obtaining greater physical insight and also to obtain real data for model validation purposes. To achieve the above goal it was also necessary to develop new measuring techniques that were suitable for surf zone application. since conventional water level and fluid velocity measuring devices perfonn poorly in the presence of wave breaker related aeration, a common feature in the surf zone. The experimental investigation of processes occurring in the surf zone was carried out using a plane beach in a 20 m long wave flume. Both spilling and plunging waves are considered. The measurement of the spatial and temporal fluid velocities, water level and aeration area was accomplished using digital video imaging and analysis techniques in conjunction with particle image velocimetry. The diagnostic equipment. employed for the measurement of water levels, consists of a monochrome video camera connected to a video cassette recorder (VCR) and to a video frame grabber residing in a Pc. The video machine is used for mass storage of video data and easy replay of the experiment. The frame grabber is used to digitise and grab a sequence of video frames. The frame grabber can accept a video signal directly from the camera or from the VCR during playback. Fluid velocities were measured using a non-interlaced digital ceo camera connected direct to a frame grabber residing in a pentium Pc. Thus, images were stored direct to the hard disk. Water level measurements were performed with the aim of measuring wave heights as well as the mean water level set-down prior to wave breaking and the subsequent set-up in the surf zone. A time series of wave evolution was obtained by tracking the air-water interface along a vertical line of pixels at a fixed horizontal position in the video image. This method has the advantage of being non intrusive, and is considered more reliable than some of the more traditional techniques employed in the surf zone. Mean water levels were measured every 0.1 m along the measurement section of the flume, which is approximately 8 m in length. The spatial and temporal particle velocities were measured initially by tracking individual neutrally buoyant particles. This technique is referred to as particle image velocimetry (PIV) . More extensive measurements were later accomplished using digital cross correlation techniques, referred to as digital correlation image velocimetry (DCIV). A longitudinal section of the flume was illuminated with a sheet of light and the flume was seeded with neutrally buoyant particles. Velocities are then estimated by tracking individual seeds or a group of seeds in consecutive video frames. The velocity flow field was measured at the break point and in the surf zone. The average and turbulent velocities are estimated through phase ensemble-averaging. The undertow current and shoreward mass flux are derived from the flow field through averaging over the wave phase. A sensitivity analysis on this data is done to ensure statistical convergence. The measured velocity flow fields were analyzed and compared with linear wave theory and measurements made using Laser Doppler anemometry (LDA). The turbulent structures generated in the surf zone are investigated through the computation of the phase ensembleaveraged turbulence intensities and vorticity. The vorticity of the averaged flow field is computed and analyzed. The time averaged Reynolds stress and the turbulent kinetic energy is also computed. The Reynolds stress near the bed is shown to increase almost linearly with distance above the bed. Spectral analysis of the spatial velocities was used to obtain estimates of the scale lengths, and a comparison with the Kolmogorov frequency scaling law in the equilibrium region of the spectrum was also performed. The measurement of the aeration/roller geometry for spilling and plunging waves was accomplished using the video techniques used for water level measurements. The normalised wave roller area, in spilling waves, is shown to be nearly constant through the inner surfzone, increasing slightly in the middle of the surf-zone. The roller slope tends to show a decreasing trend in the surf zone.