Large-scale laboratory observations of turbulence and shear stress in the surf zone

• Main Authors: Hsi Chen, 陳羲
• Other Authors: Zhi-Cheng Huang
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/15532023941855346579

碩士 === 國立中央大學 === 水文與海洋科學研究所 === 103 === Large-scale laboratory observation of the temporal and vertical evolution of turbulent properties in the surf zone is presented. The experiments were conducted in a wave flume with a slope of 1:100. Monochromatic regular waves of three different incident wave conditions were generated. More than 16 test runs with the same initial and boundary conditions were repeated for each wave condition. Because of the effect from infra-gravity waves, the wave period and breaking location in a wave train is slightly different in the surf zone even the experiment was conducted using well-controlled facilities. A phase-corrected ensemble averaging method is used to separate the turbulent velocities. Fourier transform analysis is used to get the turbulent spectrum. The results are compared to the universal turbulent spectrum proposed by Kaimal et al.(1972) (KCs). The observed turbulent spectra under wave-breaking are compared to that of boundary shear layer turbulence. We found that are the trend of the two turbulence spectra are similar. This result confirms the energy cascade process of the wave-breaking turbulence is similar to that of boundary shear layer turbulence. Vertical profile of the Turbulent Kinetic Energy (TKE) density are examined. We found that the TKE decreases with the depth in the surf zone. The temporal variation of turbulent intensity as a function of wave phase is also presented. We found that the turbulent intensity increases and lasts for a while after the broken wave crest passes. In addition, analysis for the time series of turbulent shear stress and wave shear stress were performed. It is found that the wave shear stress is one order of magnitude larger than the turbulent shear stress in the experiment. The horizontal and vertical wave-induce velocities, and were considered to be orthogonal in most of previous studies and theories. However, they may not be orthogonal and the wave shear stress may become important due to the effects of bottom slope, bottom friction, and wave breaking. The time-average wave shear stress is close to zero near the bottom; the value is negative and the intensity increases with increasing distance from the bed. We found that most of are under the wave trough phases.