Flow Field near the Artificial Sand Ripples under Wave Action

• Main Authors: Wu, Chen-Tai, 吳禎泰
• Other Authors: Hsu Tai-Wen, Lu Chen-Mou
• Format: Others
• Language: zh-TW
• Published: 1996
• Online Access: http://ndltd.ncl.edu.tw/handle/87925541707484566299

碩士 === 國立成功大學 === 水利及海洋工程學系 === 84 === The flow field near the artificial sand ripples under wave action is inve-stigated in this paper. Concerning the beach erosion problems, it isessential to investigate the flow field above the sand ripples. Free-surface progressive gravity waves train propagation over a wavy bottom, an available perturbation expansion procedure has been developed to solve the potential flow to second order. Fourier Series is used to simulate the triangular and rectangular artifi-cial sand ripples. Fiber-optic Laser Doppler Velocimeter(FLDV) was employed to measure the velocity field on artificial sand ripples bed in a wave flume. The flow structures were revealed quantitatively through the flow visualizationtechnique. The predictions of the second order nonlinear inviscid model werecompared to the measured velocity field. Theoretical solution gets well agree-ment with the FLDV experiments in the free stream region but there is diffe-rent in the boundary layer. The streamlines of theoretical solution shows good agreement with the visualization pictures of triangular artificial sand ripples.Resonanceis arising in the field from the theoretical streamlines when the wave length of the wavy bottom is one-half the surface wavelength. The vortex structures were generated at both lee sides and it caused the turbulent inten-sity to rise at the same time. The vertical velocity which always be ignored in boundary layer flow plays an important roles here. From the strength of and Reynold shear stress and the development of vortex structures, the bottom turbulent boundary layer is suggested to be 2 to 3 times of the ripple height.