Simulation of Wave Transformation and Flow Fields in Water Waves Passing over Submerged-Breakwaters and Rippled Beds Using RANS

• Main Authors: Chih-Min Hsieh, 謝志敏
• Other Authors: Tai-Wen Hsu
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/69s945

博士 === 國立成功大學 === 水利及海洋工程學系碩博士班 === 92 === The purpose of the present study is to develop a numerical model suitable for investigating the entire vortex generation and dissipation processes as water waves pass over impermeable submerged-double-breakwaters and over the rigid sand ripples. The significant benefit of the present study over the traditional way of analyzing wave propagation problems is to apply the RANS (Reynolds Averaged Navier-Stokes) by taking account of the entire nonlinear, viscous and turbulent effects on the physical problem. The model is employed to simulate the flow kinematics and the turbulence effects in the RANS. The RANS is used to simulate the flow field; and the transport equations are discretized by the finite volume method, based on a staggered grid system with variable width and height. The unsteady term is treated by an explicit method. The pressure field is obtained by a predictor-corrector procedure. In order to update the free surface configuration with every time step, the Height Function (HF) method is implemented. The proposed model is used to study three different physical problems, which include a solitary wave passing over a submerged breakwater, waves propagating over submerged double breakwaters, and periodic waves passing over the artificial rippled beds. In the first case we successfully simulated the detailed interaction between a solitary wave and a submerged breakwater. In order to establish accuracy of the numerical model, simulated results were often compared with the existing experimental data of Lee et al. (1982) and Seabra-Santos et al. (1987). The simulated wave profile and the local velocity variations were found to be in very good agreement with those reported in the experiments. Following these verifications, we made a systematic study concerning the interaction of a solitary wave and a submerged breakwater. The temporal variation of the vortices was investigated in terms of the circulation. The induced drag forces and the turbulent energy budget acting on the solitary wave are also determined and discussed. In the second part of the thesis, the interactions of water waves and submerged-double-breakwaters were investigated. The simulated results for the incident wave profiles and the associated boundary layer flow behavior were compared with the available analytical solutions to verify the accuracy of the numerical scheme. The overall agreement between the simulated results and the existing laboratory measurements appeared quite satisfactory. By using the same numerical model, we conducted a series of additional numerical experiments with various incident wave conditions and with different spatial variation between the submerged breakwaters in order to study the generation and the evolution process of the vortices, their intensity, the temporal variation of the streamlines, and the turbulent kinetic energy. To better understand the nonlinear effects following the wave propagation over the submerged double breakwaters, we make a detailed investigation concerning the wave deformation process and the change in harmonic wave amplitude. Computed results demonstrate the detail flow separation mechanism both near the upstream and the downstream edges of the submerged breakwater. It is found that the clockwise vortices are produced at the down-wave edge of an obstacle when the wave crest passes over. The vortex is seen to dissipate and diminish as the wave trough arrives at the obstacle. Conversely, a counterclockwise vortex occurs at the up-wave edge of the obstacle when the wave trough passes and dissipates as the wave crest is above the obstacle. The vortex pair is observed to shed and form again at the lee side of the obstacle. The generation of vorticity with respect to the free surface has been investigation in this study. The nonlinear interaction among the incident wave components generates higher-order harmonics. Over the breakwater the magnitude of the higher-order component is found to increase. This is mainly due to the wave energy transfer from the fundamental harmonic to the higher-order harmonics. Trajectories of the fluid particles with initial location close to the structure were determined in order to provide an understanding of the possible sediment transport around submerged breakwaters. It has been found that the present model is quite efficient in accurately simulating the flow separation and the wave deformation process for the water waves propagating over impermeable submerged-double breakwaters. Finally, the numerical model is used to study the influence of periodic wave propagation over artificial rippled beds. The numerical results concerning the wave amplitudes and the impermeable rippled bed agree very well with the analytical findings of Mase et al. (1995). We observed that the transmitted waves become small due to the presence of the rippled beds. Necessary simulations are also conducted to investigate the effects of the Bragg scattering by ripples. In case of resonance, spatial distribution of wave amplitudes is observed to attain its pick value while approaching the upstream part of an wavy ripple bed and it became minimum at the downstream side of the ripple bed. On the other hand, the local kinetic energy is found to attain its minimum value at a place where potential energy became maximum. The investigations were extended to cover flow features both with and without Bragg resonance.