- 期刊
波泥交互作用之波能衰減機制
Wave Attenuation Mechanisms under Wave-Mud Interaction
摘要
本文以實驗的方式來研究波浪通過泥質底床時，其間的交互做用與表面波衰減。一般而言，底泥的存在為此衰減主要機制。過去的研究雖有許多理論分析與數值模式探討此波浪衰減，然而實驗卻十分的稀少且僅有測量部分的參數。因此，波泥交互作用下的相關因子，如衰減係數、交界面的運動、泥層內質點速度、濃度分布與流變特性等，將在本研究中量測與分析。根據實驗結果，衰減係數主要與波高、波長與泥質黏滯度有關。此外，結果發現即使在深水波條件下，波浪衰減程度仍十分顯著。由於大部分的理論解析是根據線性波的假設，因此無法完整描述波浪與泥質的特性。當波浪尖銳度增加時，實驗與理論無法良好的吻合。本文提出新的分析方法來改善此缺失，我們根據因次分析得到的結果，並引入能量的觀點，得到新的波浪衰減參數。其能夠分離出我們所考慮的影響因子，並且比較出其重要性。接著，我們使用質點速度與流變特性來推估邊界層厚度，最大的衰減率約發生在泥層厚度為2.5到5倍的邊界層厚度。而在邊界層內的相位差可可超過1/6π，而泥水交界面處則無相位延遲的現象。最後，我們以總能量的觀點分析底泥所造成的衰減，並發現：(1)表面波能量主要受到泥層內剪力作功而消散(2)僅有部分的表面波能量被轉換成泥質的運動。
並列摘要
Propagation of water waves over a muddy bottom is experimentally examined in this study. In general, the muddy material plays a significant role in attenuating the wave energy. Though this attenuation has been calculated and observed by theoretical analyses, numerical studies and field observations, the related experiments are still few and only limited physical properties are measured. To this end, more characteristics of wave-mud interaction which include wave attenuation, amplitude of fluid-mud interface, particle velocities in the water column, concentration distribution and rheological properties are measured in our experiments.According to the experimental results, it is found that the damping coefficient depends mainly on the wave amplitude, wavelength and mud viscosity. It is also newly observed that the wave damping is measurable in the deep-water condition. Since most of analytical models based on the linear wave theory cannot describe the real characteristics of wave and mud, a new approach is adopted to improve the accuracy in comparison with experimental measurement. Herein we introduce a non-dimensional wave energy parameter into the original damping formula. This improvement can obviously increase the accuracy of the derived damping coefficient over that using only the linear theory. Next, the measured velocity distribution, concentration distribution and rheological properties were utilized to examine the boundary layer thickness. The maximum wave attenuation occurs when the thickness of the mud layer is about 2.5 to 5 times the thickness of the mud boundary layer. The measured phase shift of velocity inside the mud layer may exceed 1/6 π, while no phase shift is observed near the fluid-mud interface. Finally, total energy in the water column is analyzed. It shows that (1) most damping of energy in the water column is attributed to the effects of the strong shear stress in the mud, and (2) only a little energy is transferred from the surface wave to the induced fluid-mud interface.