有限厚度剛性多孔介質底床之剪應力量測

The Measurement of Shear Stress in Rigid Permeable Seabed with Finite Thickness

林敬樺(Jing-Hua Lin) ； 陳陽益(Yang-Yih Chen) ； 陳冠宇(Guan-Yu Chen)

《海洋工程學刊》 12卷2期 (2012/12) Pp. 171-195

https://doi.org/10.6266/JCOE.2012.1202.03

摘要

本研究利用滑移係數（slip factor）推導出冪次函數型態之簡單公式用以評估波浪層流／紊流邊界層中任意流況之波浪摩擦係數（wave friction factor），並設計一剪力計（shear apparatus）實際量測波浪通過多孔介質底床（porous medium）之底床剪應力，以驗證本模式之正確性。在本研究的理論分析中，採用零方程式模組（zero-equation model）計算合理之渦旋黏滯係數（eddy viscosity）；理論模式顯示波浪摩擦係數與（A/Ks）^(-1)成正比關係，模式預估結果亦與前人之實驗結果相符，證實本理論可用於層流區之剪應力評估上；為進一步驗證本模式於粗糙-紊流區之適用性，本研究也利用實驗室試驗與自製之剪力計直接量測有限厚度之剛性多孔介質底床之紊流剪應力。試驗結果顯示在相同條件下，多孔介質區之波浪摩擦係數約較已知粗糙床面之量測值約小30%；理論預估值與本次試驗值相當接近，在小粗糙度時之波浪摩擦係數之預估也相當合理，本研究之優點在於波浪條件及底床參數均為已知之情況下，可快速計算出對應之波浪摩擦係數而可避免使用經由試驗資料所得之經驗回歸公式；這項特點使得本研究之成果適合用在實際海床底床剪應力之評估上。

關鍵字

滑移係數 ； 波浪摩擦係數 ； 剪力計 ； 紊流剪應力

並列摘要

This paper uses the slip factor to derive a simple formula with power form estimating the wave friction factor in the different flow regimes, and applies the shear apparatus to measure waves induced the bed shear stress in the finite thickness porous medium.In the present analysis, the zero-equation model is applied to determine a reasonable eddy viscosity. The theoretical model shows that the wave friction factor is to be in proportion to (A/ks)^(-1), this result also agrees with existing experimental measurement and demonstrates that present model can be used in estimating the shear stress of laminar flow. In order to verify the usability of present model in the rough-turbulent regime, the laboratory experiment and the shear apparatus are employed to directly measure the turbulence-bed shear stress in the rigid permeable seabed with finite thickness. Based on the measurement, the wave friction factor in the porous medium is approximately 30% as small as that in the impermeable-rough seabed. The theoretical wave friction factor is identical to the experimental measurement, and the prediction in the small relative bed roughness is also valid. The advantage of present model is that the wave friction factor can be conveniently determined as wave conditions and sediment parameters are known rather than use the specific regression formula concluded by the experiment. The characteristic is essential to apply in the estimation of bed shear stress in the real seabed.

並列關鍵字

slip factor ； wave friction factor ； shear apparatus ； turbulent shear stress