計算生態流體力學在三維度自由液面植栽帶流場之應用研究

Abstract

本研究之主要目的為模擬三維度自由液面水流流場受植栽帶的阻滯效應，以及對植栽帶流場作水理分析，並以水稻田為例，探討植栽帶對調蓄洪水與下游出口流量之影響。在研究中，採用有限體積法(finite volume method)離散控制方程式，以一階準確上風法離散對流項，二階準確中央差分法展開壓力項與黏滯擴散項，由PISO法耦合速度和壓力，使整個流場符合質量守恆及動量守恆，再由κ-ε紊流模式配合體積分率法處理三維度自由液面水流流場，最後再應用孔隙介質流理論模擬水田區植栽帶對水流的阻滯效應。 本研究透過與Nepf 及Vivoni (2000)的實驗值比對，模擬具植栽帶之自由液面明渠流的平均流速(mean velocity)、紊流方均根速度(turbulent rms velocity)及紊流黏滯性(turbulent eddy viscosity)，發現驗證結果相當吻合。而植物的密度因子會因為植栽種類的不同和種植密度的差異而改變，因此本研究亦施作一組實驗量測符合台灣地區水稻田種植條件的密度因子，再由實驗所得到的參數，輸入研究案例進行數值模擬。 研究案例分為單坵塊水田區和三坵塊水田區，結果顯示水田區在休耕時期可有效地提供調蓄洪水之功能，可調節逕流抵達渠道出水口時間約渠道放水時間之20%。此外植栽高度愈高則阻水效應愈明顯，30cm以上之水稻植栽可降低約10% ~ 26%之洪峰流量，延緩洪峰到達時間為休耕時期的21.4%，而洪峰過後的流量則較休耕時期為高，洪水在洪峰過後緩緩地流出水田區，使得流量歷線重新分配，因此水田區兼具調蓄洪水與調滯洪水的功能，調洪效果良好。

The main objective of this study is to investigate the 3-D free-surface flow with plant canopies from an ecological viewpoint, and analyze the flow field within plants so as to research how the plant canopy affect the outflow in downstream of the study field. We select the finite volume method to discretize the governing equations of our model. PISO scheme is used to adjust flow field to satisfy the continuity equation. The κ-ε model is adopted to simulate the 3-D turbulence flow field in the study, and apply the volume of fluid concept in modeling multiphase flow with water and air through porous media to procure the water-damping effect of the plant . The κ-ε turbulence model is firstly verified by the experimental measurement given by Nepf and Vivoni (2000). The numerical results are in good agreement with the experimental data for the mean velocity, turbulent root-mean-square velocity, and eddy viscosity. Since the canopy density is different for various plants and plant densities, a series of experiments of the canopy density in paddy field for different rice growth periods are carried out. The experimental results are further input into the CFD model. Our study includes two cases, one is single paddy field and the other is tri-paddy field. The simulated result shows that the field of no paddy has good flood detention function, and delays the time to peak over 20%. And the higher of the plant is, the well water-damping effect of the plant is. For the paddy field having the height of rice over 30cm, it reduces the flood flow rate about 10% ~26%, and holds the time to peak over 21% compared to the field without paddy. Furthermore, paddy field redistributes flow discharge distribution. Therefore the paddy field indeed provides flood mitigation and detention function.