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標題: 土石流流槽試驗數值模擬及流變特性參數研究
Numerical Modeling of Flume Experiment and Parametric Studies on Rheological Characteristics of Debris Flow
作者: 郭建岐
Kau, Chen-Chi
關鍵字: FLO-2D
flume experiment
rheological coefficients
mud slurry
mud-gravel mixture
sediment concentration
yield stress
出版社: 水土保持學系
摘要: A series of numerical simulation of flume experiment was performed to investigate the behaviors of flow motion of debris flow. Two dimensional finite difference program FLO-2D was adopted as a numerical tool for the calculation. Incorporating with non-Newtonian fluid rheological constitutive relations, the numerical model employs central finite difference method to solve the governing equations. The rheological relations define the shear stress of sediment in terms of yield stress, viscosity and turbulent-dispersive stress. Meanwhile, the viscosity and turbulent-dispersive stress are function of shear strain rate or velocity gradient. Initially, the numerical model was established using scaling factor to magnify the scale of geometry of flume channel. Subsequently the numerical simulation of flume experiment of debris flow was implemented using the inputs of hydraulic parameters (discharge rate and Manning coefficient), material parameters (specific gravity, volumetric concentration of sediment and rheological coefficients) and controlling parameters (elapsed time of simulation and type of governing equation). The numerical results of flow motion and deposition characteristics were then compared with those from the experimental measurements and empirical calculations. The first part of the study emphasized on the comparisons of flow motion and deposition characteristics of mud slurry and mud-gravel mixture through a systematically numerical investigation of flume experiment. It was found that the mud slurry has higher potential of lateral dispersion and better capability of water conservation whereas the gravel-mud mixture possesses the characteristics of dewatering, deceleration and high invasion potential. Because of the dewatering and deceleration effects of gravel-mud mixture, an intermittent deposition frequently occurs at the deposition zone. At the meantime, the impact pressure induced by the collision of grain particle in sediment increases due to the addition of gravel material. From the viewpoint of hazard prevention, debris flow with gravel-mud mixture type may possess higher potential of catastrophic disaster. The second part of the study concentrated on the flow depth, velocity field and impact during the flow motion. The numerical result of sediment concentration was compared with those from previous studies for further verification. According to the numerical results, except the simulation of deposition thickness, the numerical calculations of deposition length, area and volume are reasonably in agreement with the experimental measurements and empirical estimations.
本研究採用FLO-2D二維有限差分程式作為數值運算工具,進行一系列之流槽試驗數值模擬,以探討土石流之運動行為。該程式採用非牛頓流體流變模式與中央有限差分法求解控制方程式。流變關係中定義土石流沉積物承受之剪應力為為降伏剪應力、黏滯力及紊流-離散應力函數,其中黏滯力及紊流-離散應力可再表示為剪應變率及速度梯度之函數。首先,利用比例因子(Scaling factor)來建立流槽幾何數值模式,其次再輸入水文參數(流量、曼寧係數)、材料參數(比重、體積濃度及流變係數)及控制參數(模擬時間、控制方程式),進行土石流流槽試驗數值模擬。土石流運動與堆積特性之模擬結果可與實際觀測值及經驗推估值進行比較。 本研究第一部份針對泥漿體與礫石-泥漿混合體二種試料進行一系列流槽試驗數值模擬。由模擬結果發現泥漿體試料之橫向擴散能力及保水性較佳。另外,礫石-泥漿混合體試料則具有脫水、減速及直進性强之特性。因此,在堆積區常發生間歇性堆積。同時礫石-泥漿混合體試料由於礫石的加入使體積濃度增加,造成顆粒頻繁碰撞並誘發相當撞擊壓力。因此,以土石流防治的觀點來看,礫石-泥漿混合型之土石流,將具有較高之潛在危害度。 第二部份研究著重在土石流流深及流場之模擬。除此之外,土石流體積濃度之數值分析結果也與過去研究成果作比較,以便獲得近一步驗證。根據數值分析結果,除堆積厚度有較大之差異外,堆積長度、面積及體積之計算值皆與試驗實際觀測值及經驗推估值相近。
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