Browsing by Author "Lien, HC"

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Bend-flow simulation using 2D depth-averaged model
(1999-10-01)
Lien, HC
;
Hsieh, TY
;
Yang, JC
;
Yeh, KC
;
土木工程學系
The purpose of this paper is to present a 2D depth-averaged model for simulating and examining flow patterns in channel bends. In particular, this paper proposes a 2D depth-averaged model that takes into account the influence of the secondary flow phenomenon through the calculation of the dispersion stresses arisen from the integration of the products of the discrepancy between the mean and the true velocity distributions. The proposed model uses an orthogonal curvilinear coordinate system to efficiently and accurately simulate the flow field with irregular boundaries. As for the numerical solution procedure, the two-step split-operator approach consisting of the dispersion step and the propagation step with the staggered grid is used to numerically solve the flow governing equations. Two sets of experimental data from de Vriend and Koch and from Rozovskii were used to demonstrate the model's capabilities. The former data set was from a mildly curved channel, whereas the latter was from a sharply curved channel. The simulations considering the secondary flow effect agree well with the measured data. Furthermore, an examination of the dispersion stress terms shows that the dispersion stresses play a major role in the transverse convection of the momentum shifting from the inner bank to the outer bank for flows in both mild and sharp bends.
Numerical simulation of the pit migration
(1997)
Lien, HC
;
Yeh, KC
;
Yang, JC
;
土木工程學系
;
Department of Civil Engineering
The purpose of this paper is to simulate the migration of a pit in the channel bed. When water flows over a pit, the velocity at the upstream end of the pit slows down rapidly owing to the increment of the flow depth, and that at the downstream end of the pit speeds up because of the sudden contraction of the now area. Under such a flow condition, the equilibrium sediment transport formula is inadequate to simulate the local deposition and scouring around the pit. In addition, the critical bed-shear stress without considering the bed slope effect may increase the deposition rate near the upstream end and the scouring rate near the downstream end of the pit. This paper proposes a numerical model to simulate the migration of a pit by considering the nonequilibrium sediment transport behavior and the modified critical bed-shear stress for the movement of sediment on the large bed slope. The capability of the model was verified by comparing the simulated results with one of the experiments executed by Lee et at. (1993).
Simulation of bend flow by a depth-averaged two-dimensional model
(1996)
Lien, HC
;
Yang, JC
;
Yeh, KC
;
交大名義發表
;
土木工程學系
The purpose of this study is to develop a depth-averaged two-dimensional model to investigate the now pattern and the development of secondary-flow as flow passes through a bend. The depth-averaged model is developed on the basis of the orthogonal curvilinear coordinate system to account for the complex geometric boundary and the irregular channels. The influence of secondary-now is taken into account by adding a nearly triangular shape of velocity profile to the transverse mean velocity. The simulated results are compared with two sets of experimental data conducted by de Vriend & Koch (1977)(2) and Rozovskii (1961)(5) and the results from the moment model (Yeh & kennedy, 1993)(8) to show the capability of the newly-developed model. Simulation result shows that there is the consistent trend with the experimental data and the results from the Yeh's moment model.
Use of two-step split-operator approach for 2D shallow water flow computation
(1999-07-15)
Lien, HC
;
Hsieh, TY
;
Yang, JC
;
土木工程學系
;
Department of Civil Engineering
The objective of this paper is to present a methodology of using a two-step split-operator approach for solving the shallow water flow equations in terms of an orthogonal curvilinear co-ordinate system. This approach is in fact one kind of the so-called fractional step method that has been popularly used for computations of dynamic flow. By following that the momentum equations are decomposed into two portions, the computation procedure involves two steps. The first step (dispersion step) is to compute the provisional velocity in the momentum equation without the pressure gradient. The second step (propagation step) is to correct the provisional velocity by considering a divergence-free velocity field, including the effect of the pressure gradient. This newly proposed method, other than the conventional split-operator methods, such as the projection method, considers the effects of pressure gradient and bed friction in the second step. The advantage of this treatment is that it increases flexibility, efficiency and applicability of numerical simulation for various hydraulic problems. Four cases, including back-water flow, reverse flow, circular basin flow and unsteady flow, have been demonstrated to show the accuracy and practical application of the method. Copyright (C) 1999 John Wiley & Sons, Ltd.