Browsing by Author "Hsieh, TY"

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A comprehensive unsteady depth-averaged model with orthogonal curvilinear coordinate system
(1998)
Hsieh, TY
;
Yang, JC
;
土木工程學系
;
Department of Civil Engineering
A mathematical model is developed here to solve unsteady depth-averaged shallow water flow equations. To enhance applicability, an orthogonal curvilinear coordinate system is used. The governing equations are solved by the two-step split-operator algorithm. Based on the finite-volume concept, all of the spatial derivatives are estimated with implicit central difference of second-order accuracy, while the convection terms are solved by a hybrid scheme. The effective stresses incorporates with a constant eddy-viscosity turbulence formulation to approximate the laminar and turbulent Reynolds stresses and with the secondary flow velocity profile proposed by de Vriend [1] to approximate the bend flow pattern. Two case studies including unsteady flow and bend flow are proposed herein to verify the model. Comparison with the experimental data shows that the model is efficient and robust. Finally, the model is applied to Tan-Shui River, a natural channel in Taiwan. The agreement between the computed results and field data is satisfactory.
An architecture for power-saving communications in a wireless mobile ad hoc network based on location information
(2004-10-04)
Tseng, YC
;
Hsieh, TY
;
資訊工程學系
;
Department of Computer Science
A mobile ad hoc network (MANET) is one consisting of a set of mobile hosts which can operate independently without infrastructure base stations. Power saving is a critical issue for MANET since most mobile hosts will be operated by battery power. In this paper, we propose an architecture for power-saving communications in an IEEE 802.11-based MANETs. Our solutions are derived by exploiting the location information of mobile hosts to achieve the goal of energy conservation. The architecture addresses the power-saving issue on several protocol layers, including network, medium access control (MAC), and physical layers. In comparison, existing protocols only exploit location information in limited layers. Similar to cellular networks, our approach is based on partitioning the network area into squares/hexagons called grids, thus leading to powerful energy and mobility management capabilities. A superframe architecture is proposed on the MAC part to support inter-grid and intra-grid communications. Simulation results are presented to demonstrate the strength of the proposed protocols. (C) 2004 Elsevier B.V. All rights reserved.
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.
Closure to "Investigation on the stability of two-dimensional depth-averaged models for bend-flow simulation" by T. Y. Hsieh and J. C. Yang
(2005-07-01)
Hsieh, TY
;
Yang, JC
;
土木工程學系
;
防災與水環境研究中心
;
Department of Civil Engineering
Fully power-aware and location-aware protocols for wireless multi-hop ad hoc networks
(2002)
Tseng, YC
;
Hsieh, TY
;
資訊工程學系
;
Department of Computer Science
A mobile ad hoc network (MANET) is one consisting of a set of mobile hosts which can operate independently without infrastructure base stations. Power saving is a critical issue for MANET since most mobile hosts will be operated by battery powers. In this paper, we address the power-saving issue for IEEE 802.11-based MANETs from several protocol layers, including physical, MAC, and network layers. Our solution is fully power-aware and location-aware in the sense that it exploits location information of mobile hosts to achieve energy conservation on all these protocol layers. In comparison, existing protocols only exploit location information in limited layers (e.g., power control is covered in [9, 18, 27], power mode management in [3, 15, 30], power-aware MAC in [2, 19, 29], and poweraware routing in [5, 20, 23]). Similar to cellular networks, our approach is based on partitioning the network area into squares/hexagons, thus leading to a powerful energy and mobility management capability.
Investigation on the suitability of two-dimensional depth-averaged models for bend-flow simulation
(2003-08-01)
Hsieh, TY
;
Yang, JC
;
土木工程學系
;
防災與水環境研究中心
;
Department of Civil Engineering
A numerical experiment is carried out to study the suitability of two-dimensional (2D) depth-averaged modeling for bendflow simulation, in which the geometry of the studied channel is rectangular. Two commonly used 21) depth-averaged models for bend-flow simulation are considered in this study of which the bend-flow model includes the dispersion stress terms by incorporating the assumption of secondary-current velocity profile, and the conventional model neglects the dispersion stress terms. The maximum relative discrepancy of the longitudinal velocity, obtained from the comparison of these two models, is used as a criterion to judge their applicability for bend-flow simulation. The analysis of simulation results indicated that the maximum relative difference in longitudinal velocity is mainly related to the relative strength of the secondary current and the relative length of the channel. Empirical relations between the maximum relative difference in the longitudinal velocity, the relative strength of the secondary current, and the relative length of the channel for both the channel-bend region and the straight region following the bend have been established. The proposed relations provide a guideline for model users to determine the proper approach to simulate the bend-flow problem by either using the conventional model or the bend-flow model. Experimental data have been adopted herein to demonstrate the applicability and to verify the accuracy of the proposed relations.
Numerical examination on the secondary-current effect for contaminant transport in curved channel
(2005)
Hsieh, TY
;
Yang, JC
;
土木工程學系
;
防災與水環境研究中心
;
Department of Civil Engineering
The purpose of this paper is to examine the effect of secondary current on contaminant transport in curved channel by using the 2D depth-averaged model. Two hypothetical cases are adopted. The results show that the secondary-current effect caused by flow is very limited and is reflected mainly from the circulatory transport term which is contained in the contaminant transport equation. A numerical experiment is carried out to judge the effect level of the secondary current. The results indicated that the maximum relative difference in concentration, MaxC*, obtained from the comparison of models with and without considering the secondary-current effect is mainly related to the relative strength of the secondary current SI and the relative length of the bend Theta b. Empirical relations including the classification on effect level of secondary current and the MaxC*-SI-theta(b) relation have been tentatively established. The former relation can serve as a guideline for model users to judge how important the secondary-current effect is and when the effect should be included in the model. The later relation can be regarded as an auxiliary relationship for the former to quantify the deviation between models with and without considering the secondary-current effect. The field data reported by Lau and Krishnappan [J. Hydraul. Div. ASCE 107 (1981) 209] and Holley and Abraham [J. Hydraul. Div. ASCE 99 (1973) 2313] are adopted herein to verify the applicability and the accuracy of the proposed relations.
Power-saving protocols for IEEE 802.11-based multi-hop ad hoc networks
(2003-10-22)
Tseng, YC
;
Hsu, CS
;
Hsieh, TY
;
資訊工程學系
;
Department of Computer Science
Power-saving is a critical issue for almost all kinds of portable devices. In this paper, we consider the design of power-saving protocols for mobile ad hoc networks (MANETs) that allow mobile hosts to switch to a low-power sleep mode. The MANETs being considered in this paper are characterized by unpredictable mobility, multi-hop communication, and no clock synchronization mechanism. In particular, the last characteristic would complicate the problem since a host has to predict when another host will wake up to receive packets. We propose three power management protocols, namely dominating-awake-interval, periodically-fully-awake-interval, and quorum-based protocols, which are directly applicable to IEEE 802.11-based MANETs. As far as we know, the power management problem for multi-hop MANETs has not been seriously addressed in the literature. Existing standards, such as IEEE 802.11, HIPERLAN, and bluetooth, all assume that the network is fully connected or there is a clock synchronization mechanism. Extensive simulation results are presented to verify the effectiveness of the proposed protocols. (C) 2003 Elsevier B.V. All rights reserved.
Power-saving protocols for IEEE 802.11-based multi-hop Ad Hoc networks
(2002)
Tseng, YC
;
Hsu, CS
;
Hsieh, TY
;
資訊工程學系
;
Department of Computer Science
Power-saving is a critical issue for almost all kinds of portable devices. In this paper, we consider the design of power-saving protocols for mobile ad hoe networks (MANETs) that allow mobile hosts to switch to a low-power sleep mode. The MANETs being considered in this paper are characterized by unpredictable mobility, multi-hop communication, and no clock synchronization mechanism. In particular, the last characteristic would complicate the problem since a host has to predict when another host will wake up to receive packets. We propose three power management protocols, namely dominating-awake-interval, periodically-fully-awake-interval, and quorum-based protocols, which are directly applicable to IEEE 802.11-based MANETs. As far as we know, the power management problem for multi-hop MANETs has not bun seriously addressed in the literature. Existing standards, such as IEEE 802.11, HIPERLAN, and bluetooth, all assume that the network is fully connected or there is a clock synchronization mechanism. Extensive simulation results are presented to verify the effectiveness of the proposed protocols.
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.