Browsing by Author "Chang, HK"

Now showing 1 - 13 of 13

A fall-velocity equation - Discussion
(2001-07-01)
Chang, HK
;
Liou, JC
;
土木工程學系
;
Department of Civil Engineering
A new quorum-based replica control protocol
(1997)
Wu, YT
;
Chang, YJ
;
Yuan, SM
;
Chang, HK
;
資訊工程學系
Data replication if an important issue in distributed systems. Many protocols are designed to achieve high availability, but some of them have restrictions on N, the number of nodes in the system. This motivates us to design a protocol suitable for arbitrary N. In this paper, we present a new quorum-based replica control protocol whose quorum size is O(root N), which is the same as that of the grid protocol. Moreover; the proposed protocol is almost symmetric, i.e., each node nearly bears the same responsibility. In particular, our protocol performs well in systems where read operations are requested more frequently than write ones.
A three-point method for separating incident and reflected waves over a sloping bed
(2002)
Chang, HK
;
土木工程學系
;
Department of Civil Engineering
This study presents a three-point method for separating incident and reflected waves to explain normally incident waves' propagating over a sloping bed. linear wave shoaling is used to determine changes in wave amplitude and phase in response to variations of bathymetry. The wave reflection coefficient and incident amplitude are estimated from wave heights measured at three fixed wave gauges with unequal spacing. Sensitivity analysis demonstrates that the proposed method can predict the reflection and amplitude of waves over a sloping bed more accurately than the two-point method.
A turbulence model of wave-induced sheet flow
(2003-12-01)
Hsu, TW
;
Chang, HK
;
Hsieh, CM
;
土木工程學系
;
Department of Civil Engineering
This paper present a turbulence model that describes the sediment concentration and turbulent boundary layer of a wave-induced sheet flow. The simultaneous interactions between oscillatory, turbulent flow, and suspended sediment are included. The model specifies the equivalent roughness as a function of the Shields parameter, which is variable with time and is more physically realistic over a mobile flat bed. A wave friction factor, which differs from the conventional rough wall law, is obtained from the velocity gradient. A comparison of the turbulent boundary layer and sediment concentration for different equivalent roughness is given. The present model is shown to provide more accurate estimates than those using an equivalent roughness. Numerical results also show that the presence of suspended sediment retards mean velocity profile and decrease corresponding turbulence dynamic quantities.
A two-point method for estimating wave reflection over a sloping beach
(2003-10-01)
Chang, HK
;
Hsu, TW
;
土木工程學系
;
Department of Civil Engineering
This work presents a frequency-domain method for estimating incident and reflected waves when normally incident waves' propagating over a sloping beach in a wave flume is considered. Linear wave shoaling is applied to determine changes of the wave amplitude and phase due to variations of the bathymetry. The wave reflection coefficient is estimated using wave heights measured at two fixed wave gauges with a distance. The present model demonstrates a high capacity of estimating reflection and shoaling coefficients from synthetic wave-amplitude data. Sensitivity tests for the present model due to measurement errors of wave amplitudes and distance of two probes can more accurately predict the reflection coefficients. The measurement error of wave amplitude affects more significantly than measurement error of distance of two probes on calculating reflection coefficient of waves over a sloping bed. (C) 2003 Elsevier Science Ltd. All rights reserved.
An explicit approximation to the wavelength of nonlinear waves
(1999-02-01)
Chang, HK
;
Lin, SC
;
土木工程學系
;
Department of Civil Engineering
An explicit and concise approximation to the wavelength in which the effect of nonlinearity is involved and presented in terms of wave height, wave period, water depth and gravitational acceleration. The present approximation is in a rational form of which Fenton and Mckee's (1990, Coastal Engng 14, 499-513) approximation is reserved in the numerator and the wave steepness is involved ill the denominator. The rational form of this approximation can be converted to an alternative form of a power-series polynomial which indicates that the wavelength increases with wave height and decreases with water depth. If the determined coefficients in the present approximation are fixed, the approximating formula can provide a good agreement with the: wavelengths numerically obtained by Rienecker and Fenton's (1981, J. Fluid Mech. 104, 119-137) Fourier series method, but has large deviations when waves of small amplitude are in deep water or all waves are in shallow water, The present approximation with variable coefficients can provide excellent predictions of the wavelengths for both long and short waves even, for high waves. (C) 1998 Elsevier Science Ltd. All rights reserved.
Bragg reflection of waves by different shapes of artificial bars
(2002)
Hsu, TW
;
Chang, HK
;
Tsai, LH
;
土木工程學系
;
Department of Civil Engineering
Experiments are performed in a wave flume to demonstrate the Bragg reflection of linear gravity waves by artificial bars. Three different artificial bars with rectangular, triangular and rectified cosinoidal shapes are placed discretely on the seabed for measurement of the Bragg reflection. A series of experimental conditions including the number of bars, the periodic bar spacing, the water depth and various wave conditions are tested. Key parameters influencing the Bragg resonances are investigated. The experimental data are compared with the values from both theoretical and numerical models. Some key parameters have proved to be effective in describing the primary resonances. Predictive equations of the characteristics for the Bragg reflection are proposed in this paper.
Long wave reflection and transmission over a sloping step
(2004)
Chang, HK
;
Liou, JC
;
土木工程學系
;
Department of Civil Engineering
This investigation examines long wave reflection and transmission induced by a sloping step. Bellman and Kalaba's (1959) invariant imbedding is introduced to find wave reflection. An alternative method matching both the surface elevation and its surface slope of each region at the junction is applied to the determination of wave reflection and transmission. The proposed methods are compared with the accurate numerical results of Porter and Porter (2000) and those of Mei (1983) for a vertical step. The wave reflection obtained for a mildly sloping step differs significantly from the result of Mei. ne wave reflection is found to fluctuate owing to wave trapping for the mild sloping step. The height and the face slope of the step are important for determining wave reflection and transmission coefficients.
Minimal cost replicated file system with an availability constraint
(1999-10-01)
Chang, HK
;
Yuan, SM
;
資訊工程學系
;
Department of Computer Science
In a replicated file system, copies of the same file are kept in different servers sb that failures of some servers can be tolerant. The goal of a replicated file system is to increase the probability that an operation tread or write) can be performed, i.e., to increase the availability of the file. Giving a minimal acceptable availability of a file, the objective is to minimize the total cost required to build a replicated file system that satisfies the availability constraint. In this paper, an efficient algorithm is proposed to find the minimal cost replicated file system. (C) 1999 Elsevier Science Inc. All rights reserved.
Neural network with multi-trend simulating transfer function for forecasting typhoon wave
(2006-03-01)
Chang, HK
;
Chien, WA
;
土木工程學系
;
Department of Civil Engineering
This study develops an NN typhoon wave model to accurately and efficiently calculate wave heights at a point of interest. Multi-trend simulating transfer functions were first introduced to exemplify the relationship between wave heights and each conceivable input factor by regressive fitting. The proposed NN-MT model can accurately forecast wave peak with an error of less 1.2 m and with time delay within 3 h and can be extended to cover the station besides the original station of interest. (C) 2005 Elsevier Ltd. All rights reserved.
Optimal binary vote assignment for replicated data
(2000-07-15)
Chang, HK
;
Yuan, SM
;
資訊工程學系
;
Department of Computer Science
Data replication can be used to improve the availability of data in a distributed database system. In such a system, a mechanism is required to maintain the consistency of the replicated data. Weighted voting is a popular solution for maintaining the consistency. The performance of a voting system is determined by the vote assignment. It was shown in previous studies that finding the optimal solution among nonnegative integer vote assignments requires O(2(N2)) time. So it is not suitable to find the optimal integer vote assignment (OIVA) for large systems. In this paper, cue propose the optimal binary vote assignment (OBVA) considering only binary vote assignments. It is shown that OBVA can achieve nearly the same availability of OIVA. On the other hand, OBVA requires only O(N(2)) time, which is preferred for large systems. (C) 2000 Elsevier Science Inc. All rights reserved.
Solving wave dispersion equation for dissipative media using homotopy perturbation technique
(2006-01-01)
Chang, HK
;
Liou, JC
;
土木工程學系
;
Department of Civil Engineering
This investigation develops a third-order explicit approximation to finding the roots of the dispersion relation for water waves that propagate over dissipative media, using homotopy perturbation. The proposed third-order approximation can give an explicit and accurate calculation on the inaccessible roots of the dispersion relation. The homotopy method yields global convergence to the roots. Interestingly, the contours of the domains of attraction of the roots resemble a fractal pattern.
Wave reflection over a sloping beach
(2001)
Chang, HK
;
Hsu, TW
;
Liou, JC
;
交大名義發表
;
National Chiao Tung University
A frequency domain method for separating Incident and reflected waves is proposed to account for normally incident waves propagating over a sloping beach in a wave flumes Linear wave shoaling is applied to determine changes of the wave amplitude and phase due to variations of the bathymetry. The wave reflection coefficient is estimated from wave heights measured by two fixed wave gauges with a separated distance. The method is applicable to both monochromatic and random waves. Brief comparisons between theories and the proposed model are made for waves propagating over a sloping seabed.