• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.7
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• pp.973-983
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• 2003

The suitability of high-order accurate, centered and upwind-biased compact difference schemes is evaluated for large eddy simulation of turbulent flow. Two turbulent flows are considered: turbulent channel flow at Re = 23000 and flow over a circular cylinder at Re = 3900. The effects of numerical dissipation on the finite differencing and aliasing errors and the subgrid-scale stress are investigated. It is shown through the simulations that compact upwind schemes are not suitable for LES, whereas the fourth order-compact centered scheme is a good candidate for LES provided that proper dealiasing of nonlinear terms is performed. The classical issue on the aliasing error and the treatment of nonlinear terms is revisited with compact difference schemes.

• Journal of the Korea Society for Simulation
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• v.5 no.1
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• pp.81-90
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• 1996

In this paper, we propose a new events ordering mechanism for the optimistic distributed simulation of DEVS models. To simulate DEVS models in a distributed environment, a synchronization protocol is required for correct simulation. Time Warp is the most well-known optimistic synchronization protocol for distributed simulation. However, employing the Time Warp protocol in distributed simulation of DEVS models incurs events ordering problem due to the semantic difference between Time Warp and DEVS, Thus, to resolve such semantic difference, we devise the time-and-priority-stamp and $\varepsilon$ -delay schemes. The proposed schemes can order simultaneous events correctly in Time Warp-based distributed simulation of DEVS models.Time Warp and DEVS, Thus, to resolve such semantic difference, we devise the time-and-priority-stamp and $\varepsilon$ -delay schemes. The proposed schemes can order simultaneous events correctly in Time Warp-based distributed simulation of DEVS models.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.9
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• pp.1227-1238
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• 2000

This study examines the effects of the discretization schemes on numerical solutions of viscoelastic fluid flows. For this purpose, a temporally evolving mixing layer, a two-dimensional vortex pair interacting with a wall, and a turbulent channel flow are selected as the test cases. We adopt a fourth-order compact scheme (COM4) for polymeric stress derivatives in the momentum equations. For convective derivatives in the constitutive equations, the first-order upwind difference scheme (UD) and artificial diffusion scheme (AD), which are commonly used in the literature, show most stable and smooth solutions even for highly extensional flows. However, the stress fields are smeared too much and the flow fields are quite different from those obtained by higher-order upwind difference schemes for the same flow parameters. Among higher-order upwind difference schemes, a third-order compact upwind difference scheme (CUD3) shows most stable and accurate solutions. Therefore, a combination of CUD3 for the convective derivatives in the constitutive equations and COM4 for the polymeric stress derivatives in the momentum equations is recommended to be used for numerical simulation of highly extensional flows.

• Proceedings of the Korea Water Resources Association Conference
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• 2006.05a
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• pp.22-27
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• 2006

This paper describes the development of the stream-tube based dispersion model for modeling contaminant transport in open channels. The operator-splitting technique is employed to separate the 2D contaminant transport equation into the pure advection and pure dispersion equations. Then the total variation diminishing (TVD) schemes are combined with the second-order Lax-Wendroff and third-order QUICKEST explicit finite difference schemes respectively to solve the pure advection equation in order to prevent the occurrence of numerical oscillations. Due to various limiters owning different features, the numerical tests for 1D pure advection and 2D dispersion are conducted to evaluate the performance of different TVD schemes firstly, then the TVD schemes are applied to experimental data for simulating the 2D mixing in a straight trapezoidal channel to test the model capability. Both the numerical tests and model application show that the TVD schemes are very competent for solving the advection-dominated transport problems.

• Korean Journal of Mathematics
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• v.18 no.3
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• pp.299-309
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• 2010

In this paper, we study the first order nonlinear neutral difference equation: $${\Delta}(x(n)+px(n-{\tau}))+f(n,x(n-c),x(n-d))=r(n),\;n{\geq}n_0$$. Using the Banach fixed point theorem, we prove the existence of bounded positive solutions of the equation, suggest Mann iterative schemes of bounded positive solutions, and discuss the error estimates between bounded positive solutions and sequences generated by Mann iterative schemes.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.807-810
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• 2002

The suitability of high-order accurate, central and upwind-biased compact difference schemes is evaluated for the large-eddy simulations of flows in complex geometry. Two flow geometries are considered: channel and circular cylinder. The effects of numerical dissipation and aliasing error on the evaluation of subgrid scale stress are investigated by extending the analysis by Ghosal (1) to centered and upwind compact schemes. It is shown that the failure of upwind schemes mainly comes from the aliasing error.

• Journal of applied mathematics & informatics
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• v.27 no.5_6
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• pp.1001-1015
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• 2009

In this paper, two hybrid difference schemes on the Shishkin mesh are constructed for solving a weakly coupled system of two singularly perturbed convection-diffusion second order ordinary differential equations with a small parameter multiplying the highest derivative. We prove that the schemes are almost second order convergence in the supremum norm independent of the diffusion parameter. Error bounds for the numerical solution and its derivative are established. Numerical results are provided to illustrate the theoretical results.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.1
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• pp.131-141
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• 1994

Three characteristics-based split-operator methods were applied to a longitudinal pollutant dispersion problem, and the results were compared with those of several Eulerian schemes. The split-operator methods consisted of generalized upwind, two-point fourth-order and sixth-order Holly-Preissmann schemes, respectively, for the advection calculation, and the Crank-Nicholson scheme for the diffusion calculation. Compared with the Eulerian schemes tested, split-operator methods using the Holly-Preissmann schemes gave much more accurate computational results. Eulerian schemes using centered difference approximations for the advection term resulted in numerical oscillations, and those using backward difference resulted in numerical diffusion, both of which were more severe for smaller value of the longitudinal dispersion coefficient.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.36 no.5C
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• pp.309-316
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• 2011

In this paper, equalization zone of two crosstalk cancellation (CC) schemes, which are the one based on only head related transfer function (HRTF) and the other one based on interaural intensity/time difference (ITD/IID) as well as HRTF is studied. To do this, the condition numbers and ITD/IID levels of two schemes are shown.

• Journal of Institute of Control, Robotics and Systems
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• v.19 no.6
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• pp.508-512
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• 2013

In recent years, sensorless drive schemes have been proposed widely and most of them are based on the ZCP (Zero Crossing Point) detection of the BEMF (Back Electro-Motive Force). These schemes have two main problems. One is that ZCP may not be detected at low speed and thus a forced drive is required. The other problem is that there is $30^{\circ}$ phase difference between ZCP and the motor commutation instant and to ensure proper operation, this gap should be accounted for. To solve these problems a circuit is devised for detecting ZCP of the BEMF difference through the line voltage difference. Experimental results show that the output of this circuit is identical to that of the Hall sensor signal, and velocity control of a BLDC motor is possible without the sensor.