• 한국전산유체공학회:학술대회논문집
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• 2000.05a
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• pp.166-171
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• 2000

A two-dimensional/axisymmetric CSCM upwind flux difference splitting Wavier-Stokes method has been developed to study the finite rate chemically react-ing invisicd and viscous hypersonic flows over blunt-body. A upwind method was chosen due to its robustness in capturing the strong bow shock waves. For the nonequilibrium chemically reacting air, NS-I species conservation equations were strongly coupled with flowfield equations through convection and species production terms. The nonequilibrium wall pressure and heat transfer rate distributions along the vehicle were compared with those from equilibrium and perfect gas calculations. The nonequilibrium species distribution shows the reduced concentrations of O and N species when compared with equilibrium species distribution. The solutions resolved strong bow shock waves md heat transfer rate very accurately when compared with central difference schemes.

• 한국전산유체공학회:학술대회논문집
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• 2005.10a
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• pp.169-177
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• 2005

A high resolution scheme for solving gas-liquid two-phase flows with cavitation is described. This scheme uses the curvilinear coordinate grid and solves the density based momentum equations for mixture of gas-liquid medium with a preconditioning method to treat both compressible and incompressible flow characteristics. The present preconditioned method is based on the Runge-Kutta explicit finite-difference scheme, and is improved by using the diagonalization, the flux difference splitting and the MUSCL-TVD schemes to save computational effort and to increase stability and resolvability, especially at gas-liquid contact surfaces. A homogeneous equilibrium cavitation model is used to treat the gas-liquid two-phase medium in cavitating flow as a locally homogeneous pseudo-single-phase medium. Therefore, it is easy to solve cavitating flow, including wave propagation, large density changes and incompressible flow characteristic at low Mach number. Some numerical results obtained by the present scheme are shown.

• Journal of applied mathematics & informatics
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• v.30 no.1_2
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• pp.235-244
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• 2012

In this paper, we solve the three-dimensional biharmonic equation with Dirichlet boundary conditions of second kind using the full multigrid (FMG) algorithm. We derive a finite difference approximations for the biharmonic equation on a 18 point compact stencil. The unknown solution and its second derivatives are carried as unknowns at grid points. In the multigrid methods, we use a fourth order interpolation to producing a new intermediate unknown functions values on a finer grid, and the full weighting restriction operators to calculating the residuals at coarse grid points. A set of test problems gives excellent results.

• Journal of computational fluids engineering
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• v.2 no.1
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• pp.8-12
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• 1997

The three-dimensional Euler equations are solved numerically for the analysis of contraction flows in wind or water tunnels. A second-order finite difference method is used for the spatial discretization on the nonstaggered grid system and the 4-stage Runge-Kutta scheme for the numerical integration in time. In order to speed up the convergence, the local time stepping and the implicit residual-averaging schemes are introduced. The pressure field is obtained by solving the pressure-Poisson equation with the Neumann boundary condition. For the evaluation of the present Euler solver, numerical computations are carried out for three contraction geometries, one of which was adopted in the Large Cavitation Channel for the U.S. Navy. The comparison of the computational results with the available experimental data shows good agreement.

• Journal of applied mathematics & informatics
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• v.22 no.1_2
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• pp.49-65
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• 2006

A robust numerical method for a singularly perturbed second-order ordinary differential equation having two parameters with a discontinuous source term is presented in this article. Theoretical bounds are derived for the derivatives of the solution and its smooth and singular components. An appropriate piecewise uniform mesh is constructed, and classical upwind finite difference schemes are used on this mesh to obtain the discrete system of equations. Parameter-uniform error bounds for the numerical approximations are established. Numerical results are provided to illustrate the convergence of the numerical approximations.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 1999.10a
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• pp.359-369
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• 1999

The paper describes the study of concurrent subspace optimization(CSSO) for coupled multidisciplinary design optimization (MDO) techniques in mechanical systems. This method is a solution to large scale coupled multidisciplinary system, wherein the original problem is decomposed into a set of smaller, more tractable subproblems. Key elements in CSSO are consisted of global sensitivity equation(GSE), subspace optimization (SSO), optimum sensitivity analysis(OSA), and coordination optimization problem(COP) so as to inquiry valanced design solutions finally, Automatic differentiation has an ability to provide a robust sensitivity solution, and have shown the numerical numerical effectiveness over finite difference schemes wherein the perturbed step size in design variable is required. The present paper will develop the automatic differentiation based concurrent subspace optimization(AD-CSSO) in MDO. An automatic differentiation tool in FORTRAN(ADIFOR) will be employed to evaluate sensitivities. The use of exact function derivatives in GSE, OSA and COP makes Possible to enhance the numerical accuracy during the iterative design process. The paper discusses how much influence on final optimal design compared with traditional all-in-one approach, finite difference based CSSO and AD-CSSO applying coupled design variables.

• Journal of Communications and Networks
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• v.14 no.5
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• pp.533-539
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• 2012

The performance analysis of wireless geolocation in a non-line-of-sight (NLOS) environment is a very important issue. Since Cramer-Rao lower bound (CRLB) determines the physical impossibility of the variance of an unbiased estimator being less than the bound, many studies presented the performance analysis in terms of CRLB. Several CRLBs for time-of-arrival (TOA), pseudo-range TOA, angle-of-arrival (AOA), and signal strength (SS) based positioning methods have been derived for NLOS environment. However, the performance analysis of time difference of arrival (TDOA) and TDOA/SS based geolocation techniques in a NLOS environment is still an opening issue. This paper derives the CRLBs of TDOA and TDOA/SS based positioning methods for NLOS environment. In addition, theoretical analysis proves that the derived CRLB for TDOA is the same as that of pseudo-range TOA and the TDOA/SS scheme has a lower CRLB than the TDOA (or SS) scheme.

• Communications of the Korean Mathematical Society
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• v.37 no.3
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• pp.939-955
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• 2022

In this paper, we study a first-order non-linear singularly perturbed Volterra integro-differential equation (SPVIDE). We discretize the problem by a uniform difference scheme on a Bakhvalov-Shishkin mesh. The scheme is constructed by the method of integral identities with exponential basis functions and integral terms are handled with interpolating quadrature rules with remainder terms. An effective quasi-linearization technique is employed for the algorithm. We establish the error estimates and demonstrate that the scheme on Bakhvalov-Shishkin mesh is O(N^-1) uniformly convergent, where N is the mesh parameter. The numerical results on a couple of examples are also provided to confirm the theoretical analysis.

• Journal of Korea Multimedia Society
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• v.9 no.7
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• pp.854-863
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• 2006

As WiBro service providers which provide a wireless multimedia communication service are selected, the commercial service will emerge in the near future. Although the use of efficient multicast technology for multimedia will be essentially required, it would be difficult to design the overall communication network in the early stage by using IP multicast due to technological or commercial issues. Application layer multicast schemes or hybrid multicast schemes also should be considered. In this paper, we propose a hybrid multicast scheme and evaluate its performance against typical IP multicast schemes on modelling of the WiBro network for one of WiBro service providers. We also propose a simulation method for the hybrid multicast on the simulation network for performance evaluation. If the number of mobile nodes reaches a certain point, the performance difference between the hybrid multicast and IP multicast is reduced significantly. We show that hybrid multicast technology can be applied for WiBro service without much performance degradation.

• The KIPS Transactions:PartC
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• v.15C no.4
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• pp.321-328
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• 2008

In this paper, we analyze web response time depending on queue managements and transmission latencies in highly congested network situation. Under FIFO scheme, the response times are for three different sizes of queue are almost the same, but the response time increases as traffic intensity increases. The performance between different queue sizes shows more different in 90% and 98% traffic intensity than in 80% traffic intensity. Especially the difference becomes bigger in short latency case than long latency case. Under RED scheme, three differently tunned RED schemes do not impact on the response time when the size of queue is relatively large. When the queue size becomes smaller, the response time of the differently tunned RED schemes becomes different for short latency case while the response times are almost same for long latency case. When comparing FIFO and RED schemes under same size of queue, RED scheme shows less response time than that of FIFO for the long latency case in high traffic intensity.