• Journal of computational fluids engineering
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• v.19 no.3
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• pp.91-97
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• 2014

A high resolution numerical method aimed at solving cavitating flow was proposed and applied to gas-liquid two-phase shock tube problem with arbitrary void fraction. The present method with compressibility effects employs a finite-difference 4th-order Runge-Kutta method and Roe's flux difference splitting approximation with the MUSCL TVD scheme. The Jacobian matrix from the inviscid flux of constitute equation is diagonalized analytically and the speed of sound for the two-phase media is derived by eigenvalues. So that the present method is appropriate for the extension of high order upwind schemes based on the characteristic theory. By this method, a Riemann problem for Euler equations of one dimensional shock tube was computed. Numerical results of high speed flow phenomena such as detailed observations of shock and expansion wave propagations through the gas-liquid two-phase media and some data related to computational efficiency are made. Comparisons of predicted results and solutions at isothermal condition are provided and discussed.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.1
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• pp.97-103
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• 2003

Numerical research has been done for the transverse jet behind a rearward-facings step in turbulent supersonic flow without chemical reaction. Purpose of transverse jet is to enhance mixing of the fuel in the combustor. Two-dimensional unsteady flowfields generated by slot injection into supersonic flow are numerically simulated with the Navier-Stokes equations with two-equation k-$\varepsilon$ turbulence model. Numerical method is used high-order upwind TVD scheme. Eight cases are computed for different slot momentum flux ratios and slot position at downstream of the step. The flow is very similar to the cavity flow, because the jet acts as an obstacle. The numerical results thus show the periodic phenomenon.

• Journal of Korea Water Resources Association
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• v.42 no.12
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• pp.1053-1067
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• 2009

The objective of this study is to develop an efficient and accurate quadratic finite element model based on Streamline Upwind/Petrov Galerkin (SU/PG) scheme for analyzing and predicting two dimensional flow features in complex natural rivers. For a development of model, quadratic tin, quadrilateral and mixed elements as well as linear tin, quadrilateral and mixed elements were used in the model. Also, this model was developed through reinforcement of Gauss Quadrature which was necessary to integral of governing equation. Several tests for bottom-rising channel and U-type channel were performed for the purpose of validation and verification of the developed model. Such results showed that solutions of second order elements are better accurate and improved than those of linear elements. Results obtained by the developed model and RMA-2 model are compared, and the results for the developed model were better accurate than those of RMA-2 model. In the future if the developed model is applied in natural rivers, it can provide better accurate results than those of existing model.

• New & Renewable Energy
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• v.4 no.4
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• pp.30-36
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• 2008

The possibility of whether the induced wind from a vehicle traveling on highway can be used in wind power generation has been verified through computational flow analysis. The bus which is presumed to accompany relatively strong and wide range of induced wind compared to passenger vehicles because of its wide frontal area has been set as the subject of research. In order to ensure the reliability of research, the flow analysis surrounding the bus on a flat road where median strip is not installed has been compared with a preceding research while the validity of grid system and interpretation method used in this research have been assured by a qualitative method. In case of the median strip type wind power generator system, because it has been verified that a strong streamwise wind speed (5 m/s) is derived from the contraction effect of flow passage between the bus and the median strip while maintaining a relatively consistent upwind wind speed (1.4 m/s) in vertical direction in the wake area after the bus passes by although the change of wind speed is intense, it was decided as having some possibility of wind power generation. In case of the traffic sign panel type wind power generator system installed at the upper top of highway, because the wind speed of 2 m/s level has been derived for a limited time only at a section equal to the length of the bus and a faint induced wind speed less than 0.5 m/s was shown at other regions, it was decided as having almost no possibility of wind power generation.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.12
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• pp.1555-1569
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• 2000

Numerical simulation of fluid flow with moving free surface has been carried out. For the free surface flow, a VOF(Volume of Fluid)-based algorithm utilizing a fixed grid system has been investigated. In order to reduce numerical smearing at the free surface represented on a fixed grid system, a new free surface tracking algorithm based on the donor-acceptor scheme has been presented. Novel features of the proposed algorithm are characterized as two numerical tools; the orientation vector to represent the free surface orientation in each cell and the baby-cell to determine the fluid volume flux at each cell boundary. The proposed algorithm can be easily implemented in any irregular non-uniform grid systems that are usual in finite element method (FEM). Moreover, the proposed algorithm can be extended and applied to the 3-D free surface flow problem without additional efforts. For computation of unsteady incompressible flow, a finite element approximation based on the explicit fractional step method has been adopted. In addition, the SUPG(streamline upwind/Petrov-Galerkin) method has been implemented to deal with convection dominated flows. Combination of the proposed free surface tracking scheme and explicit fractional step formulation resulted in an efficient solution algorithm. Validity of the present solution algorithm was demonstrated from its application to the broken dam and the solitary wave propagation problems.

• Journal of Korea Water Resources Association
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• v.44 no.6
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• pp.429-438
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• 2011

In order to simulate a free surface flow in a trench channel, a three-dimensional incompressible unsteady Reynolds-averaged Navier-Stokes (RANS) equations are closed with the ${\kappa}-{\epsilon}$ model. The artificial compressibility (AC) method is used. Because the pressure fields can be coupled directly with the velocity fields, the incompressible Navier-Stokes (INS) equations can be solved for the unknown variables such as velocity components and pressure. The governing equations are discretized in a conservation form using a second order accurate finite volume method on non-staggered grids. In order to prevent the oscillatory behavior of computed solutions known as odd-even decoupling, an artificial dissipation using the flux-difference splitting upwind scheme is applied. To enhance the efficiency and robustness of the numerical algorithm, the implicit method of the Beam and Warming method is employed. The treatment of the free surface, so-called interface-tracking method, is proposed using the free surface evolution equation and the kinematic free surface boundary conditions at the free surface instead of the dynamic free surface boundary condition. AC method in this paper can be applied only to the hydrodynamic pressure using the decomposition into hydrostatic pressure and hydrodynamic pressure components. In this study, the boundary-fitted grids are used and advanced each time the free surface moved. The accuracy of our RANS solver is compared with the laboratory experimental and numerical data for a fully turbulent shallow-water trench flow. The algorithm yields practically identical velocity profiles that are in good overall agreement with the laboratory experimental measurement for the turbulent flow.

• Wind and Structures
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• v.8 no.6
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• pp.427-442
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• 2005

Numerical and wind tunnel simulations of pollutant dispersion around rectangular obstacles with five aspect ratios have been conducted in order to identify the effects of flow patterns induced by buildings on plume dispersion in the near wake of buildings. An emission from a low source located upwind of obstacles was used in this simulation. The local flow patterns and concentrations around a cubical obstacle were initially investigated using three RANS turbulence models, (the standard $k-{\varepsilon}$, Shear Stress Transport (SST), Reynolds-Stress RSM turbulence model) and also using Large-eddy simulation (LES). The computed concentrations were compared with those measured in the wind tunnel. Among the three turbulence models, the SST model offered the best performance and thus was used in further investigations. The results show, for normal aspect ratios of width to height, that concentrations in the near wake are appreciably affected because of plume capture by the horseshoe vortex and convection by the vertical vortex pairs. These effects are less important for high aspect ratios. Vertical vortex pairs present a strong ability to exchange mass vertically and acts efficiently to reduce ground-level concentrations in the near wake.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.10
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• pp.1457-1463
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• 2003

In this paper, the response surface method using three-dimensional Navier-Stokes analysis to optimize the shape of a multi-blade centrifugal fan, is described. For numerical analysis, Reynolds-averaged Navier-Stokes equations with standard k - c turbulence model are transformed into non-orthogonal curvilinear coordinate system, and are discretized with finite volume approximations. Due to the large number of blades in this centrifugal fan, the flow inside of the fan is regarded as steady flow by introducing the impeller force models for economic calculations. Linear Upwind Differencing Scheme(LUDS) is used to approximate the convection terms in the governing equations. SIMPLEC algorithm is used as a velocity-pressure correction procedure. Design variables, location of cur off, radius of cut off, expansion angle of scroll and width of impeller were selected to optimize the shapes of scroll and blades. Data points for response evaluations were selected by D-optimal design, and linear programming method was used for the optimization on the response surface. As a main result of the optimization, the efficiency was successfully improved. It was found that the optimization process provides reliable design of this kind of fans with reasonable computing time.

• Journal of Environmental Science International
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• v.13 no.7
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• pp.655-660
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• 2004

This paper demonstrates the numerical simulation of three dimensional flow pattern for vehicular exhaust dispersion in the street canyons. The wind flow around buildings in urban is computed by the SIMPLEST method. The convection-diffusion equation was used to compute the $NO_X$ concentration level near buildings. Details are given of important boundary conditions and turbulence quantities variations. The simple turbulence model was used for unisotropic viscous effect. A control-volume based finite-difference method with the upwind scheme is employed for discretization equation. The simple turbulence model applied in this study has been verified through comparison between predicted and measured data near buildings. By the predictive results, the updraft induced by the presence of high-rise buildings is important in the transport of street level pollutant out from the street canyons. Our suggestion for reducing ground level pollution is to have high-rise buildings constructed or to reduce the channelling effect of street canyons.

• International Journal of Fluid Machinery and Systems
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• v.7 no.1
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• pp.16-27
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• 2014

Double-blade vertical axis wind turbines (DB-VAWTs) can improve the self-starting performance of lift-driven VAWTs. We here propose the quadruple-multiple streamtube model (QMS), based on the blade element momentum (BEM) theory, for simulating DB-VAWT performance. Model validity is investigated by comparison to computational fluid dynamics (CFD) prediction for two kinds of two-dimensional DB-VAWT rotors for two rotor scales with three inner-outer radius ratios: 0.25, 0.5, and 0.75. The BEM-QMS model does not consider the effects of an inner rotor on the flow speed in the upwind half of the rotor, so we introduce a correction factor for this flow speed. The maximum power coefficient predicted by the modified BEM-QMS model for a DB-VAWT is thus closer to the CFD prediction.