• 한국자동차공학회논문집
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• 제4권5호
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• pp.119-129
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• 1996

This study is concerned with the critical evaluation of predicative capability of a k-$\varepsilon$ turbulence model using the Renormalization Group(RNG) theory. The present numerical model for solution of the Navier-Stokes System is based on the modified PISO algorithms. Computations have been performed with the RNG-based K-$\varepsilon$ model for the two-dimensional flow over a backward-facing step, a confined coaxial jet, and a swirling flow in a swirl combustor. Numerical results are compared with experimental data in terms of mean flow velocities, turbulent kinetic energy, and turbulent stresses. Numerical results clearly indicate that the RNG-based K-$\varepsilon$ turbulence model shows a significant improvement over a standard K-$\varepsilon$ model in predicting the turbulent flows with flow separation and swirl.

• Wind and Structures
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• 제9권4호
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• pp.315-330
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• 2006

Flow and scalar dispersion around Cheomseongdae are numerically investigated using a three-dimensional computational fluid dynamics (CFD) model with the renormalization group (RNG) $k-{\varepsilon}$ turbulence closure scheme. Cheomseongdae is an ancient astronomical observatory in Gyeongju, Korea, and is chosen as a model obstacle because of its unique shape, that is, a cylinder-shaped architectural structure with its radius varying with height. An interesting feature found is a mid-height saddle point behind Cheomseongdae. Different obstacle shapes and corresponding flow convergences help to explain the presence of the saddle point. The predicted size of recirculation zone formed behind Cheomseongdae increases with increasing ambient wind speed and decreases with increasing ambient turbulence intensity. The relative roles of inertial and eddy forces in producing cavity flow zones around an obstacle are conceptually presented. An increase in inertial force promotes flow separation. Consequently, cavity flow zones around the obstacle expand and flow reattachment occurs farther downwind. An increase in eddy force weakens flow separation by mixing momentum there. This results in the contraction of cavity flow zones and flow reattachment occurs less far downwind. An increase in ambient wind speed lowers predicted scalar concentration. An increase in ambient turbulence intensity lowers predicted maximum scalar concentration and acts to distribute scalars evenly.

• 한국자동차공학회논문집
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• 제5권1호
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• pp.87-100
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• 1997

The ability of turbulence model to accurately describe the complex characteristics of the flow field and the fuel spray is of great importance in the optimum design of diesel engine. The numerical simulations of the flow field and the spray characteristics within the combustion chamber of direct injection model entgine are performed to examine the applicability of turbulence model. The turbulence models used are the RNG $\varepsilon$ model and the modified $\varepsilon$ model which included the compressibility effect due to the compression/expansion of the charges. In this study, the predicted results in the quiescent condition of direct injection model engine show reasonable trends comparing with the experimental data of spray characteristics, i. e., spray tip penetration, spray tip velocity. The results of eddy viscosity obtained using the $\varepsilon$ model in the spray region is significantly larger than that obtained using the RNG $\varepsilon$ model. The application of the RNG model seems to have some potential for the simulations of the spray characteristics, e. g., spray tip penetration, spray tip velocity, droplets distribution over the $\varepsilon$ model.

• Wind and Structures
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• 제17권6호
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• pp.647-670
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• 2013

The paper presents a numerical approach to study of fluid flow characteristics and to predict performance of wind turbines. The numerical model is based on Finite-volume method (FVM) discretization of unsteady Reynolds-averaged Navier-Stokes (URANS) equations. The movement of turbine blades is modeled using moving mesh technique. The turbulence is modeled using commonly used turbulence models: Renormalization Group (RNG) k-${\varepsilon}$ turbulence model and the standard k-${\varepsilon}$ and k-${\omega}$ turbulence models. The model is validated with the experimental data over a large range of tip-speed to wind ratio (TSR) and blade pitch angles. In order to demonstrate the use of numerical method as a tool for designing wind turbines, two dimensional (2-D) and three-dimensional (3-D) simulations are carried out to study the flow through a small scale Darrieus type H-rotor Vertical Axis Wind Turbine (VAWT). The flows predictions are used to determine the performance of the turbine. The turbine consists of 3-symmetrical NACA0022 blades. A number of simulations are performed for a range of approaching angles and wind speeds. This numerical study highlights the concerns with the self-starting capabilities of the present VAWT turbine. However results also indicate that self-starting capabilities of the turbine can be increased when the mounted angle of attack of the blades is increased. The 2-D simulations using the presented model can successfully be used at preliminary stage of turbine design to compare performance of the turbine for different design and operating parameters, whereas 3-D studies are preferred for the final design.

• 한국환경과학회지
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• 제23권7호
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• pp.1223-1232
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• 2014

Density currents have been easily observed in environmental flows, for instance turbidity currents and pollutant plumes in the oceans and rivers. In this study, we explored the propagation dynamics of density currents using the FLOW-3D computational fluid dynamics code. The renormalization group (RNG) $k-{\varepsilon}$ scheme, a turbulence numerical technique, is employed in a Reynold-averaged Navier-Stokes framework (RANS). The numerical simulations focused on two different types of intrusive density flows: (1) propagating into a two-layer ambient fluid; (2) propagating into a linearly stratified fluid. In the study of intrusive density flows into a two-layer ambient fluid, intrusive speeds were compared with laboratory experiments and analytical solutions. The numerical model shows good quantitative agreement for predicting propagation speed of the density currents. We also numerically reproduced the effect of the ratio of current depth to the overall depth of fluid. The numerical model provided excellent agreement with the analytical values. It was also clearly demonstrated that RNG $k-{\varepsilon}$ scheme within RANS framework is able to accurately simulate the dynamics of density currents. Simulations intruding into a continuously stratified fluid with the various buoyancy frequencies are carried out. These simulations demonstrate that three different propagation patterns can be developed according to the value of $h_n/H$ : (1) underflows developed with $h_n/H=0$ ; (2) overflows developed when $h_n/H=1$ ; (3) intrusive interflow occurred with the condition of 0 < $h_n/H$ < 1.

• 대기
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• 제21권1호
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• pp.95-108
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• 2011

The effects of an apartment complex on flow and pollutant dispersion in an urban area are numerically investigated using a computational fluid dynamics (CFD) model. The CFD model is based on the Reynolds-averaged Navier-Stokes equations and includes the renormalization group k-${\varepsilon}$ turbulence model. The geographic information system (GIS) data is used as an input data of the CFD model. Eight numerical simulations are carried out for different inflow directions and, for each inflow direction, the effects of an apartment complex are investigated, comparing the characteristics of flow and dispersion before and after construction of the apartment complex in detail. The observation data of automatic weather system (AWS) is analyzed. The windrose analysis shows that the wind speed and direction after the construction of the complex are quite different from those before the construction. The construction of the apartment complex resulted in the decrease in wind speed at the downwind region. It is also shown that the wind speed increased partially inside the apartment complex due to the channeling effect to satisfy the mass continuity. On the whole, the wind speed decreased at the downwind region due to the drag effect by the apartment complex. As a result, the passive pollutant concentration increased (decreased) near the downwind region of (within) the apartment complex compared with that before the construction.

• 한국대기환경학회지
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• 제23권2호
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• pp.214-224
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• 2007

A three-dimensional computational fluid dynamics (CFD) model with the renormalization group (RNG) $k-{\varepsilon}$ turbulence model is used to examine the effects of difference in building height on flow and pollutant dispersion in asymmetric street canyons. Three numerical experiments with different street canyons formed by two isolated buildings are performed. In the experiment with equal building height, a portal vortex is formed in the street canyon and a typical recirculation zone is formed behind the downwind building. In the experiment with the downwind building being higher than the upwind building, the ambient flow comes into the street canyon at the front of the downwind building and incoming flow diverges strongly in the street canyon. Hence, pollutants released therein are strongly dispersed through the lateral sides of the street canyon. In the experiment with the upwind building being higher than the downwind building, a large recirculation zone is formed behind the upwind building, which is disturbed by the downwind building. Pollutants are weakly dispersed from the street canyon and the residue concentration ratio is largest among the three experiments. This study shows that the difference in upwind and downwind building height significantly influences flow and pollutant dispersion in and around the street canyon.

• 한국대기환경학회지
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• 제21권5호
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• pp.525-535
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• 2005

Using a three-dimensional computational fluid dynamics (CFD) model with the $k-{\varepsilon}$ turbulence closure scheme based on the renormalization group theory, flow regimes in urban street canyons are classified according to the building and street aspect ratios. The transition between skimming flow (SF) and wake interference flow (WIF) is determined with the size of double-eddy circulation generated behind the upwind building. The transition between WIF and isolated roughness flow (IRF) is determined with the flow reattachment distance from the upwind building. The critical aspect ratios at which the flow transition occurs are found and compared with those in previous studies. The results show that the flow-regime classification method used in this study is quite reasonable and that the values of the critical aspect ratios are generally consistent with those in fluid experiments or large-eddy simulation. The regression equation describing a relation between the building and street aspect ratios at the flow-regime transition is presented.

• Wind and Structures
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• 제13권1호
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• pp.21-36
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• 2010

A new full scale testing apparatus generically named the Wall of Wind (WoW) has been built by the researchers at the International Hurricane Research Center (IHRC) at Florida International University (FIU). WoW is capable of testing single story building models subjected up to category 3 hurricane wind speeds. Depending on the relative model and WoW wind field sizes, testing may entail blockage issues. In addition, the proximity of the test building to the wind simulator may also affect the aerodynamic data. This study focuses on the Computational Fluid Dynamics (CFD) assessment of the effects on the quality of the aerodynamic data of (i) blockage due to model buildings of various sizes and (ii) wind simulator proximity for various distances between the wind simulator and the test building. The test buildings were assumed to have simple parallelepiped shapes. The computer simulations were performed under both finite WoW wind-field conditions and in an extended Atmospheric Boundary Layer (ABL) wind flow. Mean pressure coefficients for the roof and the windward and leeward walls served as measures of the blockage and wind simulator proximity effects. The study uses the commercial software FLUENT with Reynolds Averaged Navier Stokes equations and a Renormalization Group (RNG) k-${\varepsilon}$ turbulence model. The results indicated that for larger size test specimens (i.e. for cases where the height of test specimen is larger than one third of the wind field height) blockage correction may become necessary. The test specimen should also be placed at a distance greater than twice the height of the test specimen from the fans to reduce proximity effect.

• 대기
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• 제17권4호
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• pp.315-326
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• 2007

The effects of windbreaks on the reduction of suspended particles are investigated using a computational fluid dynamics (CFD) model with the ${\kappa}-{\varepsilon}$ turbulence closure scheme based on the renormalization group (RNG) theory. In the control experiment, the recirculation zones behind the storage piles are generated and, as a whole, relatively monotonous flow patterns appear. When the windbreaks with the 0% porosity are constructed, the recirculation zones are generated by the windbreaks and very complicated flow patterns appear due to the interference between the windbreaks and storage piles. The porosity of the windbreaks suppresses the generation of the recirculation zone and decreases the wind velocity in the windbreaks as well as that outside the windbreaks. As the emission of suspended particles from the storage piles are closely related with the friction velocity at the surfaces of the storage piles, variation of the friction velocity and total amount of the emission of the suspended particles with the height and porosity of the windbreaks are investigated. The results show that higher and more porous windbreaks emit less suspended particles and that the reduction effect of the porosity is still more effective than that of the height. In the case of the windbreak with 30 m height and 50% porosity, friction velocities above the storage piles are smaller than the critical friction velocity above which particles would be suspended. As a result, total amount of suspended particles are much fewer than those in other cases.