• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.2
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• pp.430-439
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• 1990

Steady state turbulent airflow and unsteady characteristics of generation, transportation, and recovery behavior of contaminate particles in the simplified 2 dimensional Vertical Laminar Flow (VLF) type clean room was numerically simulated using the low Reynolds number k-over bar.epsilon- turbulent model. Characteristics of airflow in VLF type clean room are greatly affected by the recirculation zone around working surface. The recirculation zone must be considered at the time of clean room design because the recirculation zone whose area increases with increment of inlet velocity exerts bad influence upon the performance of clean room in terms of particle contamination. The location of maximum particle concentration changes from the location of particle source to the recirculation zone, while averaged particle concentration is reduced exponentially with time. Recovery time of clean room with spontaneous particle generation source is inversely proportional to inlet velocity. We introduce nondimensionalized recovery time through the dimensional analysis, which can indicates the general performance of clean room with design structure change. It was identified that .tau. is independent of inlet velocity and background concentration. Therefore .tau. can be the simple factor to compare the different structure of clean room in terms of dynamic response to contamination and becomes larger with better structure of clean room.

• Journal of computational fluids engineering
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• v.2 no.1
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• pp.46-53
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• 1997

The flow field around a three dimensional minivan-like body has been simulated. This study solves 3-D unsteady incompressible Navier-Stokes equations on a non-orthogonal curvilinear coordinate system using second-order accurate schemes for the time derivatives, and third/second-order scheme for the spatial derivatives. The Marker-and-Cell concept is applied to efficiently solve continuity equation. A H-H type of multi-block grid system is generated around a three dimensional minivan-like body. Turbulent flows have been modeled by the Baldwin-Lomax turbulent model. To validate present procedure, the flows around the Ahmed body with 12.5° of slant angle are simulated. A good agreement with other numerical results is achived. After code validation, the flows around a mimivan-like body are simulated. The simulation shows three dimensional vortex-pair just behind body. The flow separation is also observed on the rear of the body. It has concluded that the results of present study properly agreed with physical flow phenomena.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.4 s.247
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• pp.364-372
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• 2006

The objective of this study is to evaluate the efficiency and the prediction accuracy of developed large eddy simulation (LES) program for complex turbulent flows, such as recirculating and swirling flows. To save the computational cost, a Beowulf cluster system consisting 16 processors was constructed. The flows in backward-facing step and dump combustor were examined as representative recirculating and swirling flows. Firstly, a direct numerical simulation (DNS) for laminar backward-facing step flows was previously conducted to validate the overall performance of program. Then LES was carried out for turbulent backward-facing step flows. The results of laminar flow showed a qualitative and quantitative agreement between simulations and experiments. The simulations of the turbulent flow also showed reasonable results. Secondly, LES results for non-swirling and swirling flows in a dump combustor were compared with the results of Reynolds-averaged Navier-Stokes (RANS) using standard $k-{\varepsilon}$ model. The results show that LES has a better performance in predicting the mean axial and azimuthal velocities, comer recirculation zone (CRZ) and center toroidal recirculation zone (CTRZ) than those of RANS. Finally, it was examined the capability of LES for the description of unsteady phenomena.

• International Journal of Fluid Machinery and Systems
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• v.2 no.4
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• pp.346-352
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• 2009

The objective of the present work is to improve numerical predictions of unsteady turbulent swirling flows in the draft tubes of hydraulic power plants. We present Large Eddy Simulation (LES) results on a simplified draft tube consisting of a straight conical diffuser. The basis of LES is to solve the large scales of motion, which contain most of the energy, while the small scales are modeled. LES strategy is here preferred to the average equations strategies (RANS models) because it resolves directly the most energetic part of the turbulent flow. LES is now recognized as a powerful tool to simulate real applications in several engineering fields which are more and more frequently found. However, the cost of large-eddy simulations of wall bounded flows is still expensive. Bypass methods are investigated to perform high-Reynolds-number LES at a reasonable cost. In this study, computations at a Reynolds number about 2 $10^5$ are presented. This study presents the result of a new near-wall model for turbulent boundary layer taking into account the streamwise pressure gradient (adverse or favorable). Validations are made based on simple channel flow, without any pressure gradient and on the data base ERCOFTAC. The experiments carried out by Clausen et al. [1] reproduce the essential features of the complex flow and are used to develop and test closure models for such flows.

• Nuclear Engineering and Technology
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• v.32 no.1
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• pp.46-56
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• 2000

Numerical results are presented for the 2-dimensional turbulent transient heat transfer of the shell/tube heat exchanger with a step change of the inlet temperature in the primary side. Heat transfer boundary conditions outside the pipe are given partially by the convection heat transfer conditions and partially by insulated conditions. Calculation results were obtained by solving the unsteady two-dimensional elliptic forms for the Reynolds-averaged governing equations for the mass, momentum and energy. Finite-difference method was used to obtain discretization equations, and the SIMPLER solution algorithm was employed for the calculation procedure. Turbulent model used is the algebraic model proposed by Cebeci-Smith. Results presented include the time variant Nusselt number distribution, average temperature distribution and outlet temperatures for the various inlet temperatures and flow rates.

• Journal of Advanced Marine Engineering and Technology
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• v.33 no.5
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• pp.686-695
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• 2009

Flow characteristics in linear axial cascade have been studied using large eddy simulation(LES) based on finite element method(FEM) to investigate details of the leakage flow in the tip clearance of axial flow rotor. STAR-CD(FVM) and PAT-Flow(FEM) have been adopted to solve the Navier-Stokes equations for the simulation of the unsteady turbulent flow. Numerical results from the present study have been compared with the existing experimental results to investigate a tip clearance effect on velocity profile and static pressure distribution on blade surface at various spanwise positions. Both simulation results agree well with the experimental data. However, it has been shown that the results of finite-element large-eddy simulation agree better with experimental data than $k-{\varepsilon}$ turbulent model based on finite volume method regarding the tip vortex geometry and static pressure distribution at the center of the tip vortex core. As a result of this study, it is shown that finite-element large-eddy simulation method can predict more exactly on the tip leakage vortex flow and behind flow field.

• Journal of computational fluids engineering
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• v.11 no.4 s.35
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• pp.62-66
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• 2006

High-speed flight vehicle have various cavities. The supersonic cavity flow is complicated due to vortices, flow separation, reattachment, shock waves and expansion waves. The general cavity flow phenomena includes the formation and dissipation of vortices, which induce oscillation and noise. The oscillation and noise greatly affect flow control, chemical reaction, and heat transfer processes. The supersonic cavity flow with high Reynolds number is characterized by the pressure oscillation due to turbulent shear layer, cavity geometry, and resonance phenomenon based on external flow conditions. The resonance phenomena can damage the structures around the cavity and negatively affect aerodynamic performance and stability. In the present study, we performed numerical analysis of cavities by applying the unsteady, compressible three dimensional Reynolds-Averaged Navier-Stokes(RANS) equations with the ${\kappa}-{\omega}$ turbulence model. The cavity model used for numerical calculation had a depth(D) of 15mm cavity aspect ratio (L/D) of 3, width to spanwise ratio(W/D) of 1.0 to 5.0. Based on the PSD(Power Spectral Density) and CSD(Cross Spectral Density) analysis of the pressure variation, the dominant frequency was analyzed and compared with the results of Rossiter's Eq.

• 한국전산유체공학회:학술대회논문집
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• 2010.05a
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• pp.389-396
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• 2010

In this paper, a new computational code was developed using Chorin's artificial compressibility method to solve the two-dimensional incompressible Navier-Stokes equations. In spatial derivatives, Roe's flux difference splitting was used for the inviscid flux, while central differencing was used for the viscous flux. Furthermore, AF-ADI with dual time stepping method was implemented for accurate unsteady computations. Two-equation turbulence models, Menter's $k-{\omega}$ SST model and Coakley's $q-{\omega}$ model, hae been adopted to solve high-Reynolds number flows. A number of numerical simulations were carried out for steady laminar and turbulent flow problems as well as unsteady flow problem. The code was verified and validated by comparing the results with other computational results and experimental results. The results of numerical simulations showed that the present developed code with the artificial compressibility method can be applied to slve steady and unsteady incompressible flows.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.1
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• pp.139-149
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• 1994

A numerical simulation of unsteady axisymmetric turbulent flow was performed for a reciprocating engine including port/valve assembly. The governing equations based on a nonorthogonal coordinate formulation with Cartesian velocity components were used and discretised by the finite volume method with non-staggered variable arrangements. The modified $\kappa-\xi$. turbulence model which included the effect of compressibility was used. The results of twodimensional transient calculation for the axisymmetric configuration were compared with the experimental data. Although slightly low rms velocity was predicted compared to the experimental data, predicted velocity distributions at the valve exit and in-cylinder region showed good agreements with the experimental data. The flow at the valve exit was separated at the same valve lift position with the experimental data. Two vortices incylinder region were generated during the initial intake process. The clockwise main vortex became strong and moved upward to the top wall. The counter-clockwise second vortex became weak and stick to the upper left corner of the cylinder. After middle intake process, new vortex adjacent to upper cylinder wall appeared by the piston motion and therefore, the in-cylinder flow was formed into three vortices. The cylinder pressure just before bottom dead center of piston was higher than inlet pressure and then the reverse flow occured at the valve exit. The in-cylinder flow characteristics were strongly dependent on piston motion, but insensitive to valve motion.

• 한국전산유체공학회:학술대회논문집
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• 2010.05a
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• pp.37-39
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• 2010

Aerodynamic analysis was done for a fuselage and wing configuration of a mid-sized aircraft using 3-dimensional Navier-Stokes solver. Various turbulent models including a transitional SST were implemented to observe a dynamic stall as well as cruise characteristics. Also, different mesh moving methods were evaluated. Flow hysteresis which causes dynamic stall was investigated through flow field investigations.