• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.8
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• pp.2650-2669
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• 1996

A new low Reynolds number nonlinear second moment turbulence closure was introduced to analyze a square sectioned 180.deg. bend flow. Inclusion of nonlinear return to isotropy term and cubic mean pressure strain term has brought out a marked improvement in the level of agreement with measured velocity profiles. Optimization of present closure was performed by comparison of computed velocity profiles with the experimental ones with variation of nonlinear return to isotropy term and quadratic and cubic pressure-strain model. Progressive vortex breakdown due to the interaction of primary and secondary flows was well captured by using the optimized second moment turbulence closure.

• Journal of computational fluids engineering
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• v.17 no.3
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• pp.53-58
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• 2012

Steady flow simulations through a high pressure turbine guide vanes were carried out. The main objective of the present work is to study the performance of turbulence models on the steady flow prediction from aerodynamic and aerothermal points of view. Three turbulence models were compared, namely SST, k-${\omega}$ and ${\omega}$-Reynolds stress models. The laminar results were also compared. The comparison was done with emphasis on the isentropic Mach number and heat transfer coefficient along the blade, and total pressure loss in the wake region. The calculated isentropic Mach number showed reasonable agreement with experimental data along the blade surface for all three turbulent models. For the total pressure loss in the wake region, ${\omega}$-Reynolds stress model showed the best agreement with the experimental data. However, unless using an appropriate transition model, the heat transfer coefficients of all three turbulent models showed poor agreement with experimental data.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.7
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• pp.557-570
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• 2014

The heat transfer, pressure loss, and thermal performance in a cooling channel were evaluated for various new fan-shaped pin-fin geometries using three-dimensional Reynolds-averaged Navier-Stokes equations. The turbulence was modeled using the low-Reynolds-number SST turbulence model in the Reynolds number range of 5,000-100,000. The numerical results for the area-averaged Nusselt numbers were validated by comparing them with the experimental data under the same conditions. A parametric study for three types of fan-shaped pin-fin geometries was performed with two parameters, namely, the leading and trailing reduction angles.

• Wind and Structures
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• v.13 no.6
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• pp.557-580
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• 2010

A new inflow turbulence generation method and a combined dynamic SGS model recently developed by the authors were applied to evaluate the wind effects on 508 m high Taipei 101 Tower. Unlike the majority of the past studies on large eddy simulation (LES) of wind effects on tall buildings, the present numerical simulations were conducted for the full-scale tall building with Reynolds number greater than $10^8$. The inflow turbulent flow field was generated based on the new method called discretizing and synthesizing of random flow generation technique (DSRFG) with a prominent feature that the generated wind velocity fluctuations satisfy any target spectrum and target profiles of turbulence intensity and turbulence integral length scale. The new dynamic SGS model takes both advantages of one-equation SGS model and a dynamic production term without test-filtering operation, which is particular suitable to relative coarse grid situations and high Reynolds number flows. The results of comparative investigations with and without generation of inflow turbulence show that: (1) proper simulation of an inflow turbulent field is essential in accurate evaluation of dynamic wind loads on a tall building and the prescribed inflow turbulence characteristics can be adequately imposed on the inflow boundary by the DSRFG method; (2) the DSRFG can generate a large number of random vortex-like patterns in oncoming flow, leading to good agreements of both mean and dynamic forces with wind tunnel test results; (3) The dynamic mechanism of the adopted SGS model behaves adequately in the present LES and its integration with the DSRFG technique can provide satisfactory predictions of the wind effects on the super-tall building.

• Journal of Advanced Marine Engineering and Technology
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• v.35 no.7
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• pp.938-945
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• 2011

Unsteady flow simulations through a transonic turbine vane were carried out for an isentropic Mach number of 1.02 and a Reynolds number of $10^6$. The main objective of the study is to investigate the effect of unsteadiness due to vortex shedding on the flow in transonic regime. The steady and the time-averaged unsteady results by employing three different turbulence models: shear stress transport (SST), k-${\omega}$, and ${\omega}$ Reynolds stress models were compared. The comparisons were emphasized on the isentropic Mach number along the blade and total pressure loss at the cascade exit. The results showed that both steady and unsteady calculations have good agreement with experimental data along the blade surface. However, at cascade exit, the unsteady calculations have much better agreement with experimental data than steady calculations. Based on these, we conclude that the unsteady flow calculations are essential for these types of problems.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.9
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• pp.1172-1177
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• 1999

Comparisons of measured turbulence properties in the unburned gas region of turbulent premixed flame stabilized by pilot flame, in cases of combusting and non-combusting flow conditions, are presented. Methane-air premixed jet at fuel equivalence ratio of 0.6 and 1.0 and Reynolds number of 7,000 was diagnosed using two-color laser velocimeter to obtain turbulence statistics. Same set of measurements was repeated at 21 locations within the unburned gas region of both combusting and non-combusting conditions. Velocity data were analyzed to evaluate the spatial distribution of turbulence properties including Reynolds stress, probability densities, joint probability densities and auto correlations. Contrary to assumptions of current theoretical models, significant influence of flame was observed in every property that was studied in the present investigation. The effective viscosity increased ten-fold when flame was on from cold flow values. The effect of mixing on joint probability as well as in turbulence intensity was suppressed by the flame. The measurements suggest that common assumptions of premixed flame model may result in sizable error in prediction of flame length and temperature distribution in near-field.

• Transactions of the Korean Society of Automotive Engineers
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• v.2 no.1
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• pp.38-50
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• 1994

To improve the efficiency of internal combustion engines, it is necessary to understand mixed air-fuel in-cylinder flow processes accurately at intake and compression strokes. There is experimental and numerical methods to analyse in-cylinder flow process. In numerical method, standard $k-{\varepsilon}$ model with wall function was mostly adopted in in-cylinder flow process. But this type model was not efficiently predicted in the near wall region. Therefore in the present study, low Reynolds number $k-{\varepsilon}$ model was adopted near the cylinder wall and standard $k-{\varepsilon}$ model in other region. Also QUICK scheme was used for convective difference scheme. This study takes axisymmetric reciprocating model engine motored at 200rpm with a centrally located valve, incorporated 60 degree seat angie, and flat piston surface excluding inlet port. Because in-cylinder flow processes are undergoing unsteady and compressible, averaged cylinder pressure and inlet velocity at arbitrary crank angle are determined from thermodynamic analytic method and incylinder states at that crank angle are iteratively determined from the numerical analytic method.

• 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.

• Journal of Mechanical Science and Technology
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• v.15 no.3
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• pp.392-402
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• 2001

The paper reports a multiple source modeling of low-Reynolds-number dissipation rate equation with aids of DNS data. The key features of the model are to satisfy the wall limiting conditions of the individual source terms in the exact dissipation rate equation using the wall damping functions. The wall damping functions are formulated in term of dimensionless dissipation length scale ι(sup)+(sub)D(≡ι(sub)D($\upsilon$$\xi$)(sup)1/4/$\upsilon$) and the invariants of small and large scale turbulence anisotropy tensors. $\alpha$(sub)ij(=$\mu$(sub)i$\mu$(sub)j/$\kappa$-2$\delta$(sub)ij/3) and e(sub)ij(=$\xi$(sub)ij/$\xi$-2$\delta$(sub)ij/3). The model constants are optimized with aids of DNS data in a plane channel flow. Adopting the dissipation length scale as a parameter of damping function, the applicabilities of $\kappa$-$\xi$ model are extended to the turbulent flow calculation of complex flow passages.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.16 no.9
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• pp.804-810
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• 2004

This paper reports the characteristics of the three dimensional turbulent flow by numerical method in the 180 degree bends with increasing cross-sectional area. Calculated pressure and velocity, Reynolds stress distributions are compared to the experimental data. Turbulence model employed are low Reynolds number $textsc{k}$-$\varepsilon$ model and algebraic stress model(ASM). The results show that the main vortex generated from the inlet part of the bend maintained to outlet of the bend and vortices are continually developed at the inner wall region. The distribution of turbulent kinetic energy along the bend are increase up to 120$^{\circ}$ because of increment of cross-sectional area. Secondary flow strength of the flow is lower about 60% than that of square duct flow.