• 대한기계학회논문집B
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• 제24권2호
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• pp.224-232
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• 2000

Four turbulent $k-{\varepsilon}$ models (i.e., standard model, modified models with streamline curvature modification and/or preferential dissipation modification) are applied in order to analyze the turbulent flow of wall-attaching offset jet. For numerical convergence, this paper develops a method of slowly increasing the convective effect induced by skew-velocity in skew-upwind scheme (hereafter called Partial Skewupwind Scheme). Even though the method was simple, it was efficient in view of convergent speed, computer memory storage, programming, etc. The numerical results of all models show good prediction in first order calculations (i.e., reattachment length, mean velocity, pressure), while they show some deviations in ·second order (i.e., kinetic energy and its dissipation rate). Like the previous results obtained by upwind scheme, the streamline curvature modification results in better prediction, while the preferential dissipation modification does not.

• 대한기계학회논문집
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• 제13권5호
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• pp.1052-1060
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• 1989

본 연구에서는 0-방정식 난류모형과 2-방정식 난류모형 그리고 역압력 구배와 수렴과 발산의 영향을 고려한 2-방정식 난류모형을 사용하여 대칭단면에서의 난류유동 을 해석하고, 계산 결과를 비교하여 난류모형의 적절성을 평가하고자 한다.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2002년도 제24회 KOSCO SYMPOSIUM 논문집
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• pp.113-122
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• 2002

The purpose of this study is to verify that the modified Lagrangian model can predict temperature, flow and scalar fields in the high temperature recirculation region of swirling confined diffusion flame. In the meantime numerical results from EBU and Equilibrium PDF models as well as experimental results are compared with those from the modified Lagrangian model. Adaption of three different turbulent models were accompanied with this procedure. Look-up table of the ignition characteristic time scale which is one of important factors of the Lagrangian model was referred to the 11-step reduced mechanism. Eventually, results with the Lagrangian model show a good accordance with experimental results, which shows the validity of this model. Results from Chen's model differ from those of the others. Numerical results of ${\widetilde{k}$ show significant deviation from experimental results for three models.

• International Journal of Aeronautical and Space Sciences
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• 제13권2호
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• pp.188-198
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• 2012

An improvement to the k-${\varepsilon}$ turbulence model is presented and is shown to lead to better agreement with data regarding supersonic base flows. The improvement was achieved by imposing a grid-independent realizability constraint in the Launder-Sharma k-${\varepsilon}$ model. The effects of compressibility were also examined. The numerical results show that the modified Launder-Sharma model leads to some improvement in the prediction of the velocity and turbulent kinetic energy profiles. Compressibility corrections also lead to better agreement in both the turbulent kinetic energy and the Reynolds stress profiles with the experimental data.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2003년도 The Fifth Asian Computational Fluid Dynamics Conference
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• pp.13-14
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• 2003

Two different ${\kappa}-{\varepsilon}-\overline{{\nu}{\nu}}-f$ turbulence models together with the two-layer model are evaluated for natural convection in a rectangular cavity. The numerical problem and accuracy of the turbulence models are discussed. The original $\overline{{\nu}{\nu}}-f$ model suffers from the numerical stiffness problem when used with the segregate solution procedure like the SIMPLE algorithm, and a remedy for this problem is proposed. It is shown that original $\overline{{\nu}{\nu}}-f$ model best predicts the mean velocity, Reynolds stresses and the turbulent heat flux while the modified $\overline{{\nu}{\nu}}-f$ model (N=6) overpredicts the turbulent quantities.

• 대한기계학회논문집B
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• 제25권3호
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• pp.305-314
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• 2001

A modified model is proposed for calculation of transitional boundary layer flows. In order to develop the eddy viscosity model for the problem, the flow is divided into three regions; namely, pre-transition region, transition region and fully turbulent region. The pre-transition eddy-viscosity is formulated by extending the mixing length concept. In the transition region, the eddy-viscosity model employs two length scales, i.e., pre-transition length scale and turbulent length scale pertaining to the regions upstream and the downstream, respectively, and a universal model of stream-wise intermittency variation is used as a function bridging the pre-transition region and the fully turbulent region. The proposed model is applied to calculate three benchmark cases of the transitional boundary layer flows with different free-stream turbulent intensity (1%∼6%) under zero-pressure gradient. It was found that the profiles of mean velocity and turbulent intensity, local maximum of velocity fluctuations, their locations as well as the stream-wise variation of integral properties such as skin friction, shape factor and maximum velocity fluctuations are very satisfactorily predicted throughout the flow regions.

• 한국자동차공학회논문집
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• 제2권5호
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• pp.48-57
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• 1994

The specific power output and thermal effeciency of any two-stroke engine are dependent on its scavenging behavior. Among the many factors which influence on the scavenging process, the cylinder head shape is one of the important factor. Hence in this study three different type models of cylinder head shape which are the cylindrical, the spherical and the arbitrary shape are studied to show the effects of the turbulent scavenging process in the cylinder with one inlet port, two side ports and one exhaust port. A modified version of KIVA-II which strip out of or add planes of cells across the mesh above the piston for flow simulation of two-stroke engine is used. The $k-{\varepsilon}$ turbulent model is used. The results show that the flow in a two-stroke engine cylinder of the spherical head shape among the three different type model is a desirable for efficient scavenging.

• 한국연소학회:학술대회논문집
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• 한국연소학회 1997년도 제15회 KOSCO SYMPOSIUM 논문집
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• pp.163-173
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• 1997

This article presents an application of a large-scale structural mixing model (Broadwell et al. 1984) to the blowout of turbulent reacting jets discharging perpendicularly into an unconfined cross air-flow. In an analysis of a common stability curve, a plausible explanation can be made that the phenomenon of blowout is related only to the mixing time scale of the two flows. The most notable observation is that the blowout distance is traced at fixed positions at all times according to the velocity ratio R. Measurements of the lower blowout limits in the liftable flame agree qualitatively with the blowout parameter ${\varepsilon}$, proposed by Broadwell et al. Good agreement between the results calculated by a modified blowout parameter ${\varepsilon}^'$ and experimental results confirms the important effect of a large-scale structure in specifying the stabilization feature of blowouts.

• Nuclear Engineering and Technology
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• 제5권1호
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• pp.30-37
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• 1973

양극산화 피막의 임피단스(impedance)에 대한 니시다니식은 p-i-n 모델에 기초를 두고 $\omega$$\varepsilon$$\rho$$_{ο}$<<4$\pi$<<$\omega$$\varepsilon$$\rho$$_{\omega$}$의 가정하에서 유도된 것이다. 여기서 $\omega$는 각 주파수, $\varepsilon$는 유전상수, $\rho$$_{ο}$ and $\rho$$_{\omega}$는 양극산화피막의 계면과 중간영역의 비저항이다. 그러나 이 식의 파라데타를 전부 계산할 수 없기 때문에 이 식으로 양극산화피막의 물리적 모형을 분명히 할 수가 없다. 그러므로 $\omega$$\tau$$_{\omega}$1과 In(1+$\omega$$^2$$\tau$$^2$$_{ο}$)<<1이란 가정을 하여 임피단스에 대한 수정된 식을 유도하였다. 여기서 $\tau$$_{\omega}$=$\varepsilon$$\rho$$_{\omega}$/(4$\pi$) 및 $\tau$$_{ο}$=$\varepsilon$$\rho$$_{ο}$/(4$\pi$)로 정의된다. 양극산화피막을 가열하였을 때의 주파수톡성의 변화를 이 수정된 식으로 설명하였다. 양극산화피막을 가열하였을 매 양극산화피막의 임피단스의 변화는 주로 양극산화피막의 확산층의 증가와 비저항의 감소때문이라고 해석하였다.

• Wind and Structures
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• 제30권1호
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• pp.69-83
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• 2020

Modelling incompressible, neutrally stratified, barotropic, horizontally homogeneous and steady-state atmospheric boundary layer (ABL) is an important aspect in computational wind engineering (CWE) applications. The ABL flow can be viewed as a balance of the horizontal pressure gradient force, the Coriolis force and the turbulent stress divergence. While much research has focused on the increase of the wind velocity with height, the Ekman layer effects, entailing veering - the change of the wind velocity direction with height, are far less concerned in wind engineering. In this paper, a modified k-ε model is introduced for the ABL simulation considering wind veering. The self-sustainable method is discussed in detail including the precursor simulation, main simulation and near-ground physical quantities adjustment. Comparisons are presented among the simulation results, field measurement values and the wind profiles used in the conventional wind tunnel test. The studies show that the modified k-ε model simulation results are consistent with field measurement values. The self-sustainable method is effective to maintain the ABL physical quantities in an empty domain. The wind profiles used in the conventional wind tunnel test have deficiencies in the prediction of upper-level winds. The studies in this paper support future practical super high-rise buildings design in CWE.