• 대한기계학회논문집B
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• 제27권6호
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• pp.813-820
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• 2003

The purpose of this research is to develop the model of turbulence modulation due to the presence of particles in various types of particle-laden flows Available experimental data are surveyed and the dependence of turbulence modulation of carrier-phase on particle size, concentration and. particle Reynolds number are examined. This study takes into account the effect of wake produced by particle, the drag between phases and the velocity gradient in the wake to estimate the production of turbulence. The model of turbulence modulation using the mixing length theory under the assumption of equilibrium flow is proposed. Numerical results show that the model is successful in predicting the characteristics of the particle-laden in various flow conditions both qualitatively and quantitatively.

• Wind and Structures
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• 제10권6호
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• pp.511-532
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• 2007

The present study aims to generate turbulent inflow data to more accurately represent the turbulent flow around a square cylinder when the inflow turbulence level is significant. The modified random flow generation (RFG) technique in conjunction with a previously developed LES code is successfully adopted into a finite element based fluid flow solver to generate the required inflow turbulence boundary conditions for the three-dimensional (3-D) LES computations of transitional turbulent flow around a square cylinder at Reynolds number of 22,000. The near wall region is modelled without using wall approximate conditions and a wall damping coefficient is introduced into the calculation of sub-grid length scale in the boundary layer of the cylinder wall. The numerical results obtained from simulations are compared with each other and with the experimental data for different inflow turbulence boundary conditions in order to discuss the issues such as the synthetic inflow turbulence effects on the 3-D transitional flow behaviour in the near wake and the free shear layer, the basic mechanism by which stream turbulence interacts with the mean flow over the cylinder body and the prediction of integral flow parameters. The comparison among the LES results with and without inflow turbulence and the experimental data emphasizes that the turbulent inflow data generated by the present RFG technique for the LES computation can be a viable approach in accurately predicting the effects of inflow turbulence on the near wake turbulent flow characteristics around a bluff body.

• 대한토목학회논문집
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• 제31권5A호
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• pp.341-349
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• 2011

본 연구에서는 다양한 풍동실험을 통하여 기류 조건에 따른 트윈박스 거더 교량의 공기역학적 특성을 파악하는데 그 목적이 있다. 이를 위하여 자연적인 난류를 생성할 수 있는 능동 난류 발생장치를 개발하였고 검증하였다. 그리고 능동 난류 및 격자 난류 조건하에서 정적공기력, 비정상공기력 그리고 버페팅 응답 측정 실험을 수행하였다. 풍동실험 결과를 보면, 난류 적분길이는 교량의 정적공기력과 $A_1^*$를 제외한 플러터계수에는 영향을 주지 않는 것으로 나타났다. 그리고 난류 강도는 비정상공기력에 일부 영향을 미치고, 난류 적분길이 또한 일부 수직 방향 성분에 영향을 주는 것으로 나타났다.

• 한국환경과학회지
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• 제19권2호
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• pp.137-148
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• 2010

This paper investigates the characteristics of turbulence schemes. Turbulence closures are fundamental for modeling the atmospheric diffusion, transport and dispersion in the boundary layer. In particular, in non-homogeneous conditions, a proper description of turbulent transport in planetary boundary layer is fundamental aspect. This study is based on the Regional Atmospheric Modeling System (RAMS) and combines four different turbulence schemes to assess if the different schemes have a impact on simulation results of vertical profiles. Two of these schemes are Isotropc Deformation scheme (I.Def) and Anisotropic deformation scheme (A.Def) that are simple local scheme based on Smagorinsky scheme. The other two are Mellor-Yamada scheme (MY2.5) and Deardorff TKE scheme (D.TKE) that are more complex non-local schemes that include a prognostic equation for turbulence kinetic energy. The simulated potential temperature, wind speed and mixing ratio are compared against radiosonde observations from the study region. MY2.5 shows consistently reasonable vertical profile and closet to observation. D.TKE shows good results under relatively strong synoptic condition especially, mixing ratio simulation. Validation results show that all schemes consistently underestimated wind speed and mixing ratio but, potential temperature was somewhat overestimated.

• 대한기계학회논문집
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• 제18권4호
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• pp.920-932
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• 1994

The combustion phenomena of a reciprocating engine is one of the most important processes affecting performance and emissions. One effective way to improve the engine combustion is to control the motion of the charge inside a cylinder by means of optimum induction system design, because the flame speed is mainly determined by the turbulence at compression(TDC) process in S.I. engine. It is believed that the tumble and swirl motion generated during intake breaks down into small-scale turbulence in the compression stroke of the cycle. However, the exact nature of their relationship is not well known. This paper describes cycle resolved LDV measurement of turbulent flow inside the cylinder of a 4-valve engine under motoring(non-firing) conditions, and studies the effect of intake port configurations on the turbulence characteristics using following parameters ; Eulerian temporal autocorrelation coefficient, turbulence energy spectral density function, Taylor micro time scale, integral time scale, and integral length scale.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2008년도 춘계학술대회논문집
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• pp.652-655
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• 2008

Despite of the laminar-turbulent transition region co-exist with fully turbulence region around the leading edge of an airfoil, still lots of researchers apply to fully turbulence models to predict aerodynamic characteristics. It is well known that fully turbulent model such as standard k-${\varepsilon}$ model couldn't predict the complex stall and the separation behavior on an airfoil accurately, it usually leads to over prediction of the aerodynamic characteristics such as lift and drag forces. So, we apply correlation based transition model to predict aerodynamic performance of the NREL (National Renewable Energy Laboratory) Phase IV wind turbine. And also, compare the computed results from transition model with experimental measurement and fully turbulence results. Results are presented for a range of wind speed, for a NREL Phase IV wind turbine rotor. Low speed shaft torque, power, root bending moment, aerodynamic coefficients of 2D airfoil and several flow field figures results included in this study. As a result, the low speed shaft torque predicted by transitional turbulence model is very good agree with the experimental measurement in whole operating conditions but fully turbulent model(k-${\varepsilon}$) over predict the shaft torque after 7m/s. Root bending moment is also good agreement between the prediction and experiments for most of the operating conditions, especially with the transition model.

• 한국연소학회:학술대회논문집
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• 한국연소학회 1998년도 제17회 KOSCI SYMPOSIUM 논문집
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• pp.23-30
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• 1998

The stabilization characteristics of diffusion flame formed in the wake of a cylindrical flame holder were investigated. Distribution of turbulence intensity, concentration distribution of combustion gas, and ion currents were measured. The turbulence intensity in the wake of cylindrical- game holder is increased with increase of diameter or blockage ratio of grid. If the auxiliary fuel is injected into recirculation zone, the concentration of $C_3H_8$ is high, but the concentration of $CO_2$ is low at the boundary of recirculation zone. The region with highest average value of ion currents in the middle of flame is moved to the upstream side by the turbulent components of main stream. The flame mass with partially active reaction is moved fast for uniform flow and turbulence generator G3, but the flame mass with relatively slow reaction is moved slowly for turbulence generator G1.

• 한국추진공학회지
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• 제2권2호
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• pp.56-63
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• 1998

본 논문에서는 주유동의 난류강도 변화에 따른 복합각도로 분사되는 단일 막냉각홀 주위에서의 국소 열/물질전달계수 특성을 살펴보기 위하여 실험을 수행하였다. 단일 막냉각홀 시편은 평면에 대하여 $30^{\cire}$의 경사각을 가지고 있으며, 횡방향으로는 주유동에 대해 $45^{\cire}$의 각도로 분사하여 복합각도 분사시 주유동의 난류강도 변화에 따른 효과를 살펴보았다. 또한 막냉각제트의 분사율을 0.5에서 2.0까지 변화시켜가며, 주유동의 높은 난류강도조건에서 분사율 변화시 막냉각홀 주위의 열/물질전달특성에 미치는 효과를 살펴보았다. 주유동의 난류강도를 변화시키기 위하여 막냉각홀 상류에 난류발생격자를 설치하였다. 격자를 설치하지 않은 경우, 주유동의 난류강도는 0.5%이며, 난류발생격자의 종류 및 설치위치를 달리하여 난류강도를 3%에서 10%까지 변화시켜가며 실험하였다. 막냉각홀 주위에서 국소적인 열/물질전달계수 값을 얻기 위하여 물질전달 실험방법인 나프탈렌 승화법을 사용하였다. 주유동의 난류강도가 낮은 경우 분사홀 주변에서 막냉각제트 혹은 주유동에 의한 열/물질전달 촉진영역이 뚜렷한 경계를 갖지만, 난류강도가 증가하면서 전 영역에 걸쳐 열/물질전달이 촉진되었으며 주유동과 막냉각제트의 활발한 혼합작용으로 인해 영역의 구분이 점차 소멸되었다. 또한 주유동의 높은 난류강도 효과는 막냉각제트의 분사율이 높은 경우 뚜렷이 나타났다.

• 한국자동차공학회논문집
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• 제11권3호
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• pp.115-123
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• 2003

With the advent of high performance computers and more efficient numerical algorithms, computational fluid dynamics(CFD) has come out as a modem alternative for reducing the use of wind tunnels test in automotive engineering. However, in spite of the fact that many competent researchers have made all their talents in developing turbulence model over since the past dozen or more years, it has been an important impediment in using the CFD effectively to design machinery and to diagnose or to improve engineering problems in the industry since the turbulence model has been acting as the Achilles' tendon in aspect of the reliability even to this time. In this study, Reynolds-averaged Wavier-Stokes equations were solved to simulate an incompressible turbulent flow around a bus-like bluff body near ground plane. In order to investigate the effect of the discretisation schemes and turbulence model on the aerodynamic forces several turbulence models with five convective difference schemes are adopted. From the results of this study, it is clear that choice of turbulence model and discretisation scheme profoundly affects the computational outcome. The results also show that the adoption of RNG $k-\varepsilon$ turbulence model and nonlinear quadratic turbulence model with the second order accurate discretisation scheme predicts fairly well the aerodynamic coefficients.

• Wind and Structures
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• 제38권4호
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• pp.309-323
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• 2024

The gust factor and turbulence intensity are two crucial parameters that characterize the properties of turbulence. In tropical cyclones (TCs), these parameters exhibit significant variability, yet there is a lack of established formulas to account for their probabilistic characteristics with consideration of their inherent connection. On this condition, a probabilistic analysis of gust factors and turbulence intensities of TCs is conducted based on fourteen sets of wind data collected at the Sutong Cable-stayed Bridge site. Initially, the turbulence intensities and gust factors of recorded data are computed, followed by an analysis of their probability densities across different ranges categorized by mean wind speed. The Gaussian, lognormal, and generalized extreme value (GEV) distributions are employed to fit the measured probability densities, with subsequent evaluation of their effectiveness. The Gumbel distribution, which is a specific instance of the GEV distribution, has been identified as an optimal choice for probabilistic characterizations of turbulence intensity and gust factor in TCs. The corresponding empirical models are then established through curve fitting. By utilizing the Gumbel distribution as a template, the nexus between the probability density functions of turbulence intensity and gust factor is built, leading to the development of a generalized probabilistic model that statistically describe turbulence intensity and gust factor in TCs. Finally, these empirical models are validated using measured data and compared with suggestions recommended by specifications.