• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.7
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• pp.1866-1872
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• 1994

Turbulent flow in a channel with a square rib periodically mounted on one wall is studied by large-eddy simulation(LES). An efficient 3D Navier-Stokes solver has been written for this geometry using a fractional step method and a multi-grid technique. The Reynolds number considered is 82, 000 based on the mean velocity above the obstacle height. Near-wall turbulence is approximated by a wall-layer model based on the turbulence intensity at the grid point nearest a solid wall. The results show a good qualitative agreement with experiments currently available for a single rib, indicating that LES can be a useful tool in simulating complex turbulent flows.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.4
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• pp.504-510
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• 2003

Simultaneous measurements of turbulent velocity and concentration field in a stirred mixer tank are carried out by using PIV/LIF technique. Instantaneous velocity fields are measured with a 1K$\times$1K CCD camera adopting the frame straddle method while the concentration fields are obtained by measuring the fluorescence intensity of Rhodamine B tracer excited by the second pulse of Nd:Yag laser light. Image distortion due to the camera view-angle is compensated by a mapping function. It is found that the general features of the mixing pattern are quite dependent on the local flow characteristics during the rapid decay of mean concentration. However, the small scale mixing seems to be independent on the local turbulent velocity fluctuation.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.5
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• pp.1717-1726
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• 1991

본 연구에서는 낮은 레이놀즈수 영역에도 적용될 수 있는 레이놀즈응력모델의 개발을 위해, 우선 벽근처 영역에서 사용되는 실험식(벽법칙)을 Hassid와 Poreh에 의 해 개발된 1-방정식모델로 대체하고 이를 레이놀즈응력모델과 접속시키는 방식을 사용 하였다. Hassid-Poreh의 1-방정식모델은 이미 Gibson등에 의해 그 성능이 평가되어 압력구배가 크지 않은 경계층유동의 낮은 레이놀즈수 영역에서 매우 좋은 결과를 보여 줌이 밝혀졌다. 본 연구에서는 곡면위의 난류경계층에 대해 위에서 설명한 바 있는 난류모델을 적용함에 있어 Gillis등과 Gibson등에 의해 실험된, 각각 곡률이 큰 경우 와 작은 경우의 대표적인 유동을 선택하여 모델의 성능을 시험하였다. 1-방정식모델 내에 포함된 길이차원(length scale)에 대해서는 곡률을 고려한 수정이 이루어졌다.

• Journal of Mechanical Science and Technology
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• v.18 no.1
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• pp.173-182
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• 2004

The present study has focused on numerical investigation on the flame structure, flame lift-off and stabilization in the partially premixed turbulent lifted jet flames. Since the lifted jet flames have the partially premixed nature in the flow region between nozzle exit and flame base, level set approach is applied to simulate the partially premixed turbulent lifted jet flames for various fuel jet velocities and co-flow velocities. The flame stabilization mechanism and the flame structure near flame base are presented in detail. The predicted lift-off heights are compared with the measured ones.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.7
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• pp.973-983
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• 2003

The suitability of high-order accurate, centered and upwind-biased compact difference schemes is evaluated for large eddy simulation of turbulent flow. Two turbulent flows are considered: turbulent channel flow at Re = 23000 and flow over a circular cylinder at Re = 3900. The effects of numerical dissipation on the finite differencing and aliasing errors and the subgrid-scale stress are investigated. It is shown through the simulations that compact upwind schemes are not suitable for LES, whereas the fourth order-compact centered scheme is a good candidate for LES provided that proper dealiasing of nonlinear terms is performed. The classical issue on the aliasing error and the treatment of nonlinear terms is revisited with compact difference schemes.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.9
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• pp.1227-1236
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• 2001

A systematic analysis is made of suboptimal control for drag reduction. The influence of the amplitude of actuation (A) and the time scale of actuation ($\Delta$t(sub)a(sup)+) is evaluated. Two wall sensing variables are employed (∂w/∂y│(sub)w and ∂p/∂z│(sub)w) with two wall actuations (${\Phi}$$_2$and ${\Phi}$$_3$). To test the suboptimal control, direct numerical simulations of turbulent channel flow at Re(sub)$\tau$=100 are performed in a spectral domain. It is found that the effect of A and $\Delta$t(sub)a(sup)+∼1. The near-wall behaviors of flow structure are analyzed to characterize the drag reduction. The size effect of the sensor/actuator is examined.

• International Journal of Aeronautical and Space Sciences
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• v.11 no.1
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• pp.41-48
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• 2010

In the present study, the turbulent flow fields around a circular cylinder at $Re=3.6{\times}10^6$ were investigated based on an unstructured mesh technique, and the comparisons between URANS(S-A, SST) and hybrid RANS/LES(DES, SAS) methods for the simulation of high Reynolds number flow have been conducted. For this purpose, unsteady characteristics of vortex shedding and time-averaged quantities were compared. A quasi-steady solution-adaptive mesh refinement was also made for the URANS and hybrid RANS/LES approaches. The results showed that the simple changes in the turbulent length scale or source term of turbulent models made the flow fields less dissipative and more realistic in hybrid RANS/LES methods than the URANS approaches.

• 한국연소학회:학술대회논문집
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• 2015.12a
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• pp.69-72
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• 2015

Computational fluid dynamics (CFD) modeling of large-scale coal-fired boilers requires a complicated set of flow, heat transfer and combustion process models based on different degrees of simplification. This study investigates the influence of coal devolatilization, char conversion and turbulent gas reaction models in CFD for a tangential-firing boiler at 500MWe capacity. Devolatilization model is found out not significant on the overall results, when the kinetic rates and the composition of volatiles were varied. In contrast, the turbulence mixing rate influenced significantly on the gas reaction rates, temperature, and heat transfer rate on the wall. The influence of char conversion by the unreacted core shrinking model (UCSM) and the 1st-order global rate model was not significant, but the unburned carbon concentration was predicted in details by the UCSM. Overall, the effects of the selected models were found similar with previous study for a wall-firing boiler.

• 한국가시화정보학회:학술대회논문집
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• 2002.11a
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• pp.101-104
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• 2002

Simultaneous measurement of turbulent velocity and concentration field in a stirred mixer tank is carried out by using PIV/LIF technique. Instantaneous velocity fields are measured by a $1K\times1K$ CCD camera adopting the frame straddle method while the concentration fields are obtained by measuring the fluorescence intensity of Rhodamine B tracer excited by the second pulse of Nd:Yag laser light. Image distortion due to the camera view-angle is compensated by a mapping function. It is found that the general features of the mixing pattern are quite dependent on the local flow characteristics during the rapid decay of mean concentration. However, the small scale mixing seems to be independent on the local turbulent velocity fluctuation.

• Journal of Environmental Science International
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• v.12 no.3
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• pp.265-274
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• 2003

The Pasquill-Gifford stability category is a very important scheme of the Gaussian type dispersion model defined the complex turbulence state of the atmosphere by A grade(very unstable) to F grade(very stable). But there has been made a point out that this stability category might decrease the predictability of the model because it was each covers a broad range of stability conditions, and that they were very site specific. The APSM (Air Pollution Simulation Model) was composed of the turbulent parameters, i.e. friction velocity(${\mu}$$\_$*/), convective velocity scale($\omega$$\_$*/) and Monin-Obukhov length scale(Ｌ) for the purpose of the performance increasing on the case of the unstable atmospheric conditions. And the PDF (Probability Density Function)model was used to express the vertical dispersion characteristics and the profile method was used to calculate the turbulent characteristics. And the performance assessment was validated between APSM and EPA regulatory models(TEM, ISCST), tracer experiment results. There were very good performance results simulated by APSM than that of TEM, ISCST in the short distance (<1415 m) from the source, but increase the simulation error(%) to stand off the source in others. And there were differences in comparison with the lateral dispersion coefficient($\sigma$$\_$y/) which was represent the horizontal dispersion characteristics of a air pollutant in the atmosphere. So the different calculation method of $\sigma$$\_$y/ which was extrapolated from a different tracer experiment data might decrease the simulation performance capability. In conclusion, the air pollution simulation model showed a good capability of predict the air pollution which was composed of the turbulent parameters compared with the results of TEM and ISCST for the unstable atmospheric conditions.