초음속 유동장에서 기저 유동의 Detached Eddy Simulation

신재렬(J.R. Shin),원수희(S.H. Won),최정열(J.Y. Choi) 한국전산유체공학회 2008 한국전산유체공학회 학술대회논문집 Vol.2008 No.-

Detached Eddy Simulation (DES) is applied to an axisymmetric base flow at supersonic mainstream. DES is a hybrid approach to modeling turbulence that combines the best features of the Reynolds-averaged Navier-Stokes (RANS) and large-eddy simulation (LES) approaches. In the Reynolds-averaged mode, the model is currently based on either the Spalart-Allmaras (S-A) turbulence model. In the large eddy simulation mode, it is based on the Smagorinski subgrid scale model. Accurate predictions of the base flowfield and base pressure are successfully achieved by using the DES methodology with less computational cost than that of pure LES and monotone integrated large-eddy simulation (MILES) approaches. The DES accurately resolves the physics of unsteady turbulent motions, such as shear layer rollup, large-eddy motions in the downstream region, small-eddy motions inside the recirculating region. Comparison of the results shows that it is necessary to resolve approaching boundary layers and free shear-layer velocity profiles from the base edge correctly for the accurate prediction of base flows. The consideration of an empirical constant CDES for a compressible flow analysis may suggest that the optimal value of empirical constant CDES may be larger in the flows with strong compressibility than in incompressible flows.

건물주위의 가스 확산사고에 대한 CFD 난류 해석기법 검토

고민욱,오창보,한용식,도규형 한국화재소방학회 2015 한국화재소방학회논문지 Vol.29 No.5

Three simulation approaches for turbulence were applied for the computation of propane dispersion in a simplified realscaleurban area with one building:, Large Eddy Simulation (LES), Detached Eddy Simulation (DES), and UnsteadyReynolds Averaged Navier-Stokes (RANS). The computations were performed using FLUENT 14, and the grid systemwas made with ICEM-CFD. The propane distribution depended on the prediction performance of the three simulationapproaches for the eddy structure around the building. LES and DES showed relatively similar results for the eddy structureand propane distribution, while the RANS prediction of the propane distribution was unrealistic. RANS was found tobe inappropriate for computation of the gas dispersion process due to poor prediction performance for the unsteady turbulence. Considering the computational results and cost, DES is believed to be the optimal choice for computation of the gasdispersion in a real-scale space. 도심지역을 단순 모델링한 실규모 공간에서 누출된 프로판 확산과정에 대해 대와동모사(Large Eddy Simulation:LES), 분리와동모사(Detached Eddy Simulation: DES) 및 비정상 레이놀즈평균기법(Reynolds Averaged Navier-Stokes:RANS)을 이용한 3가지 전산해석을 수행하였다. 전산해석은 FLUENT 14를 이용하였고 격자계는 ICEM-CFD를 이용하여 구성하였다. 그 결과 건물 주변의 프로판 농도분포는 주변 와구조와 밀접한 관련이 있어 이러한 와구조를 합리적으로예측하는 정도에 따라 농도분포가 크게 차이날 수 있음을 알았다. LES와 DES는 비교적 유사한 와구조와 프로판 농도분포를 보이지만 RANS는 너무 부드러운 농도분포를 보여 복잡한 비정상적인(Unstedy) 난류 유동장을 재현하는데 한계가 있어 가스연료 누출 초기의 농도분포 전개과정을 예측하는 데에는 어려움이 있을 것으로 판단된다. 해석결과와 계산시간까지 고려한다면 DES 방법이 실규모에서의 가스연료 누출 확산과정에 대한 CFD 해석방법으로 적합할 것으로 판단된다.

화물차 주위 유동의 성긴 격자 큰에디모사

이상승(S. Lee),김명균(M. Kim),유동현(D. You),김정재(J.J. Kim),이상준(S.J. Lee) 한국전산유체공학회 2016 한국전산유체공학회지 Vol.21 No.1

In order to investigate effects of grid resolution on large-eddy simulation of flow over a heavy vehicle, large-eddy simulations over the vehicle with coarse grid and fine grid are conducted. In addition, comparison of drag coefficients with the experimental data obtained by a wind tunnel experiment is conducted. Both of the drag coefficients of coarse grid and fine grid large-eddy simulation show good agreement with the experimental data. Flow fields obtained by the coarse and the fine grid large-eddy simulation are compared in the vehicle frontal-face region, the vehicle rear wheel region, and the vehicle base region. Coarse grid large-eddy simulation shows good agreement with the fine grid large-eddy simulation in the vehicle front face region and the vehicle rear wheel region, since the flow over the present vehicle is dominated by flow separation which is geometrically pre-determined, not by the skin friction which is known to be sensitive to grid resolution.

Large eddy simulation of shock vector control using bypass flow passage

Deng, Ruoyu,Setoguchi, Toshiaki,Kim, Heuy Dong Elsevier 2016 The International journal of heat and fluid flow Vol.62 No.2

<P><B>Abstract</B></P> <P>A detail description of the unsteady phenomena of three-dimensional shock vector control (SVC), including recirculation zones and shear layer regions, has been presented in this study. Shock vector control is a really efficient way to achieve flight direction control of high speed vehicle. Large eddy simulation (LES) has been applied to capture the unsteady characteristics of SVC method using bypass flow passage. Comparison of RANS and LES has been conducted in this study. LES model shows better results than others and it is able to capture the unsteady process very well. In this study, the separation bubble upstream of the injection port is the main source of flow unsteadiness. Large scale eddies in the whole flow field have been resolved by the LES model. Unsteady characteristics of SVC method at different nozzle pressure ratios (NPR) have been investigated. The time histories of thrust vector angle at different NPRs have been recorded by the LES model. The results indicate that it is possible to achieve SVC with the range of bypass mass flow ratio less than 7%. It is also revealed that nozzle pressure ratio has a strong effect on the unsteady phenomenon of SVC system.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Unsteady phenomenon of shock vector control. </LI> <LI> Large eddy simulation. </LI> <LI> Vortexes in the separation bubble. </LI> </UL> </P>

Large-eddy simulation of reactive pollutant exchange at the top of a street canyon

Han, Beom-Soon,Baik, Jong-Jin,Kwak, Kyung-Hwan,Park, Seung-Bu Elsevier 2018 Atmospheric environment Vol.187 No.-

<P><B>Abstract</B></P> <P>The exchange of reactive pollutants (NO, NO<SUB>2</SUB>, and O<SUB>3</SUB>) at the top (roof level) of a street canyon are investigated using the parallelized large-eddy simulation model (PALM). The transport equations of NO, NO<SUB>2</SUB>, and O<SUB>3</SUB> with simple photochemical reactions are combined within the LES model for this study. NO and NO<SUB>2</SUB> are emitted from an area source located near the canyon floor, and O<SUB>3</SUB> is included within the ambient air and inflow. A clockwise-rotating vortex appears in the street canyon and transports NO, NO<SUB>2</SUB>, and O<SUB>3</SUB>. NO and NO<SUB>2</SUB> are transported along the ground and leeward wall and escape from the canyon at the roof level. O<SUB>3</SUB> enters the canyon at the roof level and is transported along the windward wall. The mean O<SUB>3</SUB> production rate is generally negative with large magnitudes at and near the roof level and near the windward wall. The chemical reactions reduce the mean NO and O<SUB>3</SUB> concentrations in the canyon by 31% and 84%, respectively, and increase the mean NO<SUB>2</SUB> concentration in the canyon by 318%. The exchange of reactive pollutants at the roof level is significantly affected by small-scale eddies at the roof level and low- or high-speed streaks above the canyon. Air in the canyon with high NO and NO<SUB>2</SUB> concentrations escapes from the canyon when low-speed air parcel appears due to small-scale eddies at the roof level or low-speed streak above the canyon. In contrast, air outside the canyon with a high O<SUB>3</SUB> concentration enters the canyon when high-speed air parcel appears because of small-scale eddies at the roof level or high-speed streak above the canyon. The time-lagged correlation analysis reveals that NO, NO<SUB>2</SUB>, and O<SUB>3</SUB> concentrations near the ground are affected by low- or high-speed streaks above the canyon but not significantly affected by small-scale eddies at the roof level.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Reactive pollutant exchange in a street canyon is examined using an LES model. </LI> <LI> Small-scale eddies at the roof level affect reactive pollutant exchange. </LI> <LI> High- or low-speed streaks above the canyon affect reactive pollutant exchange. </LI> <LI> High- or low-speed streaks affect reactive pollutant concentration near the ground. </LI> </UL> </P>

Large-eddy simulation and wind tunnel study of flow over an up-hill slope in a complex terrain

Tsang, C.F.,Kwok, Kenny C.S.,Hitchcock, Peter A.,Hui, Desmond K.K. Techno-Press 2009 Wind and Structures, An International Journal (WAS Vol.12 No.3

This study examines the accuracy of large-eddy simulation (LES) to simulate the flow around a large irregular sloping complex terrain. Typically, real built up environments are surrounded by complex terrain geometries with many features. The complex terrain surrounding The Hong Kong University of Science and Technology campus was modelled and the flow over an uphill slope was simulated. The simulated results, including mean velocity profiles and turbulence intensities, were compared with the flow characteristics measured in a wind tunnel model test. Given the size of the domain and the corresponding constraints on the resolution of the simulation, the mean velocity components within the boundary layer flow, especially in the stream-wise direction were found to be reasonably well replicated by the LES. The turbulence intensity values were found to differ from the wind tunnel results in the building recirculation zones, mostly due to the constraints placed on spatial and temporal resolutions. Based on the validated mean velocity profile results, the flow-structure interactions around these buildings and the surrounding terrain were examined.

대기경계층 난류유동의 Large Eddy Simulation 해석기법의 알고리즘에 관한 연구

조창주,서성규,박종혁 國立 麗水大學校 環境問題硏究所 1998 環境硏究論文集 Vol.1 No.-

LES(Large eddy simulation) is an intermediate technique between the direct simulation of turbulent flows and the solution of the Reynold-averaged equations. The method is initiated by the introduction of a filtering operation which seperates the large-scale and small scale structures. The large scales are computed explicitly, whereas the small scale necessarily are modeled. In this study, a LES model used three-dimensional incompressible Navier-Stokes equation is calculated for the numerical simulation of turbulent flow in the PBL. Instantaneous and detailed flow of velocity component war obtain with LES and horizontal mean vertical profile of potential temperature was indicated typical profile in the PBL.

Large-eddy simulation analysis of turbulent flow over a two-blade horizontal wind turbine rotor

Taeyoung Kim,Donghyun You 대한기계학회 2016 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.30 No.11

Unsteady turbulent flow characteristics over a two-blade horizontal wind turbine rotor is analyzed using a large-eddy simulation technique. The wind turbine rotor corresponds to the configuration of the U.S. National Renewable Energy Laboratory (NREL) phase VI campaign. The filtered incompressible Navier-Stokes equations in a non-inertial reference frame fixed at the centroid of the rotor, are solved with centrifugal and Coriolis forces using an unstructured-grid finite-volume method. A systematic analysis of effects of grid resolution, computational domain size, and time-step size on simulation results, is carried out. Simulation results such as the surface pressure coefficient, thrust coefficient, torque coefficient, and normal and tangential force coefficients are found to agree favorably with experimental data. The simulation showed that pressure fluctuations, which produce broadband flow-induced noise and vibration of the blades, are especially significant in the mid-chord area of the suction side at around 70 to 95 percent spanwise locations. Large-scale vortices are found to be generated at the blade tip and the location connecting the blade with an airfoil cross section and the circular hub rod. These vortices propagate downstream with helical motions and are found to persist far downstream from the rotor.

Large Eddy Simulation of a High Reynolds Number Swirling Flow in a Conical Diffuser

Duprat, Cedric,Metais, Olivier,Laverne, Thomas Korean Society for Fluid machinery 2009 International journal of fluid machinery and syste Vol.2 No.4

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.

Wind load and wind-induced effect of the large wind turbine tower-blade system considering blade yaw and interference

S.T. Ke,X.H. Wang,Y.J. Ge 한국풍공학회 2019 Wind and Structures, An International Journal (WAS Vol.28 No.2

The yaw and interference effects of blades affect aerodynamic performance of large wind turbine system significantly, thus influencing wind-induced response and stability performance of the tower-blade system. In this study, the 5MW wind turbine which was developed by Nanjing University of Aeronautics and Astronautics (NUAA) was chosen as the research object. Large eddy simulation on flow field and aerodynamics of its wind turbine system with different yaw angles (0°, 5°, 10°,20°, 30° and 45°) under the most unfavorable blade position was carried out. Results were compared with codes and measurement results at home and abroad, which verified validity of large eddy simulation. On this basis, effects of yaw angle on average wind pressure, fluctuating wind pressure, lift coefficient, resistance coefficient, streaming and wake characteristics on different interference zone of tower of wind turbine were analyzed. Next, the blade-cabin-tower-foundation integrated coupling model of the large wind turbine was constructed based on finite element method. Dynamic characteristics, wind-induced response and stability performance of the wind turbine structural system under different yaw angle were analyzed systematically. Research results demonstrate that with the increase of yaw angle, the maximum negative pressure and extreme negative pressure of the significant interference zone of the tower present a V-shaped variation trend, whereas the layer resistance coefficient increases gradually. By contrast, the maximum negative pressure, extreme negative pressure and layer resistance coefficient of the non-interference zone remain basically same. Effects of streaming and wake weaken gradually. When the yaw angle increases to 45°, aerodynamic force of the tower is close with that when there’s no blade yaw and interference. As the height of significant interference zone increases, layer resistance coefficient decreases firstly and then increases under different yaw angles. Maximum means and mean square error (MSE) of radial displacement under different yaw angles all occur at circumferential 0° and 180° of the tower. The maximum bending moment at tower bottom is at circumferential 20°. When the yaw angle is 0°, the maximum downwind displacement responses of different blades are higher than 2.7 m. With the increase of yaw angle, MSEs of radial displacement at tower top, downwind displacement of blades, internal force at blade roots all decrease gradually, while the critical wind speed decreases firstly and then increases and finally decreases. The comprehensive analysis shows that the worst aerodynamic performance and wind-induced response of the wind turbine system are achieved when the yaw angle is 0°, whereas the worst stability performance and ultimate bearing capacity are achieved when the yaw angle is 45°.