Earticle

본 연구는 Large Eddy Simulation (LES) 기법을 활용하여 수직 원통형 해양 구조물 주변에서의 동역학적 난류 유동 특성을 수치적으로 분석하였다. OpenFOAM의 olaFlow와 overOlaDyMFlow를 사용하여 물과 공기 사이의 자유수면 상호작용을 재현하였으며, 이를 덴마크 DHI 수리모형실험 데이터를 통해 검증하였다. DHI 수리모형실험은 수직 실린더 주변에서 파랑에 의해 발생하는 파랑 하중과 수면 변위 시계열을 측정한 데이터를 기반으로 한다. 수리모형실험과 비교를 통해 수치모형의 신뢰성을 평가하였다. 본 연구에서는 snappyHexMesh와 oversetMesh 두 가지 격자 생성 기법을 비교하여 파랑-실린더 간의 상호작용 모의에서 각 기법의 성능을 평가하였다. snappyHexMesh는 실린더 근처의 경계층 유동과 와류구조를 정밀하게 포착하는 데 탁월한 성능을 보였으며, oversetMesh는 복잡한 경계 조건과 넓은 계산 영역을 효과적으로 처리하는 데 강점을 보였다. 두 기법 모두 파고와 파랑 하중에 대해 실험 결과와 높은 일치성을 나타내었지만, 난류 유동과 전단 응력 분포에서는 미세한 차이가 확인되었다. 본 연구는 해양 구조물 주변에서 파랑에 의해 유발되는 힘과 난류를 수치적으로 모의하는 데 격자 기법에 대한 통찰을 제공하며, 이러한 결과는 향후 해양 구조물 설계 및 안정성 평가 연구에 기여할 수 있다.

This study analyzes turbulent flow characteristics generated by the interaction between regular sinusoidal waves and a vertical cylinder using large eddy simulation methods. Numerical simulations are conducted with olaFlow and overOlaDyMFlow. An interface compression method is used to resolve the water and air interactions. Model results are validated against experimental data obtained from DHI hydraulic facilities in Denmark. Additionally, two meshing techniques, snappyHexMesh and oversetMesh are evaluated relative to their performance in modeling wave-cylinder interactions. Model results demonstrate that the unstructured grid with snappyHexMesh accurately captures the detailed boundary-layer flow and vortex structures near the cylinder, while the multi-layer features in oversetMesh effectively handles complex boundary conditions in larger computational domains. Both meshing methods show good agreement with experimental results for wave height and fluid forces. However, slight discrepancies in shear stress distribution highlight the unique strengths of each technique. This research provides critical insights for numerical modeling of wave-induced forces and turbulence evolution around offshore structures, offering guidance for future designs and safety assessments.

Large eddy simulation of turbulent premixed flame in turbulent channel flow is studied by using G-equation. A flamelet model for the premixed flame is combined with a dynamic subgrid combustion model for the filtered propagation flame speed. The objective of this work is to investigate the validity of the dynamic subgrid G-equation model to a complex turbulent premixed flame. The effect of model parameters of the dynamic sub grid G-equation on the turbulent flame speed is investigated. In order to consider quenching of laminar flames on the wall, wall-quenching damping function is employed in this calculation. In the present study, a constant density turbulent channel flow is used. The calculation results are evaluated by comparing with the DNS results of Bruneaux et al.

The dynamic mixed model (DMM) combined with a box filter of Zang et. al. (1993) has been generalized for passive scalar transport and applied to large eddy simulation of turbulent channel flows with Prandtl number up to 10. Results from a priori test showed that DMM is capable of predicting both subgrid-scale (SGS) scalar flux and dissipation rather accurately for the Prandtl numbers considered. This would suggest that the favorable feature of DMM, originally developed for the velocity field, works equally well for scalar transport problem. The validity of the DMM has also been tested a posteriori. The results of the large eddy simulation showed that DMM is superior to the dynamic Smagorinsky model in the prediction of scalar field and the model performance of DMM depends to a lesser degree on the ratio of test to grid filter widths, unlike in the a priori test.

본 연구에서는 OpenFOAM에서 제공하는 소스 코드를 이용하여 폭-수심비가 2인 직사각형 개수로 흐름에 대해 수치모의를 수행하였다. 여과된 연속 방정식과 운동량 방정식을 해석하기 위하여 큰 와 수치모의를 이용하였고, 비등방성 잔여 응력항을 산정하기 위하여 Smagorinsky 모형(1963)을 사용하였다. LES 모형을 Tominaga et al. (1989)의 폭-수심비가 2인 실험수로에 적용하고 평균흐름 및 난류량을 비교하였다. 추가로 Nezu and Rodi (1985)의 실험 결과와 Shi et al. (1999)의 LES 모의 결과와 함께 비교를 수행하였다. 비교 결과 평균흐름 및 난류량 모두 기존 실험 및 모의 결과를 잘 재현하는 것으로 확인되었다. 특히 이차흐름 분포도에서 측벽과 자유수면의 접합부에서 발생하는 내부이차흐름이 발생하는 것을 확인하였다. 또한 수심방향 난류강도의 경우 측벽과 바닥벽에서 난류강도의 등치선도가 측벽과 바닥벽의 접합부 방향으로 편향되는 현상을 확인하였다.

This study presents numerical simulation of turbulent flows in a rectangular open-channel that has a width-to-depth ratio of 2 using the source code provided by OpenFOAM. Large eddy simulations are carried out by solving the filtered continuity and momentum equations numerically. For the non-isotropic residual stress term, Smagorinsky's (1963) model is used. The flow in the open-channel whose width-to-depth ratio is 2, from experiment of Tominaga et al. (1989), is simulated numerically. Simulation results are compared with measured data by Tominga et al. (1989) and Nezu and Rodi (1985) and with LES data by Shi et al. (1999). Comparisons revealed that the model simulates the mean flow and turbulence statistics well. Specifically, the model reproduced the inner secondary currents located at the corner of sidewall and free surface successfully. In addition, the vortical component of turbulence intensity shows bulged contours towards the bottom edge.

In this work, mixing processes of hot and cold fluids of three different jet types are predicted by large-eddy simulation (LES) on FLUENT platform. Temperature at different positions of internal wall and mixing conditions of T-junctions at different times are obtained, then the simulated normalized mean and root-mean square (RMS) temperature, temperature contour and velocity vector of every case are compared. The results indicate that, the mixing regions in the tee junction is related to the jet type, and temperature fluctuations on the pipe wall in the type of the deflecting jet is the least.

In order to verify the feasibility of the method of numerical simulation of supersonic noise, the flow field and noise at far field of a small solid fuel rocket engine F tested by National Aeronautics and Space Administration's Langley Research Center (NASA) was simulated by computational fluid dynamics (CFD) and computational aeroacoustics (CAA). The conjunction with large eddy simulation and FW-H integral method is adopted to simulate spectrums of the noise. The conjunction with large eddy simulation and Möhring sound analogy is adopted to predict the direction of the noise at the near-field. Comparing the result with the experimental data, the flow field coincides well, and in the sound field, the low and medium-frequency sound pressure level coincides well too, and the high-frequency sound pressure level is slightly lower than the estimated value. The error of overall sound pressure level of the measure point is small. The sound field resulted by high temperature, high pressure and high velocity fluid is simulated successfully.

The multi-environment PDF model coupled with flamelet generated manifolds(FGM) has been developed for a large eddy simulation of turbulent partially premixed lifted flame. This approach has a capability to realistically account for the transport and evolution of probability density function for mixture fraction and progress variable with the manageable computational burden. Using the tabulated chemistry, it is possible to track radical distributions which is important to predict autoignition process with the vitiated coflow environment. Numerical results indicate that the present yields the good agreement with experimental data in terms of mixture fraction, temperature, and species mass fractions.

The turbulent flow behavior of air supply and exhaustion in the Shin-gum-ho subway station is analyzed for ordinary and emergency state. The depth of Shin-gum-ho station is 43.6m which consists of the island-type platform(8th floor in underground) and a two-story lobby (first & second floor in underground). An emergency stairway connects between the platform and the lobby. Ventilation operation mode for ordinary state is set up as a combination of air supply and exhaustion in the lobby and platform, while for emergency state it is set up as a full air supply in the lobby and a full exhaustion in the platform. The entire station is covered for simulation. The ventilation diffusers are modeled as 95 square shapes of $0.6m{\times}0.6m$ in the lobby and as 222 square shapes of $0.6m{\times}0.6m$ and 4 rectangular shapes of $1.2m{\times}0.8m$ in the platform. The total of 7.5million grids are generated and whole domain is divided to 22 blocks for MPI efficiency of calculation. Large eddy simulation(LES) is applied to solve the momentum equation and Smagorinsky model($C_s$=0.2) is used as SGS(subgrid scale) model. The time-averaged velocity fields are compared to experimental data and show a good agreement with it.

스월 연소기의 비반응 난류 유동특성을 파악하기 위하여 3차원 Large Eddy Simulation(LES)을 수행하였다. 연소기는 GEAE LM6000 연소기를 이용하였으며, 실제 실험 결과에 따른 인젝터 유입 형상을 적용하였다. 주 흐름 부분에서 강한 vortex breakdown, 중심 재순환영역, 모서리 재순환영역, 축방향으로 전진하는 스월링 형상, 주기적으로 나타나는 난류 구조를 관찰하였다. 계산된 결과는 실제 실험결과와 선행연구자들의 LES 계산결과와 비교하여 잘 맞음을 확인하였다.

Production and dissipation of turbulent structure in a swirl stabilized combustor was investigated using three-dimensional Large Eddy Simulation analysis. The combustor of concern is the LM6000, lean premixed dry low-NOx annular combustor, developed by GEAE. Inlet condition was based on experimental data. Strong vortex breakdown in main stream, vortex ring proceeding downstream, and the turbulent structure periodically oscillating have been observed. Reasonable agreement was obtained by comparison of the results with experiments and previous LES studies.

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.

연소실 안으로 분출되는 스월 유동의 vortex breakdown mechanism에 대한 연구를 하였다. 3차원 유한 체적기법과 Runge-Kutta 시간 적분법이 적용되었으며, 난류모델은 dynamic large eddy simulation (DLES)이 적용되었다. 계산 시간의 효율성과 기억용량을 효과적으로 사용하기 위하여 message passing interface (MPI) 병렬계산 기법이 적용되었다. 스월 난류 유동에 있어서 vortex breakdown 거동을 가시적으로 표착 하였는데, 이는 스월 유동에 의한 난류 응력 증대, 난류 생성/소산율 증대 및 혼합율 증대에 대한 실험적 근거를 뒷받침하는 매우 중요한 결과이다. 또한 평균 속도와 난류 운동에너지에 대한 계산 결과도 실험 결과와 비교하였다.

Two aspects of Large-Eddy Simulation(LES) are investigated in order to improve its performance. The first one is on how to determine the model coefficient in conjunction with a dynamic subgrid-scale model, and the second one is on a wall-layer model(WLM) which allows one to skip near-wall regions to save a large number of grid points otherwise required. Especially, a WLM suitable for a separated flow is considered. Firstly, an averaging technique to calculate the model coefficient of dynamic subgrid-scale modeling(DSGSM) is introduced. The technique is based on the concept of local averaging, and useful to stabilize numerical solution in conjunction with LES of complex turbulent flows using DSGSM. It is relatively simple to implement, and takes very low overhead in CPU time. It is also able to detect the region of negative model coefficient where the "backscattering" of turbulence energy occurs. Secondly, a wall-layer model based on a local turbulence intensity is considered. It locally determines wall-shear stresses depending on the local flow situations including separation, and yields better predictions in separated regions than the conventional WLM. The two techniques are tested for a turbulent obstacle flow, and show the direction of further improvements.rovements.

The hydrogen behavior in a nuclear containment vessel is a significant issue when discussing the potential of hydrogen combustion during a severe accident. After the Fukushima-Daiichi accident in Japan, we have investigated in-depth the hydrogen transport mechanisms by utilizing experimental and numerical approaches. Computational fluid dynamics is a powerful tool for better understanding the transport behavior of gas mixtures, including hydrogen. This paper describes a Large-eddy simulation of gas mixing driven by a high-buoyancy flow. We focused on the interaction behavior of heat and mass transfers driven by the horizontal high-buoyant flow during density stratification. For validation, the experimental data of the Containment InteGral effects Measurement Apparatus (CIGMA) facility were used. With a high-power heater for the gas-injection line in the CIGMA facility, a high-temperature flow of approximately 390 ℃ was injected into the test vessel. By using the CIGMA facility, we can extend the experimental data to the high-temperature region. The phenomenological discussion in this paper helps understand the heat and mass transfer induced by the high-buoyancy flow in the containment vessel during a severe accident.

Subchannel code is one of the effective simulation tools for thermal-hydraulic analysis in nuclear reactor core. In order to reduce the computational cost and improve the calculation efficiency, empirical correlation of turbulent mixing coefficient is employed to calculate the lateral mixing velocity between adjacent subchannels. However, correlations utilized currently are often fitted from data achieved in central channel of fuel assembly, which would simply neglect the wall effects. In this paper, the CFD approach based on spectral element method is employed to predict turbulent mixing phenomena through gaps in 3 × 3 bare tight lattice rod bundle and investigate the flow pulsation through gaps in different positions. Re = 5000,10000,20500 and P/D = 1.03 and 1.06 have been covered in the simulation cases. With a well verified mesh, lateral velocities at gap center between corner channel and wall channel (W-Co), wall channel and wall channel (W-W), wall channel and center channel (W-C) as well as center channel and center channel (C-C) are collected and compared with each other. The obvious turbulent mixing distributions are presented in the different channels of rod bundle. The peak frequency values at W-Co channel could have about 40%-50% reduction comparing with the C-C channel value and the turbulent mixing coefficient β could decrease around 25%. corrections for β should be performed in subchannel code at wall channel and corner channel for a reasonable prediction result. A preliminary analysis on fluctuation at channel gap has also performed. Eddy cascade should be considered carefully in detailed analysis for fluctuating in rod bundle.

The appearance of a rivulet at the upper surface of a stay cable is responsible for rain-wind-induced vibration (RWIV) of cables of cable-stayed bridges. However, the formation mechanism of the upper rivulet and its aerodynamic effects on the stay cable has not been fully understood. Large eddy simulation (LES) method is used to investigate flow around and aerodynamics of a circular cylinder with an upper rivulet at a Reynolds number of 140,000. Results show that the mean lift coefficients of the circular cylinder experience three distinct stages, zero-lift stage, positive-lift stage and negative-lift stage as the rivulet located at various positions. Both pressure-induced and friction-induced aerodynamic forces on the upper rivulet are helpful for its appearance on the upside of the stay cable. The friction-induced aerodynamic forces, which have not been considered in the previous theoretical models, may not be neglected in modeling the RWIV. In positive-lift stage, the shear layer separated from the upper rivulet can reattach on the surface of the cylinder and form separation bubbles, which result in a high non-zero mean lift of the cylinder and potentially induces the occurrence of RWIV. The separation bubbles are intrinsically unsteady flow phenomena. A serial of small eddies first appears in the laminar shear layer separated from the upper rivulet, which then coalesces and reattaches on the side surface of the cylinder and eventually sheds into the wake.

GASFLOW-MPI is a widely used scalable computational fluid dynamics numerical tool to simulate the fluid turbulence behavior, combustion dynamics, and other related thermal-hydraulic phenomena in nuclear power plant containment. An efficient scalable linear solver for the large-scale pressure equation is one of the key issues to ensure the computational efficiency of GASFLOW-MPI. Several advanced Krylov subspace methods and scalable preconditioning methods are compared and analyzed to improve the computational performance. With the help of the powerful computational capability, the large eddy simulation turbulent model is used to resolve more detailed turbulent behaviors. A backward-facing step flow is performed to study the free shear layer, the recirculation region, and the boundary layer, which is widespread in many scientific and engineering applications. Numerical results are compared with the experimental data in the literature and the direct numerical simulation results by GASFLOW-MPI. Both time-averaged velocity profile and turbulent intensity are well consistent with the experimental data and direct numerical simulation result. Furthermore, the frequency spectrum is presented and a -5/3 energy decay is observed for a wide range of frequencies, satisfying the turbulent energy spectrum theory. Parallel scaling tests are also implemented on the KIT/IKET cluster and a linear scaling is realized for GASFLOW-MPI.

A large eddy simulation (LES) of a turbulent channel flow is performed by using the third order low-storage Runge-Kutta method in time and second order finite difference formulation in space with staggered grid at a Reynolds number, $Re_{\tau}=590$ based on the channel half width, ${\delta}$ and wall shear velocity, $u_{\tau}$. To reduce the calculation cost of LES, algebraic wall model (AWM) is applied to approximate the near-wall region. The computation is performed in a domain of $2{\pi}{\delta}{\times}2{\delta}{\times}{\pi}{\delta}$ with $32{\times}20{\times}32$ grid points. Standard Smagorinsky model is used for subgrid-scale (SGS) modeling. Essential turbulence statistics of the flow field are computed and compared with Direct Numerical Simulation (DNS) data and LES data using no wall model. Agreements as well as discrepancies are discussed. The flow structures in the computed flow field have also been discussed and compared with LES data using no wall model.

Due to rapid progress in the performance of high-end computers, numerical prediction of fluid flow and flow-induced sound is expected to become a vital tool for aero- and hydro- dynamic design of various flow-related products. This presentation focuses on the applications of large-scale numerical simulations to complex engineering problems with a particular emphasis placed on the low-speed flows. Flow field computations are based on a large eddy simulation that directly computes all active eddies in the flow and models only those eddies responsible for energy dissipations. The sound generated from low-speed turbulent flows are computed either by direct numerical simulation or by decoupled methods, according to whether or not the feedback effects of the generated sound onto the source flow field can be neglected. Several numerical examples are presented in order to elucidate the present status of such computational methods and discussion on the future prospects will also be given.

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.

A Large Eddy Simulation (LES) of the flow in an inducer is carried out under flow rate oscillations. The present study focuses on the dynamic response of the backflow and the unsteady pressure performance to the flow rate fluctuations under non-cavitation conditions. The amplitude of angular momentum fluctuation evaluated by LES is larger than that evaluated by RANS. However, the phase delay of backflow is nearly the same as RANS calculation. The pressure performance curve exhibits a closed curve caused by the inertia effect associated with the flow rate fluctuations. Compared with simplified one dimensional evaluation of the inertia component, the component obtained by LES is smaller. The negative slope of averaged performance curve becomes larger under unsteady conditions. From the conservations of angular momentum and energy, an expression useful for the evaluation of unsteady pressure rise was obtained. The examination of each term of this expression show that the apparent decrease of inertia effects is caused by the response delay of Euler's head and that the increase of negative slope is caused by the delay of inertial term associated with the delay of backflow response. These results are qualitatively confirmed by experiments.