• Proceedings of the Korea Water Resources Association Conference
• /
• 2021.06a
• /
• pp.244-244
• /
• 2021

난류흐름 거동은 지형이나 수공구조물과 같은 고체 경계면의 변화에 민감하게 반응하며 특징 또한 다양하다. 보나 여수로 등과 같은 단차 구조물을 통과하는 흐름은 구조물의 모서리 같은 흐름 경계면이 급변하는 지점에서는 흐름분리(flow separation)가 발생하는 것이 특징이다. 이러한 흐름분리로 인해 전단층이 발생하며 흐름 재순환(recirculation)이 구조물 하류부에 형성된다. 이 연구에서는 낙차공 형식의 단차 구조물 하류부에서의 흐름 거동을 이해하기 위해 CFD모델링을 통하여 계산된 3차원 유동장을 분석한다. 난류 모의는 하이브리드 LES(large-eddy simulation)/RANS 계산 기법인 IDDES(improved delayed detached-eddy simulation)기법을 적용한다. IDDES의 기본 모형으로는 k-ω SST모형과 Spalart-Allmaras모형을 이용하여 두 모형의 성능을 평가한다. 자유수면의 변동은 VoF(volume of fluid)기법을 이용하여 계산하며, 각 지배방정식은 최소의 수치분산을 유지하면서 수치해의 안정성을 확보할 수 있는 2차 정확도의 유한체적법을 이용하여 이산화하였다. 수치해석 결과는 레이놀즈수 23,400과 후르드수 0.22의 조건에서 기존에 계측된 자료와 비교하여 수치모형의 정확도를 평가하고 하상 단차 하류부에서의 흐름 거동 특성을 분석한다. 계산 결과는 공학적으로 널리 사용되는 RANS 수치모의에서 볼 수 없는 전단층과 난류구조의 동적 거동 특성과 이에 따른 레이놀즈 응력분포의 특성을 설명해준다.

• Wind and Structures
• /
• v.35 no.6
• /
• pp.419-430
• /
• 2022

In wind-resistant designs, wind velocity is assumed to be a Gaussian process; however, local complex topography may result in strong non-Gaussian wind features. This study investigates the non-Gaussian wind features over complex terrain under atmospheric turbulent boundary layers by the large eddy simulation (LES) model, and the turbulent inlet of LES is generated by the consistent discretizing random flow generation (CDRFG) method. The performance of LES is validated by two different complex terrains in Changsha and Mianyang, China, and the results are compared with wind tunnel tests and onsite measurements, respectively. Furthermore, the non-Gaussian parameters, such as skewness, kurtosis, probability curves, and gust factors, are analyzed in-depth. The results show that the LES method is in good agreement with both mean and turbulent wind fields from wind tunnel tests and onsite measurements. Wind fields in complex terrain mostly exhibit a left-skewed Gaussian process, and it changes from a softening Gaussian process to a hardening Gaussian process as the height increases. A reduction in the gust factors of about 2.0%-15.0% can be found by taking into account the non-Gaussian features, except for a 4.4% increase near the ground in steep terrain. This study can provide a reference for the assessment of extreme wind loads on structures in complex terrain.

• Wind and Structures
• /
• v.34 no.1
• /
• pp.59-72
• /
• 2022

In this work the numerical results of the flow around a 5:1 rectangular cylinder at Reynolds numbers 3 000 and 40 000, zero angle of attack and smooth incoming flow condition are presented. Implicit Large Eddy Simulations (ILES) have been performed with a high-order accurate spatial scheme and an implicit high-order accurate time integration method. The spatial approximation is based on a discontinuous Galerkin (dG) method, while the time integration exploits a linearly-implicit Rosenbrock-type Runge-Kutta scheme. The aim of this work is to show the feasibility of high-fidelity flow simulations with a moderate number of DOFs and large time step sizes. Moreover, the effect of different parameters, i.e., dimension of the computational domain, mesh type, grid resolution, boundary conditions, time step size and polynomial approximation, on the results accuracy is investigated. Our best dG result at Re=3 000 perfectly agrees with a reference DNS obtained using Nek5000 and about 40 times more degrees of freedom. The Re=40 000 computations, which are strongly under-resolved, show a reasonable correspondence with the experimental data of Mannini et al. (2017) and the LES of Zhang and Xu (2020).

• Proceedings of the KSME Conference
• /
• 2007.05b
• /
• pp.2737-2742
• /
• 2007

Strong wind flow around a building complex was numerically studied by LES. The original motivation of this work stemmed from the efforts to develop a risk assessment technique for windstorm hazards. Lagrangian-averaged scale-invariant dynamic subgrid-scale model was used for turbulence modeling, and a log-law-based wall model was employed on all the solid surfaces including the ground and the surface of buildings to replace the no-slip condition. The shape of buildings was implemented on the Cartesian grid system by an immersed boundary method. Key flow quantities for the risk assessment such as mean and RMS values of pressure on the surface of the selected buildings are presented. In addition, characteristics of the velocity field at some selected locations vital to safety of human beings is also reported.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.23 no.5
• /
• pp.349-355
• /
• 2011

In this study, numerical simulation has been conducted to investigate dispersion of a pollutant released from a new furniture, a kind of Sick Building Syndrome (SBS). A sofa which generates formaldehyde is implemented by using an immersed boundary method. Large Eddy Simulation (LES) is employed to obtain time-dependent velocity and concentration fields. It is shown that the ventilation efficiency in this room can be improved by changing inlet angle of diffuser, even though other conditions still remain unchanged. Both active diffusion near a sofa and air flow pattern are important parameters to enhance the ventilation efficiency.

• Proceedings of the KSME Conference
• /
• 2003.11a
• /
• pp.664-669
• /
• 2003

Recently, the recording density of hard disk drives has improved at an annual percentage rate of 100%. Therefore for faster access, higher disk rotational speeds will be required. The influence of the airflow produced by the rotation of a disk on the positioning accuracy has become a serious topic of research and the aerodynamic aspect of hard disk drives is now quite considerable with the increases in recording density and higher rotational speeds. Unsteady airflow in an actual hard disk drive is numerically simulated by using LES(Large Eddy Simulation) technique, we could predicted and aerodynamic mechanism that was related actuators' surroundings in HDD. At a result, with modifying the various shapes of the E-block and Damper, we estimated the characteristic of the influence of airflow in HDDs.

• 유체기계공업학회:학술대회논문집
• /
• 2005.12a
• /
• pp.223-230
• /
• 2005

The unsteady nature of vortex structures has been investigated by a large eddy simulation (LES) in an axial flow fan with a shroud covering only the rear region of its rotor tip. The simulation shows that the tip vortex plays a major role in the structure and unsteady behavior of the vortical flow in the fan. The movements of the vortex structures induce high-pressure fluctuations on the rotor blade and in the blade passage. Frequency characteristics of the fluctuating pressure on the rotor blade are analyzed using wavelet transform. The dominant frequency of the real-time pressure selected at the high pressure fluctuation region corresponds well to that of the fluctuating rotor torque and the experimental result of fan noise. It is mainly generated due to the unsteady behavior of the vortical flow, such as the tip vortex and the leading edge separation vortex.

• Journal of Power System Engineering
• /
• v.18 no.3
• /
• pp.5-13
• /
• 2014

In order to control emissions from engine, it is necessary to understand the mixture formation process of diesel spray. In this study, analysis of diesel fuel(n-Tridecane, $C_{13}H_{28}$) spray under a high temperature and pressure was performed by a general-purpose program, ANSYS CFX release 11.0, and the results of these are compared with experimental results of diesel fuel spray using the Exciplex Fluorescence Method. The simulation results of diesel spray is analyzed by using the combination of Large-Eddy Simulation(LES) and Lagrangian Particle Tracking(LPT), and then injection pressure was selected as an analysis parameter. Consequently, it was found that the experimental results and the numerical results are consistent with each other, and then in order to investigate the behavior characteristics of evaporative diesel spray, the effectiveness of the use of CFX of commercial code is definitely validated.

• Proceedings of the Korea Institute of Fire Science and Engineering Conference
• /
• 2012.04a
• /
• pp.58-61
• /
• 2012

엔진나셀 화재를 둔각물체에서 분사된 연료제트 화염으로 모사하여 화염안정화 및 소화특성을 조사하기 위해 실험과 수치해석을 수행하였다. 연료제트는 공기유동에 동측류인 경우와 대향류인 경우에 사각의 둔각물체에서 분사하였고, 소화약제는 이산화탄소와 질소를 사용하여 공기유동에 희석시켜주었으며 연료로는 메탄을 사용하였다. 본 실험의 결과를 해석하고 보충하기 위하여 LES(Large Eddy Simulation)을 기반으로 하는 FDS(Fire Dynamics Simulator)를 이용하여 비반응 유동장에서의 혼합특성과 둔각물체 후류의 유동특성을 살펴보았다.

• Proceedings of the KSR Conference
• /
• 2008.06a
• /
• pp.1809-1815
• /
• 2008

This experiment simulated the fire driven flow of an underground station through parallel processing method. Fire analysis program FDS(Fire Dynamics Simulation), using LES(Large Eddy Simulation), has been used and a 6-node parallel cluster, each node with 3.0Ghz_2set installed, has been used for parallel computation. Simulation model was based on the Kwangju-geumnan subway station. Underground station, and the total time for simulation was set at 600s. First, the whole underground passage was divided to 1-Mesh and 8-Mesh in order to compare the parallel computation of a single CPU and Multi-CPU. With matrix numbers($15{\times}10^6$) more than what a single CPU can handle, fire driven flow from the center of the platform and the subway itself was analyzed. As a result, there seemed to be almost no difference between the single CPU's result and the Multi-CPU's ones. $3{\times}10^6$ grid point one employed to test the computing time with 2CPU and 7CPU computation were computable two times and fire times faster than 1CPU respectively. In this study it was confirmed that CPU could be overcome by using parallel computation.