• 대한토목학회논문집
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• 제34권5호
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• pp.1435-1442
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• 2014

본 연구에서는 OpenFOAM에서 제공하는 소스코드를 이용하여 매끄러운 하상의 직사각형 개수로 흐름에 대해 수치모의를 수행하였다. 난류 해석을 위해 LES를 수행하였는데, 비등방성 잔여 응력항을 모델링하기 위해서 Germano et al. (1991)이 제시한 Dynamic Subgrid-scale 모형을 이용하였다. 조직구조를 분석하기 위하여 Lu and Willmarth (1973)가 제시한 uw 사분면기법을 이용하여 순간레이놀즈 응력이 레이놀즈 응력에 미치는 영향을 기여율과 시간비로 나누어 분석하였다. LES 모의 결과를 토대로 기존 실험 및 DNS 모의 결과와 비교하고 분석하였다. 매끈한 하상을 가진 개수로 흐름에서 완충층 이후의 구간에서 분출현상이 쓸기현상에 비해 레이놀즈 응력의 양의 생성에 기여하는 바가 크지만, 분출현상에 비해 쓸기현상의 발생빈도가 큰 것으로 확인되었다.

• Wind and Structures
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• 제5권2_3_4호
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• pp.291-300
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• 2002

An efficient large eddy simulation algorithm is used to compute surface pressure distributions on an eleven story (target) building on the NIST campus. Local meteorology, neighboring buildings, topography and large vegetation (trees) all play an important part in determining the flows and therefore the pressures experienced by the target. The wind profile imposed at the upstream surface of the computational domain follows a power law with an exponent representing a suburban terrain. This profile accounts for the flow retardation due to friction from the surface of the earth, but does not include fluctuations that would naturally occur in this flow. The effect of neighboring buildings on the time dependent surface pressures experienced by the target is examined. Comparison of the pressure fluctuations on the single target building alone with those on the target building in situ show that, owing to vortices shed by the upstream buildings, fluctuations are larger when such buildings are present. Even when buildings are lateral to or behind the target, the pressure disturbances generate significantly different flows around this building. A simple grid-free mathematical model of a tree is presented in which the trunk and the branches are each represented by a collection of spherical particles strung together like beads on a string. The drag from the tree, determined as the sum of the drags of the component particles, produces an oscillatory, spreading wake of slower fluid, suggesting that the behavior of trees as wind breakers can be modeled usefully.

• Wind and Structures
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• 제13권6호
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• pp.557-580
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• 2010

A new inflow turbulence generation method and a combined dynamic SGS model recently developed by the authors were applied to evaluate the wind effects on 508 m high Taipei 101 Tower. Unlike the majority of the past studies on large eddy simulation (LES) of wind effects on tall buildings, the present numerical simulations were conducted for the full-scale tall building with Reynolds number greater than $10^8$. The inflow turbulent flow field was generated based on the new method called discretizing and synthesizing of random flow generation technique (DSRFG) with a prominent feature that the generated wind velocity fluctuations satisfy any target spectrum and target profiles of turbulence intensity and turbulence integral length scale. The new dynamic SGS model takes both advantages of one-equation SGS model and a dynamic production term without test-filtering operation, which is particular suitable to relative coarse grid situations and high Reynolds number flows. The results of comparative investigations with and without generation of inflow turbulence show that: (1) proper simulation of an inflow turbulent field is essential in accurate evaluation of dynamic wind loads on a tall building and the prescribed inflow turbulence characteristics can be adequately imposed on the inflow boundary by the DSRFG method; (2) the DSRFG can generate a large number of random vortex-like patterns in oncoming flow, leading to good agreements of both mean and dynamic forces with wind tunnel test results; (3) The dynamic mechanism of the adopted SGS model behaves adequately in the present LES and its integration with the DSRFG technique can provide satisfactory predictions of the wind effects on the super-tall building.

• Structural Engineering and Mechanics
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• 제46권2호
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• pp.197-212
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• 2013

With more and more high-rise building being constructed in recent decades, bluff body flow with high Reynolds number and large scale dimensions has become an important topic in theoretical researches and engineering applications. In view of mechanics, the key problems in such flow are high Reynolds number turbulence and fluid-solid interaction. Aiming at such problems, a parallel fluid-structure interaction method based on socket parallel architecture was established and combined with the methods and models of large eddy simulation developed by authors recently. The new method is validated by the full two-way FSI simulations of 1:375 CAARC building model with Re = 70000 and a full scale Taipei101 high-rise building with Re = 1e8, The results obtained show that the proposed method and models is potential to perform high-Reynolds number LES and high-efficiency two-way coupling between detailed fluid dynamics computing and solid structure dynamics computing so that the detailed wind induced responses for high-rise buildings can be resolved practically.

• Wind and Structures
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• 제26권2호
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• pp.89-98
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• 2018

For most full-scale tall buildings the Reynolds number of a flow field around a circular cylinder under strong wind is usually greater than $2{\times}10^7$, which is difficult to achieve in most wind tunnel tests. To explore the wind characteristics of tall cylindrical buildings with equidirectional grooves from subcritical to transcritical flow ($6.6{\times}10^4{\leq}Re{\leq}3.3{\times}10^5$ and $9.9{\times}10^6{\leq}Re{\leq}7.2{\times}10^7$), wind tunnel tests and full-scale large eddy simulations were carried out. The results showed that the rectangular-grooves narrow the wake width due to the downstream movement of the separation point and the deeper grooves cause smaller mean and fluctuating pressure while the peak pressure is little affected. Furthermore, the grooves lead to lower frequency of vortex shedding but the Strouhal number remains at the range from 0.15 to 0.35. The drag coefficient of the cylinders with grooves was found to be 2~3 times as large as that of smooth cylinders.

• 국제초고층학회논문집
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• 제8권4호
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• pp.255-264
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• 2019

This paper describes large eddy simulation of wind pressures on a square cylinder in a uniform flow and a high-rise building immersed in an atmospheric turbulent boundary layer. For the atmospheric boundary layer case, the inflow turbulence is generated by a numerical wind tunnel. In the numerical simulation, particular attention is devoted to the performance of an auto hexahedral non-structural mesh. Both simulations are performed for three grid systems: an auto hexahedral non-structured grid, a structured Cartesian grid and a non-structured triangular prism grid, and for three grid numbers. The present study shows that the auto hexahedral unstructured mesh achieves the best simulation results for wind pressures on the square cylinder and the high-rise building. When the grid number is sufficiently large, the differences among the results obtained from the three investigated grid systems are not significant. However, the advantage of the auto hexahedral unstructured mesh becomes clear when the grid number decreases, because it enables a balanced distribution of orthogonal grids. The results described in this paper demonstrate that the auto hexahedral non-structured mesh has good potential applicability to simulation of urban flows.

• 대한토목학회논문집
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• 제34권3호
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• pp.833-840
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• 2014

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.

• 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.

• 한국추진공학회지
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• 제16권1호
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• pp.55-63
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• 2012

동축제트분사기에 대한 난류유동의 특징이 비선형 $k-{\varepsilon}-f_{\mu}$ 모형[1]과 큰에디모사법에 의해서 조사되었다. 비연소조건에서 밀도가 다른 유체가 혼합될 때 레이놀즈수가 일정한 조건에서 리세스와 운동량비가 변화되었다. 비선형 $k-{\varepsilon}-f_{\mu}$ 모형은 리세스와 운동량비의 다양한 조건에서 의미있는 상관관계를 제안하였다. LES결과는 리세스에 의해서 난류유동 구조의 변화를 잘 묘사해 주었다. 리세스가 있는 경우 난류운동에너지의 발달은 리세스가 없는 경우보다 빠르게 나타났다. 또한, 혼합특성은 전단변형률의 변화가 지배적이었지만 국부적인 혼합은 리세스에 의해서 변화되었다.

• 대한기계학회논문집B
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• 제22권7호
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• pp.1030-1040
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• 1998

One of the main unresolved issues in large-eddy simulation(LES) of wall-bounded turbulent flows is the requirement of high spatial resolution in the near-wall region, especially in the spanwise direction. Such high resolution required in the near-wall region is generally used throughout the computational domain, making simulations of high Reynolds number, complex-geometry flows prohibitive. A grid-embedding strategy using a nonconforming spectral domain-decomposition method is proposed to address this limitation. This method provides an efficient way of clustering grid points in the near-wall region with spectral accuracy. LES of transitional and turbulent channel flow has been performed to evaluate the proposed grid-embedding technique. The computational domain is divided into three subdomains to resolve the near-wall regions in the spanwise direction. Spectral patching collocation methods are used for the grid-embedding and appropriate conditions are suggested for the interface matching. Results of LES using the grid-embedding strategy are promising compared to LES of global spectral method and direct numerical simulation. Overall, the results show that the spectral domain-decomposition grid-embedding technique provides an efficient method for resolving the near-wall region in LES of complex flows of engineering interest, allowing significant savings in the computational CPU and memory.