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Combined model of structured and unstructured-grid system for wind pressure estimation of a tall building

T. Nozu1, T. Tamura2, T. Kishida3and A. Tatsumura3

1Shimizu Corporation, 3-4-17, Etchujima, Koto-ku, Tokyo, JAPAN

nozu@shimz.co.jp 2Tokyo Institute of Technology, JAPAN

3Wind Engineering Institute, JAPAN

Abstract

This paper discusses the use of LES in wind-resistant design of buildings in cities. In order to accurately predict the wind pressure on actual building with a complicated shape, we introduce the unstructured-grid system which is formulated on the open source CFD code. Especially the combinedmethodis employed consisting of the structured-grid for the accurate turbulence structures in the urban canopy, and the unstructured-grid for the exact wake patterns around the specified building inside the densely arrayed buildings.In this study, we have applied this model to the flow around the high rise buildings in two different city models.

1 Introduction

This paper discusses the highly accurate LES (Large eddy simulation) for the wind loading estimation on a specified building in cities. Thus far Urban winds have been often simulated by RANS model from an environmental point of view, where the wind velocity prediction is essential. In this case, good performance was shownby RANS model (Blockenet al., 2004). On the LES of the urban wind, the Cartesian grid system with equal increment was often used and its accuracy was generally enough for the wind velocity prediction (Tamura et al., 2010) (Nozuet al., 2012).However the predictive accuracy of the Cartesian grid system is ambiguous for the estimation of the wind pressures and forces on the specified building which is focused on in the city.Therefore, this paper introduces the overset grid with finer mesh for the near region of the specified building by employing the unstructured grid system and realizes the accurate aerodynamic prediction by LES.

2 Problem Formulations

2.1 Combined model

For the LES analysis of the specified building in the city center, it is important to set up how to resolve the flow around the building and its near region or the urban area consisting of many other buildings. As far as the prediction of wind

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velocity, it is not so significant to reproduce the shape of the building. The Cartesian grid method with high-order accurate scheme has been applied to this problem (see Figure 1).For generating turbulent boundary layer, we set up the driver region (Nozawa& Tamura, 2002).In order to predict the wind pressures and forceson the specified building in the city,we introducethe combined model (Tamura &Nozu, 2012)which is employed consisting of the Cartesian grid and the unstructured-grid formulated on OpenFOAM which is widely used as an open source code. Figure 2 illustrates the numerical model where the area including the specified building and its circumstances are clipped for the unstructured grid area obtained by the mesh generation software (SnappyHexa).

2.2 Unstructured grid embedded in the near region of the specified building

According to unstructured grid in Figure2, the curved surface of the building can be represented smoothly. On the inflow boundary of the unstructured grid area, turbulent flow is given by one-way method, which is obtained using LES on the Cartesian coordinate grid system. Appropriate resolution to the near region of the building, neighboring area and the surrounding buildings area is selected making use of the property of the unstructured grid such as the variability of the resolution on each area.

2.3 Two different city models

Figure 3 shows two different city models. Model A has an urban aspect with locally high-rise buildings in the vicinity of the specified building, and Model B has the aspect with widespread layout of high-rise buildings, including the specified building. In Model B, two wind directions are calculated, one is wind direction South, which corresponds to main streets, and the other is wind direction NW, which does not corresponds to main streets.

Figure 3: City models.

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Figure 2: Unstructured grid for near-region of the building.

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3 LES results by the Open FOAM

3.1 Model A

Figure 4 depicts the LES results for the wind velocity distributions among the densely arrayed tall buildings. It can be recognized that the flow with fully developed turbulence comes from the inflow boundary into the computational region.

Figures 5 and 6 indicate details of the pressure distributions on the plate-shaped building. The pressures at higher level, since the section becomes smaller, are different from that of the lower level. The higher zone has another stagnation point separately and generates the very local negative pressures on both the upstream corners. Its magnitude is larger at the side of inside the pack where the separated shear layer is bending due to the surrounding buildings and approaching to the side of the plate-shape of building. Figure 6 shows the comparison of LES and experimental data for mean pressure coefficients at each height indicated by the dashed lines in Fig. 5. Correspondence of LES results (lines) and experimental data (symbols) is definitely good in quantitative sense at almost area.

3.2 Model B in wind direction S

Figure 7shows the LES results for the wind velocity distributions among the densely arrayed tall buildings on the unstructured grid. It can be recognized that the flow with fully developed turbulence comes from the inflow boundary into the computational region.

Figures 8 indicate the comparison between wind tunnel test and LES results of pressure distributions on the surface of the specified building. The distribution of experimental results has unsymmetrical shape, and this tendency is good agreement with LES results.

Figure 9illustrates the comparison of LES and experimental data

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Figure 4: LES results of wind velocity around the densely arrayed buildings in cities.

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Figure 5: Computed time-averaged pressure contours.

Figure 6: Computed and experimental time-averaged pressure distributions at each height on the tall building.

Figure 7: Computed wind velocity field on vertical section.

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for mean and r.m.s. response accelerations at one year period of the specified building at the top floor in both X and Y direction. LES results are corresponding to the experimental results.

3.3 Model B in wind direction NW

Figure 10shows the LES results for the wind velocity distributions among the densely arrayed tall buildings on Cartesian grid.

Figure 11 indicate the comparison between wind tunnel test and LES results of pressure distributions on the surface of the specified building.

4 Conclusions

The conclusions of this study can be shown as follows:

Using the LES model of the method overlaid by the unstructured grid system, the appropriate turbulence structures at inflow and among a pack of tall buildings can be simulated.

The pressure distribution on the specified building obtained by the present LES is appropriate in order to realize the practical use of CFD technique for wind-resistant design of the buildings.

References

B. Blocken, S. Roels, J. Charmeliet,Modification of pedestrian wind comfort in the SilvertopTower passages by an automatic control system. J. Wind Eng.Ind.Aerodyn.,92, (2004) 849-873. T. Tamura, Y. Okuda, T. Kishida, O. Nakamura, K. Miyashita, A. Katsumura, M. Tamari, LES for aerodynamic characteristics of a tall building inside a dense city district, CWE2010, (2010) T. Nozu, T. Tamura, LES of turbulent wind and gas dispersion in a city, J. Wind Eng.Ind.Aerodyn., (2012) K. Nozawa, T.Tamura, Large eddy simulation of the flow around a low-rise building in a rough-wall turbulent boundary layer, J. Wind Eng. Ind. Aerodyn., 90, (2002) 1151-1162. T.Tamura, T. Nozu,Introduction of unstructured-grid system on LES for wind pressure estimation on a building in cities,BBAA7, (2012)

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Figure 8: Computed and experimental pressure distributions on the tall building.

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