CFD Prediction of Wind Environment around a High-rise Building Located in an Urban Area

Ryuichiro Yoshiea Akashi Mochidab Yoshihide Tominagac Hiroto Kataokad Masaru Yoshikawae

a Tokyo Polytechnic University, Iiyama 1583, Atsugi-City, Kanagawa, Japan

b Tohoku University, Aoba 06, Aramaki, Aoba-ku, Sendai-City, Miyagi, Japan c Niigata Institute of Technology, Fujihashi 1719, Kashiwazaki-City, Niigata, Japan

d Technical Research Institute, Obayashi Corp., Shimokiyoto 4-640, Kiyose-City, Tokyo, Japan e Niigata Institute of Technology, Fujihashi 1719, Kashiwazaki-City, Niigata, Japan

ABSTRACT: Flow-field around a high-rise building surrounded by low-rise urban blocks in a turbulent boundary layer was calculated, and the results were compared with those of the wind tunnel experiments conducted by the present authors. In the strong wind region, the calculated wind speeds with standard k-εmodel were somewhat lower than the experimental results, while this was improved by using revised k-εmodels. Influence of various calculation conditions on CFD results was also investigated. This paper focuses on the influence of reproduction range of the surrounding urban blocks on the flow-field around the high-rise building and the profile of the turbulent boundary layer.

KEYWORDS: Pedestrian wind environment, CFD, Urban block, 1 INTRODUCTION Progress in high-speed processing by personal computer and rapid propagation of software for numerical analysis of fluid dynamics in recent years have enabled prediction of the pedestrian wind environment around high-rise buildings based on CFD (Computational Fluid Dynamics). However, prediction accuracy of CFD simulations and influence of various calculation condi-tions on the CFD results have not yet been thoroughly understood.

Thus, a working group named “Working Group for Preparation of Wind Environment Evaluation Guideline based on CFD” was organized by the Architectural Institute of Japan. Since its inception, this group has been working continuously to prepare a guideline for proper use of CFD for calculation of the wind environment. Comparative and parametric studies have been carried out on several building configurations to elucidate the problems on setting or select-ing calculation conditions and turbulence models. [1-4].

The present article introduces one of the results (flow-field around a high-rise building located in a city) and discusses influence of reproduction range of the surrounding urban blocks on the flow-field around the high-rise building and the profile of the turbulent boundary layer.

2 OUTLINES OF WIND TUNNEL EXPERIMENT The flow-field analyzed here is that around a high-rise building in a simple urban area, for which the wind tunnel experiment was carried at the Niigata Institute of Technology. The low-rise ur-ban block was assumed to be 40m square and 10m high as shown in Figure 1 (simulating a con-dition where low-rise houses are densely jammed), with a high-rise building 25m square and 100m high (1:1:4) in a block at the center of this area. One urban block is assumed to be en-closed by two roads (each 10m wide) and roads 20m and 30m wide.

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The wind velocity measuring points are shown in Figurer 2. The scale of the experimental model was 1/400 and the measuring height was 5mm above the floor of the wind tunnel (2m above ground in real scale). Wind speed was measured using a split film probe.

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Figure 1. Outline of wind tunnel experiment Figure 2. Measuring points

3 OUTLINES OF CFD CALCULATION In the working group, the conditions shown in Table 1 and Figures 3 were given as the standard calculation conditions for the comparative studies. In addition to the standard calculation condi-tions, we investigated the influence on the calculation results of changing the boundary condi-tions, the computational domain, the grid resolution, and the turbulence models, etc.

Table 1. Standard Calculation conditions （Calculation was carried out in experimental scale.） Computational Domain 1.8m×1.8m×1.8m (The size of the test section of the wind tunnel)Grid resolution 132(x)×130(y)×76(z)=1,304,160 mesh (Fig. 3)Scheme for advection term Quick scheme for U,V,W,k,εBuilding wall and ground surface Logarithmic law for smooth surface wallUpper and side surface of computational domain Free slip wall condition (symmetrical wall)Turbulence model Standard k-εmodel

Outflow boundary condition Zero gradient condition

Inflow boundary conditionInterpolated values of U and k from the experimental approaching flowε＝Cμ 1/2 k･dU/dz 　(ε＝Pk ), Cμ=0.09

x

y

1.8m

1.8m

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1.8m

(a) Whole computational domain and grid resolution

※High-rise buildingwas divided into

12(x)×12(y)×27(z)

(b) Macrograph of central area Figure 3. Computational domain and grid resolution for standard calculation conditions

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4 RESULTS OF EXPERIMENT AND CALCULATION

4.1 Comparison of experimental results with CFD results The calculation results based on the standard calculation conditions and the experimental results are compared in Figure 4. The wind speed ratio between the scalar wind velocity at each measur-ing point and UH (inflow wind speed at the height of the central high-rise building) is represented on the ordinate. In strong wind region such as measuring points 35, 38, 44, the calculation results were ten or so % lower than the experimental results. However this was improved by using modified k-εmodels [4].

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Figure 4. Comparison between CFD and experiment (Wind direction＝0°)

4.2 Influence of reproduction range of surrounding urban blocks Figure 6 shows the results of calculation with two rows and three rows each deleted from the pe-ripheral region of the surrounding urban blocks, as shown in Figure 5. The difference from the standard case was very small except at measuring points 1, 2, 3 and 4 on the roads on the wind-ward side. Figure 7 shows the vertical profiles of wind velocity U for the three cases. The inflow velocity profiles upper than the high-rise building (the height of the high-rise building is 0.25m) are maintained from the inflow boundary (point A in Fig.7) to the outflow boundary (point J in Fig.7) regardless of the reproduction range of surrounding urban blocks. And there are almost no differences in the three cases of the wind velocities higher than twice the height of the low-rise surrounding urban blocks (the height of the low-rise urban block is 0.025m). At the point D which is behind the urban block the vertical profiles of the wind velocities for the standard case and for the case of deleting two rows are almost the same even in the region near the floor. Also, at the point E, F and G which are behind the urban blocks or the high-rise building, much differ-ence can not be seen among the three cases regardless of the reproduction range of surrounding urban blocks. Therefore it is concluded that pedestrian wind in urban blocks is almost deter-mined by the blocks near the target area. Two or more surrounding urban blocks each of several tens of meters around the region to be evaluated would be satisfactory for practical application.

(a) deleting

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(b) deleting 3 rows

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io StandardDeleting 2 rowsDeleting 3 rows

Figure 5 Reproduction range Figure 6 Influences of reproduction range of surrounding

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of surrounding urban blocks urban blocks

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(a) Standard (b) deleting 2 rows (c) deleting 3 rows

Figure 7. Vertical wind velocity profiles (Wind direction=0°)

5 CONCLUSIONS According to the results of the present study, the influence on the calculation results of the com-putational domain, the grid resolution, and the reproduction range of the surrounding urban block is relatively low. In the strong wind region, the wind speed ratios of modified k-εmodels agreed well with the experimental results compared with the standard k-ε model. Two or more urban blocks each of several tens of meters should be reproduced in the area surrounding the re-gion to be evaluated

6 REFERENCES

1 Mochida, A., Tominaga, Y., Murakami, S., Yoshie, R., Ishihara, T., and Ooka, R. (2002), Comparison of vari-ous k-ε models and DSM applied to flow around a high-rise building -report on AIJ cooperative project for CFD prediction on wind environment, Wind&Structures, Vol.5, No.2～4, pp.227-244.

2 Tominaga, Y., Mochida, A., Murakami, S. (2003), Large Eddy Simulation of Flowfield around a High-rise Building, 11th International Conference on Wind engineering, B10-5.

3 Tominagada, Y., Mochida, A., Shirasawa, T., Yoshie, R., Kataoka, H., Harimoto, K. and Nozu, T. (2004), Cross Comparisons of CFD results of wind environment at pedistrian level around a high-rise building and within a building complex, Journal of Asian Architecture and Building Engineering, vol.3 no.1, pp.63-70.

4 R. Yoshie, A. Mochida, Y. Tominaga, H. Kataoka, K. Harimoto, T. Nozu, T. Shirasawa (2005): Cooperative Project for CFD Prediction of Pedestrian Wind Environment in the Architectural Institute of Japan, The Fourth European - African Conference on Wind Engineering, Prague.

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