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A prediction model for wind speed ratios at pedestrian level with simplified urban canopies

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Abstract

The purpose of this study is to review and improve prediction models for wind speed ratios at pedestrian level with simplified urban canopies. We adopted an extensive database of velocity fields under various conditions for arrays consisting of cubes, slender or flattened rectangles, and rectangles with varying roughness heights. Conclusions are summarized as follows: first, a new geometric parameter is introduced as a function of the plan area index and the aspect ratio so as to express the increase in virtual density that causes wind speed reduction. Second, the estimated wind speed ratios in the range 0.05 < z/h < 0.3, where h is the building height, are consistent with those derived from the database to within an error of ±25%. Lastly, the effects of the spatial distribution of the flow were investigated by classifying the regions near building models into areas in front of, to the side of, or behind the building. The correlation coefficients between the wind speeds averaged over the entire region, and the front or side region values are larger than 0.8. In contrast, in areas where the influence of roughness elements is significant, such as behind a building, the wind speeds are weakly correlated.

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Abbreviations

A N  , B N :

Empirical constants in prediction models (subscript indicates authors)

A(z) , a(z) , b(z):

Height-dependent empirical coefficients

a M :

Attenuation coefficient in Eq. (3.4)

H :

Actual building height in simulation domain (=1L – 3L)

h :

Assumed building height (30 m for a ten-story, 15 m for a five-story, and 6 m for a two-story building)

h av :

Average building height for nonuniform array

h max :

Maximum building height for nonuniform array

L :

Standard length (=25 m)

l x :

Streamwise distance between two blocks

n :

index number of power law for wind speed

Re* :

Roughness Reynolds number (\( ={u}^{*}{z}_o/\nu \))

T :

Time scale defined by \( H/{u}^{*} \)

U :

Spatially averaged streamwise wind speed (subscript indicates defined height)

u * :

Friction velocity

u , v , w:

Wind velocity components in the x , y , z directions, respectively

V :

Magnitude of spatially averaged wind velocity vector (subscript indicates defined height)

w p :

Width of pedestrian space (=3.0 m)

X , Y:

Streamwise and spanwise block lengths, respectively

x , y , z:

Coordinates in the streamwise, transverse, and vertical directions, respectively

z o :

Roughness length

z p :

Pedestrian height (=1.5 m)

α p :

Aspect ratio (\( ={\lambda}_f/{\lambda}_p \))

α px  , α py  :

Streamwise and spanwise aspect ratios, respectively

γ N :

Prediction model of wind speed ratio (subscript indicates authors)

γ(z, ζ):

Proposed prediction model for wind speed ratio

\( {\gamma}_{h_{av}} \) :

Wind speed ratio defined by h av

\( {\gamma}_{h_{\max }} \) :

Wind speed ratio defined by h max

γfront , γ side , γ behind :

Wind speed ratio in pedestrian spaces

λ f :

Frontal area index

λ p :

Plan area index (=building coverage ratio)

ν :

Dynamic viscosity

ζ :

New geometric parameter [=\( 1-{\left(1-{\lambda}_p\right)}^{\alpha_p^{a(z)}} \)]

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Acknowledgments

This work was supported by Japan Society for Promotion of Science KAKENHI Grant Numbers 25289196 and 25820282.

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Authors and Affiliations

  1. Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga-koen 6-1, Kasuga-shi, Fukuoka, 816-8580, Japan

    N. Ikegaya, Y. Ikeda, A. Hagishima & J. Tanimoto

  2. Facility of Mechanical Engineering, Universiti Teknologi MARA, Shah Alam, Malaysia

    A. A. Razak

Authors
  1. N. Ikegaya
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  2. Y. Ikeda
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  3. A. Hagishima
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  4. A. A. Razak
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  5. J. Tanimoto
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Correspondence to N. Ikegaya.

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Ikegaya, N., Ikeda, Y., Hagishima, A. et al. A prediction model for wind speed ratios at pedestrian level with simplified urban canopies. Theor Appl Climatol 127, 655–665 (2017). https://doi.org/10.1007/s00704-015-1655-z

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  • DOI: https://doi.org/10.1007/s00704-015-1655-z

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