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December 2020

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Luma Bendini

Including trees in the numerical simulations of the wind flow in urban areas: Shinjuku Case Study

Vegetation impacts urban microclimate in many forms – from humidity levels, to accessory in pollution dispersion. And when it comes to winds, it’s not different. The green elements are a great tool to mitigate the undesirable effects that high-rises have on wind behavior. Because they function as a porous obstacle of airflow, vegetation affects wind direction and wind speed significantly.

Here’s a practical example of how the inclusion of trees in a certain urban design lowers the wind velocities, offering a greater experience for pedestrians.

Area of interest: Shinjuku Ward, Tokyo

The model used for this case is inspired in municipality of Shinjuku, in Tokyo – a dense commercial and administrative centre that houses one of the busiest train stations in the city. With a distinctive skyline, Shinjuku has an estimated population of over 300.000 people.

area-of-interest-shinjuku-v2

The model used for this case is inspired in municipality of Shinjuku, in Tokyo – a dense commercial and administrative centre that houses one of the busiest train stations in the city.

Simulation Summary:

8 Wind Directions
Weather data considered from 3 years preceding the current year
Implicit Large Eddy Simulations CFD Model applied
Urban Wind Profile used with a visualisation at 1.5 meters high
Leaf Area Index (LAI) 5.1

Full report: CFD Simulation without trees
Full report: CFD Simulation with trees

Wind Rose: high wind activity

 wind-rose-q1-q2wind-rose-q3-q4

A quick analysis of the weather data in the region shows a significant wind activity all over the year. Winds are naturally assuming velocities between 2 and 4 meter/second, which are considered light or gentle breezes in Beaufort scale. But considering the significant impact that urban density plays on wind behaviour, the natural velocities can be accelerated greatly, especially considering the volume of skyscrapers in the area.

Read about the main Wind Effects caused by high-rise building here. 

Pedestrian Comfort and Safety 

All evaluations of wind comfort should be done without any vegetation present. That allows for a conservative scenario and a “raw” image of how the buildings of interest and surroundings are affecting the wind behaviour.

A first analysis of pedestrian comfort and safety in the Shinjuku area using Lawson Criterion shows significant wind acceleration in the area of interest - with wind speeds reaching 15 meter/second during a few days of the year.

full-year-no-trees

Wind mitigation with Porous Media

The inclusion of trees has a significant impact in spreading the winds on ground level and enhancing the Lawson Criteria analysis for Pedestrian Safety and Comfort.

Worthy of notice is the reduction of red areas across the highlighted area bellow. Narrow areas between two or more high-rises tend to experience greater wind acceleration – due to airflow being compressed between the structures, therefore arising its velocity.

full-year-trees

Worthy of notice is the reduction of red areas across the highlighted area bellow. Narrow areas between two or more high-rises tend to experience greater wind acceleration – due to airflow being compressed between the structures, therefore arising its velocity.

passage-effect-v1

Consider wind, consider trees

Wind analysis has never before been so accessible for urban designer and architects. Urban projects are getting more and more complex, and running accurate wind simulation in early design phase can smooth things up by saving time and money for designers and developers.