IDENTIFYING THE FULL-SCALE URBAN WIND CORRIDOR

This is an example of the identification process of urban wind corridors in Taichung City.

1. Wind in summer at night (mainly from the South) is used as the reference wind direction for the urban wind corridor in this case.

2. The cooling effect of urban wind corridor mainly affects the dense areas. We have therefore defined the study domain as 20 x 20 square kilometres of Taichung City center, a dense built environment.

3. Assume the low RL length (<1 m) facilitated the wind passage, and that the urban wind corridors deflect in advance when encountering large areas of high roughness (>2 m, in this case) built-up areas.

4. The angle of the wind deflection should not exceed 30° (according to a Japanese research of wind corridors).

STUDY DOMIAN ( 40 GRIDS )

To define the Primary and Secondary Wind Corridor, we calculate the number of high-RL-value grids that each route passes through. If a wind corridor flows alone the rougher route, then it become weaker.

According to the ratio of high RL(>1m) grids*, it is classified as follows:

Type II Wind Corridor: 35%~50%

Type I Wind Corridor:<35%

The explicited formula is given by: *P (%)= N / T

P : Ratio.

N : The number of high RL grids the wind passes through.

T : The number of grids in the North South direction* of the study domain (40 in this case).

*The direction is case dependant.

Type III Wind Corridors are used to reduce the scale of study to a smaller block for the more specific implementation of the heat mitigation strategy. At this scale, Computational Fluid Dynamics (CFD) simulations were allowed to used to simulate the impact and variability of different cooling strategies on the environment, using the microclimate measurements as input parameters, which enable us to confirm the effectiveness of the cooling strategy.

Our first step, identifying the local urban wind corridors, is the same as doing the full-scale ones. The reference grid size was based on the studying domain. We found that refining the side lengths of the grid in the domain of interest (e.g. 20 m² , see the upper left figure) and generalising the outer area (100 x 100 m², see the upper left figure) led to very efficient judgement information.

In the result of the CFD simulations, we eliminated the areas where the wind speed was too low, then identified the unobstructed paths with a width greater than 10m as wind corridors. The CFD simulations were then manipulated with the aim of verifying the above steps. It can of course be used as an accreditation method for wind corridors, but we have confirmed that it is more costly in all aspects.

Comparison of 2 Methods for Identifying Wind Corridors (Roughness Length Grids LCP and CFD Simulations)

Identification Methods Properties

Reference Height (Cutting Plane)

Input Information

Accuracy Base

Features limitation

A range, according to RL value

Site plan with height information (shapefile), Wind direction

Depending on grid size, relatively general

Definition of wind corridors by roughness of wind passage

More flexible

1. The width of the wind corridor is limited by the grid size.

2. Identification may be influenced by the grid size, the input position and others.

3. Judgement may be affected due to the boundaries.

Tools

Easier to operate (QGIS, Excel)

A fixed values

Site models in 3D, Wind direction, Oiginal wind speed

Depending on the input parameters, allow finer details

Definition of wind corridors according to wind speed and direction differences

More accurate but time consuming

1. High cost for large scale analysis (high computer performance required to create large scale 3D model).

2. The result is affected by the boundary. The closer to the model boundary, the lower the reference value.

Complex tools (Grasshopper, rhino, flowdesigner, photoshop)