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How Does Pedestrian Wind Comfort Analysis Type Deal With Complex Geometry at the Boundary?

Atmospheric Boundary Layer

By definition, the Atmospheric Boundary Layer (ABL) is the lowest part of the troposphere and is directly influenced by the existence of the earth’s surface and responds to the surface roughness. Due to these factors, the wind conditions vary with the height and type of surface available.

atmospheric boundary layer profiles in various areas
Figure 1: Wind velocity profile with different areas.

In Pacefish®\(^1\) Pedestrian Wind Comfort simulations, the ABL is automatically generated and takes into consideration the topography and the defined surface roughness. The ABL is generated on the limits of the flow domain. It follows the topology of the model, as shown in this example:

atmospheric boundary layer following the topology
Figure 2: Atmospheric boundary layer following the topology in the SimScale platform.

Different Wind Exposures

The ABL assumes the following values for the following conditions:

velocity curves profiles
Figure 3: Velocity curves.

These types of terrains describe the areas we can find outside of the flow domain and influence the velocities that will be inputted into the ABL. As a consequence, generate different flow results on the region of interest.

When there is a need to apply extra roughness to the flow domain, the platform has a function that can be activated with the toggle button, as demonstrated in the picture bellow.

add surface roughness option toggle
Figure 4: Add surface roughness option.

This added roughness is applied to the surface inside of the flow domain and outside of the region of interest. The conditions used to define the roughness are the ones previously used to define the ABL on the different directions of the model.

Recommendations When Using SimScale

As shown before, the ABL is generated by cutting the model on the limits of the flow domain. This means that if we have a part of the model crossing the Flow Domain that does not represent the topography or the ground level of the model. The ABL is applied to those faces and may influence the results of the simulation. In the case of a building intersecting the Flow Domain, the ABL generated by the Platform is applied on the roof and walls of that building.

atmospheric boundary layer following the topology and the building
Figure 5: Atmospheric boundary layer following the topology and the building in the SimScale platform.

For this reason, when preparing a model for simulation in SimScale, the buildings that intersect the Flow Domain should be removed from the model to avoid possible deviation from the correct result that might come from this intersection of the flow domain.

Last updated: July 5th, 2021

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