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How Does Surface Roughness Affect PWC Results?

In Pedestrian Wind Comfort (PWC) analysis, the main target is to generate wind comfort results at the pedestrian level. This article explains if the surface roughness will affect the PWC results and, if yes, then how.

cutting plane visualizing air pressure around a building
Figure 1: Slice visualizing air pressure in a PWC analysis.

1. Wind Exposures

In PWC analysis, we simulate the effect of wind over the model with respect to multiple wind directions simultaneously. In reality, wind speed varies with respect to height. This is called the Atmospheric Boundary Layer (or ABL). ABL depends on the height of the obstacles (buildings, trees, terrain, etc.).

In order to reduce the computational expense, we model only a sufficient portion of the geometry. In other words, we simplify the model by removing obstacles outside of the flow domain. After that, we define a wind exposure category to each wind direction, with respect to the unmodeled region.

visualization of wind category related to downwind direction
Figure 2: Wind exposure categories to model ABL.

It is important to keep in mind that exposure only affects the ABL profile, not the physical terrain roughness! In other words, by defining the wind exposure categories, we modify the ABL inlet profile in each wind direction.

visualization of relationship between the atmoshperic boundary layer, flow domain and terrain in SimScale
Figure 3: ABL inlet, flow domain, and terrain. ABL helps in the accurate modelling of the wind shear.

For more information about the wind exposure categories, please visit this documentation.

2. Roughness

Wind exposure categories only modify the incoming wind profiles to the domain, but this is good enough for most cases. If you have cleaned the buildings outside of the region of interest (ROI) or cleaned some small obstacles, such as trees, you might need to add roughness.

visualization of relationship between wind profile, terrain and region of interest
Figure 4: Wind profile, terrain and region of interest (ROI).

2.1 Add Surface Roughness in Wind Conditions

If your model does not include the terrain, it will be assumed as a plain ground. This means that the bottom of the flow domain will be the ground level. In this case, you can activate the Add surface roughness option to add roughness effects on the ground. Roughness value will be taken as ground surface value, and will be automatically added with respect to the corresponding wind direction.

steps on how to add surface roughness in wind condition settings for pedestrian wind comfort simulation in simscale
Figure 5: Steps to add surface roughness in wind conditions settings.

2.2 Surface Roughness Under Advanced Modelling

If your model includes the terrain (an uneven ground), then the surface roughness setting under the Wind conditions will not work. If this is the case, you will need to add the roughness under the Advanced modelling tab. The roughness value added here is the equivalent sand grain roughness height.

view of simscale workbench and steps on how to add surface roughness with the surface roughness settings
Figure 6: Steps to add surface roughness in surface roughness settings under advanced modelling.

Best Practices

Here are methods that we recommend regarding this topic:

  • Examine the region you are simulating and assign realistic wind exposure categories.
  • Model the buildings not only inside the ROI, but also inside the flow domain.
  • Use porous mediaTree feature to add vegetation.
  • Create the CAD model with separate surfaces to assign multiple roughness values.

The following picture compares pedestrian wind comfort analysis results of Bristol City based on two scenarios. On the left, no additional surface roughness is applied, meanwhile, the picture on the right has individual roughness values applied to the different terrain types such as roads, green fields, and other man-made structures.

comparison of simulation results between excluding surface roughness and applying surface roughness
Figure 7: Visualization of wind comfort criterion with and without roughness.

We can see that the addition of surface roughness slightly affects the results because there are areas where the category changes to a more comfortable category.

Note

If none of the above suggestions solved your problem, then please post the issue on our forum or contact us.

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