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Documentation

Advanced Tutorial: Pedestrian Wind Comfort Simulation

This article will provide you with a step-by-step tutorial on how to create a Pedestrian Wind Comfort (PWC) simulation and post-process the results within SimScale.

Visualization of nen8100 comfort criterion simulated with pedestrian wind comfort simulation in simscale
Figure 1: Visualization of the NEN8100 comfort criterion

Note

This tutorial uses the Lattice Boltzmann method (LBM) by Pacefish®\(^1\) to solve the simulation, which is only accessible through professional licenses. Therefore, this tutorial cannot be performed using the community license. Learn more.

If you want to perform external aerodynamics with a community license, please check out the fluid dynamics section of the main tutorial page.

Overview:

This tutorial will teach you how to:

  • Set up and run a pedestrian wind comfort simulation
  • Define trees as porous objects in your model
  • Export results

We are following the common workflow in SimScale:

  1. Prepare the CAD model for the simulation
  2. Set up the simulation
  3. Run the simulation and analyze the results

1. CAD Mode and Analysis Selection

You can import the tutorial project into your SimScale Workbench by clicking the button below.

By importing the tutorial project, the Workbench will open with the prepared tutorial geometry already loaded into the viewer.

view of simscale workbench after importing the tutorial project
Figure 2: View of the Workbench after the import, showing the .stl geometry

This CAD model follows good practices for PWC simulations: to make the simulation set up simpler, we recommend always importing the CAD models for PWC simulations following the guidelines below:

  • The sky should be in the positive z-direction
  • The geographic North should be in the positive y-direction

Helpful Tips on Importing Your Model

Creating a new simulation is simple – please select the geometry and click on ‘Create Simulation’:

creating a new simulation in simscale
Figure 3: Creating a new simulation in SimScale
  • To create a new simulation, click on the ‘Create Simulation’ button on the geometry panel.
  • Now, the SimScale analysis type widget will pop up:
analysis type widget
Figure 4: The analysis type library contains all the available options, including the Pedestrian Wind Comfort module.

From the list, select ‘Pedestrian Wind Comfort’ and proceed by clicking on ‘Create Simulation’. Now a new simulation tree appears on the left-hand side panel containing the configuration options.

simulation tree set up simscale
Figure 5: Simulation tree, in the left-hand side panel, containing configuration options.

In SimScale, you will typically go through the simulation tree from the top to the bottom, adjusting the settings when needed. In the next sections, we will go through the entries in the simulation tree.

2. Set up the Simulation

2.1 Geometry

The geometry is automatically assigned to this simulation, so no changes are required here.

2.2 Region of Interest

Firstly, you will need to define the region of interest for the simulation results, which can be done by clicking the ‘Region of Interest’ menu in the simulation tree. It is worth noting that the pedestrian comfort results are only shown within the region of interest, so you need to be careful with its placement.

Here is a brief explanation of the available settings:

  • Disc Radius: The radius of the disc that represents the region of interest
  • Center point: The center point of the region of interest
  • Ground height: The ground height for the wind tunnel for the simulation. The ground height automatically receives the minimum z-coordinate of the CAD model
  • North angle: The north angle that will be used in the simulation. As previously discussed, the default direction for the north angle is the positive y-direction.

For this simulation, you can follow the parameters defined below.

region of interest dialog box for pedestrian wind comfort simulation
Figure 6: In the Workbench, the shaded disc represents the region of interest, with the north direction indicated by the arrow.

You can find more information regarding the region of interest in the attached page below:

2.3 Wind Conditions

Under Wind conditions, you can define the wind rose at the location of your site. This process can be automatic, by using the Meteoblue®\(^2\) plugin, or manual (for example, uploading .stat files).

In this tutorial, we will use the automatic approach with the steps below:

wind condition dialog box in simscale for pedestrian wind comfort simulation
Figure 7: With the automatic workflow, you can obtain wind data from any location on the planet.
  1. Select ‘Wind conditions’
  2. In the location box, type in ‘Rotterdam, Netherlands’ and pick the entry from the list.
  3. For this tutorial, we are going to use the ‘AS/NZS 1170.2’ Wind Engineering Standard.
  4. Finally, click on the ‘Import wind data’ button to load the wind rose.

A wind rose with 16 directions will load by default. Please adjust the Number of directions to ‘8’. Furthermore, you can also adjust the Wind Exposures to control the incoming wind profile from each of the directions. Since our CAD model depicts a part of the city of Rotterdam, the wind exposure is ‘City center (4)’ in all directions.

wind rose configuration pedestrian wind comfort
Figure 8: Number of directions and wind exposure configurations.

You can find more information regarding wind conditions in the following article:

Number of Wind Directions

If you are not sure about the wind directions required for the analysis, our knowledge base article can answer your question, “How many wind directions should I run for a pedestrian wind comfort analysis?”

2.4 Pedestrian Comfort Map

The next simulation tree entry is the Pedestrian comfort map, where you can define the faces and height of interest for the pedestrian comfort calculation. As a reminder, only the parts inside the region of interest are displayed in the default simulation results.

For this tutorial, click on the ‘+’ button next to Pedestrian comfort map and assign a ‘Ground absolute’ configuration. With this setting, you will receive the pedestrian comfort results 1.5 meters off the bottom face of the wind tunnel, inside the region of interest.

pedestrian comfort map configuration
Figure 9: Defining ground level for simulation

The ground absolute option works well when the CAD model doesn’t contain a face representing the terrain. For a detailed description of all setup options, please check the page below:

2.5 Simulation Control

In Simulation control, the default values will be enough for the simulation. The default setting ensures that particles travel three times from the inlet to the outlet during the simulation, which is the minimum recommended amount.

simulation control for pedestrian wind comfort simulation
Figure 10: Setting the Simulation control panel by applying 3 fluid passes, a convenient amount to extract useful results.

2.6 Advanced Modelling – Trees

We can define porous objects and surface roughness of selected volumes in the domain under advanced concepts. We will specifically discuss how to set up trees for this simulation.

A common use of modeling porous objects in a pedestrian wind comfort simulation is when trees are acting as a natural barrier against the wind. SimScale has built a feature to model trees with the following steps:

tree porous object configuration pwc
Figure 11: SimScale offers an option to set up trees as porous objects
  1. Firstly, click the ‘+’ button beside Porous objects and select ‘Tree’ as the Porous object type
  2. In the Type of tree window, select ‘Plane tree’ with ‘5’ meters of Average tree height. SimScale has built a list of general existing trees and the physical characteristics will change accordingly. It is also possible to define a custom tree if required
  3. Lastly, assign the ‘Tree – Foliage’ volume to the porous object

Note that this .stl CAD model conveniently contains one layer for the trees. Adding layers for certain parts of interest allows you to speed up the setup process. For interested readers, this article contains workflows to import .stl files to SimScale with layers.

Furthermore, you can find more information regarding advanced modelling and porous objects for a PWC simulation below:

2.7 Additional Result Export

We can also define additional results that we want to have by the end of the simulation. The additional results that are available are:

  • Forces and Moments: calculates the forces and moments on a selected face, or group of faces
  • Probe points: place probe points on the domain to monitor velocity and pressure
  • Transient output: transient results of the simulation
  • Statistical averaging: time-averaged result for each wind direction simulated and over a selected fraction of the time selected

Note

Keep in mind that the Transient output and Statistical averaging outputs are automatically applied to your region of interest. However, we will add a different region to each output to show how you can assign these to other regions.

a. Forces and Moments

For this tutorial, we will add Forces and Moments into our simulation, specifically for the face of the tallest building that is facing the southwest direction. A Forces and Moments result export can be added by following these steps:

forces and moments dialog box for pedestrian wind comfort simulation
Figure 12: Forces and moments result export configuration. The Center of rotation is used to calculate the moments.
  1. Firstly, click the ‘+’ button beside Forces and Moments
  2. Adjust the Center of rotation as required. In this tutorial, we will calculate the moments about the base of the façade. The coordinates are ‘30.8’ meters in x, ‘-18.77’ meters in y, and ‘4.5’ meters in the z-direction.
  3. Define the face where the forces and moments will be calculated. We will select the face of the highest building that is facing the southwest direction.

b. Transient Output

It is important to remember for Transient output that the larger the region the longer it takes for the simulation to run. Therefore, we will apply a smaller size box. You can follow the steps below to determine the location of your transient output.

  • Firstly, click ‘Transient output’
  • Click the ‘+’ sign beside Geometry Primitives
  • Select ‘Local cartesian box’
  • Create your geometry primitives based on the dimensions below:
cartesian box size for transient output of pedestrian wind comfort simulation
Figure 13: Setting the cartesian box’s dimensions for transient output

When you save the local cartesian box settings, they will be automatically assigned to the transient output.

c. Statistical Averaging

For statistical averaging, we will use the same settings as the ones we used for Transient Output. However, all you need to do now is to slide the toggle for the cartesian box created before.

2.8 Mesh

Under mesh settings, the fineness of the mesh can be selected. The selection is ranged from very coarse to very fine, but a coarse mesh will be sufficient for this simulation.

mesh settings for pedestrian wind comfort simulation
Figure 14: Choosing a coarse fineness for the mesh creation

The PWC analysis type also supports local mesh refinement, which is especially useful if there are regions that require additional mesh elements. A more in-depth view of the PWC mesh settings is available in the following article:

3. Start Simulation

To start the computing process, click on the ‘+’ button next to Simulation Runs. This will create a snapshot of the current simulation tree and will start the computing process.

creating a pedestrian wind comfort simulation run
Figure 15: After setting up the simulation, you can start a new run.

In SimScale you can leverage cloud computing to run several simulations in parallel, further speeding up the design process.

4. Results

After the simulation finishes running, you will receive a notification via email. For this simulation you will have two main result sets:

  • The statistical surface solution contains all the pedestrian comfort and safety plots. These are taken from the frequencies provided by the weather data in the wind rose, as well as the average velocity results in each of the directions.
  • Results from individual wind directions: the results from each wind direction will depend on the result export definition. The individual direction results consist of:
    • Average result: averaged results from each wind direction, sampled over the final 20% of timesteps
    • Transient result: compilation of individual results, sampled over the last 20% of timesteps

You can access the results of the simulation in two ways:

  • When the simulation is finished, a dialog box will show that your simulation has finished. You can click on the ‘Post-process results’ button to post-process your results, and it will take you to the averaged statistical results of the simulation.
  • Select your simulation run in the simulation tree, and the results will be shown below the run.
how to access the simscale post-processor
Figure 16: Options to access the post-processing environment

4.1 Statistical Surface Solution

The Statistical surface solution shows the wind comfort results. There are several criteria available to choose from, depending on your needs. For example, the figure below is a visualization of the Lawson wind comfort criteria using mean velocities as a base.

visualization of lawson pedestrian wind comfort criteria done with simscale
Figure 17: Visualization of Lawson pedestrian wind comfort criteria across the region of interest.

The following comfort criteria are available!

  • Davenport
  • London LDDC
  • Lawson
  • Lawson 2001
  • Lawson LDDC Comfort
  • NEN8100 Comfort
  • NEN8100 Danger

Furthermore, it’s possible to select Mean velocities, Gust equivalent mean velocities (GEM), or Max velocities for the pedestrian comfort calculation. For further information, please visit this page.

4.2 Forces and Moments

By clicking on each wind direction, you will be given a temporal graph of the calculated Forces and Moments for the assignments. You can choose which data you want to display by clicking the legends, and this will then toggle the visibility of the data.

Below is an image of the forces and moments for the 0-degree wind direction results:

calculated forces of highest building in pedestrian wind comfort simulation
Figure 18: Forces and moments exerted onto one face of the tallest building

It’s also possible to view the forces and moments results in Statistical mode. With this option, the user can check minimum, maximum, average, root mean square (RMS), and the standard deviation of forces and moments:

statistical data for forces and moments pwc
Figure 19: Accessing statistical data for the forces and moments results

4.3 Average Result

As explained previously, you can also access averaged results on the pedestrian level from each wind direction. After selecting ‘Average result’, you will be directed to the post-processor.

In the post-processor, you can select what parameter you want to plot. The image below shows the velocity magnitude at the pedestrian level, however, you can also see other parameters by changing the input:

average solution pwc pedestrian level
Figure 20: Visualization of the averaged wind velocity on the pedestrian level for the 0 degrees direction. The blue arrow indicates the button to change the input

4.4 Transient Results

You can access the transient results on the pedestrian level by clicking on ‘Transient results’. After creating an animation using the top filter ribbon, this is what the velocity results look like at the pedestrian level:

animation pwc transient results pedestrian level
Video 1: Animation of the transient results for velocities at the pedestrian level (0-degree direction).

4.5 Cutting Planes

In section 2.7 from this tutorial, we defined Additional result exports within a cartesian box. Therefore, we can access the averaged and transient data under Additional results. For instance, we can open the ‘Averaged solution’ and create a cutting plane filter:

cutting plane filter pwc
Figure 21: All post-processing filters can be applied to the cartesian box region

In this configuration, the cutting plane is normal to the z-direction and it shows the velocity magnitude. It is easy to change the orientation and coloring of interest – for example, the image below shows pressure levels on a cutting plane normal to the x-direction:

pressure cutting plane pwc results
Figure 22: Pressure levels on a cutting plane around the building of interest

4.6 Particle Traces

Still looking at the averaged results, let’s now create a Particle Trace filter. This filter allows you to track the path of fluid particles as they travel across the domain. Please follow the steps below:

steps to create a new particle trace filter
Figure 23: Creating a new particle trace filter for a pedestrian wind comfort simulation
  1. Select a ‘Particle Trace’ filter from the top ribbon.
  2. Make sure that the fluid volume is visible by clicking on the ‘eye icon’.
  3. Select a face of the fluid volume to generate the traces from.

Filters in SimScale are loaded with default settings, however, they are highly customizable. For example, for this simulation, the particle traces are too large. Let’s adjust the Size of the traces and hide the fluid domain to improve the visualization:

particle traces adjusting settings
Figure 24: Particle trace filter, tracking the path of particles around the building of interest

A complete description of filters and their settings is available on this documentation page.

Congratulations! You finished the pedestrian wind comfort simulation tutorial!

Note

If you have questions or suggestions, please reach out either via the forum or contact us directly.

Last updated: May 16th, 2022

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