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Documentation

Pedestrian Wind Comfort Analysis

The Pedestrian Wind Comfort (PWC) analysis type is used to assess the effects of building’s external aerodynamics on pedestrians which typically returns a comfort map in an exported environment representing the activities that can be done and where. This is usually done to demonstrate that new development (e.g. building) does not interfere with pedestrian comfort, or if it can be used to experiment with mitigation features (trees, screens, and canopies for example) to ensure that pedestrians remain comfortable. 

pedestrian wind comfort mean velocities
Figure 1: Mean wind velocities at pedestrian level in a typical urban environment computed in SimScale

The PWC analysis type is extremely streamlined to allow the user to set up and run a simulation from geometry upload in minutes with 3 simple steps. The setup is automated using best practices and validation exercises to ensure that fast results are complemented with accuracy. Although this highly automated workflow is in place, much of the parameters can be manually adjusted if required.

pwc set up
Figure 2: A basic structure of pwc analysis setup in three simple steps

The underlying technology used to solve the fluid flow around the buildings is the Lattice Boltzmann Method (LBM), by Pacefish®\(^1\). There are several key things that make this stand out in comparison to its competition. LBM is able to solve almost regardless of the geometry quality given to it, and therefore, geometry preparation and cleaning are almost removed provided some guidelines are followed. In some real cases, this has reduced weeks of CAD preparation work to a matter of hours.

LBM method also uses a different technology to solve on. Where the standard OpenFOAM solvers use CPUs, LBM uses GPUs (Graphics processors) enabling many more parallel processes to occur, and therefore solve times are rarely longer than a few hours for a complex case. This puts the PWC solver in a good place where users can expect an entire turnaround in a day once they are familiar with the processes. This solver uses the k-omega SST DDES turbulence model.

wind comfort criteria in simscale
Figure 3: Multiple common wind comfort criteria are automatically evaluated and can be interrogated fully in 3D within the integrated post-processing environment.

Geometry for Pedestrian Wind Comfort

Our PWC analysis type takes all the standard geometry formats that we advertise in the geometry upload section of the documentation. Typically, however, geometry comes from one of the common tools used by architect firms and wind engineers, which are mainly Revit and Rhino (others exist and if they can export STL then they are also fine). Therefore the main format types relevant here are Rhino (.3dm) and STL (.stl). 

More details regarding geometry requirements and how to prepare the CAD for best results with Pedestrian Wind Comfort can be found here.

Region of Interest

The Region of interest is a very important stage of simulation setup. The core functionality of the region of interest is to define the area around the main building/area on which the pedestrian comfort should be evaluated. Additionally, it is used as a reference to automatically size and orient the virtual wind tunnel.

The settings panel has the following parameters:

region of interest in pedestrian wind comfort
Figure 4: Region of interest definition on a city model with north direction indicated

More details on how to correctly define the region of interest parameters can be found here.

Wind Conditions

Under Wind conditions not only the local wind statistics for the target area are defined, but also the wind engineering standard, according to which the analysis will be run and the results will be evaluated.

Currently, four different Wind Engineering Standards are available on SimScale – Eurocode  EN 1991-1-4, the standard in Europe including the UK; AS/NZS 1170.2, the standard in Australia and New Zealand; NEN8100, the standard for the Netherlands and most recently also the London City Wind Microclimate Guidelines, which were specifically designed for the City of London region.

interface for automatically importing wind data for pwc in simscale
Figure 5: Wind Conditions settings with wind engineering standard definition and wind statistics import

More details about the correct setup of the wind conditions can be found here.

Pedestrian Wind Comfort Map

Wind comfort study should be assessed at the mean pedestrian level since the comfort level is correlated to pedestrian-level winds. Height above ground input should be defined by taking pedestrian height into account. A ground reference can be set as an absolute reference height or as a relative reference height.

The reference height used will be the height at which the comfort criteria will calculated. Currently, SimScale has implemented a number of comfort criteria, for example: Lawson criterion, Davenport criterion, NEN8100 Comfort criterion and others.

More details regarding the setup of the pedestrian height can be found here.

Simulation Control

Simulation control contains all the necessary parameters to control the run-time of the simulation. The maximum run time and the number of fluid passes is defined in this part.

The maximum runtime defines the maximum runtime for the simulation in physical time and the number of fluid passes defines how long a transient simulation runs (counted in seconds).

More details on how to define the runtime and the number of fluid passes can be found here.

Advanced Modelling

Additional physical parameters can be defined in advanced modelling. The additional parameters which can be defined are Surface roughness and Porous objects.

If the effect of friction on the flow wants to be observed then it is necessary to define the surface roughness. The value defined for the surface roughness is the equivalent sand-grain roughness and can be set manually by the user.

Porous media is used to model permeable obstructions such as trees, hedges, windscreens, and other wind mitigation measures. When air flows through a porous body, a pressure gradient along the direction of the flow is generated. Using porous media simplification reduces CAD and mesh complexity, and saves computational time and expenses. 

More details on defining surface roughness and porous media can be found here.

Additional Result Export

Under Additional result export, besides Forces and moments and Probe points, one can define additional filters for transient result output data along with statistical averaging.

The settings for transient output and statistical averaging are the same for PWC and Incompressible LBM analysis types and thus can be found detailed here.

Mesh Settings

The mesh generated for a pedestrian wind comfort simulation is based on the lattice Boltzmann method. Here a Cartesian background mesh is generated, that is composed of cube elements that are not necessarily aligned with the geometry of the buildings or the terrain.

The necessary fineness of the mesh can be defined in the global mesh settings and refinements can be added to the mesh in the refinements settings.

More details regarding the setup of the mesh can be found here.

Additionally, the mesh and geometry guidelines for the City of London are described here.

Starting a Run and Run Information

When all of the settings and physical conditions are applied, a simulation run can begin. The solver runs a simulation in each direction in parallel and will be compiled into the statistical results based on the comfort criteria once it is finished.

More details regarding how to start a run and the information of the run can be seen here.

Residuals and Convergence

You won’t see the residuals or convergence plots in a PWC or an Incompressible LBM analysis, because these are explicit solvers.

Comfort Analysis Simulation Result Assessment

Once all individual wind directions have been computed and also the statistical analysis on the wind comfort had been carried out, the simulation run changes to ‘Finished‘ state, and the user is informed via email that the results of the run are now ready for visualization.

comfort analysis post processor simscale
Figure 6: Results structure of a finished Pedestrian Wind Comfort analysis. Either click on ‘Statistical surface solution’ or ‘Post-process results’ to access the post-processor.

Clicking either the ‘Post-process results‘ button, or the highlighted item ‘Statistical surface solution‘ will open the comfort analysis results in the online post-processor.

Note

Results for individual wind direction along with aditional results exported will be stored under Directions below Statistical surface solution.

post processor pwc simscale comfort analysis
Figure 7: Post-processor interface for the Pedestrian Wind Comfort results evaluation

Above highlighted are the 5 main comfort analysis interaction items for the post-processor:

  1. Filters panel: This is the main result field and appearance selection panel. In the Pedestrian Wind Comfort filter, the individual comfort criteria can be selected for visualization. The Parts Color filter enables setting the color and opacity for the buildings.
  2. Post-processor toolbar: The toolbar allows the switch between multiple views and renders modes, toggling the legend visibility, picking specific locations for result inspection, and taking a screenshot of the whole viewer scene.
  3. Result topology tree: Here the individual visibility of the result parts as well as the pedestrian zones can be toggled on or off.
  4. Result legend: It shows the relation of the wind comfort criteria to the viewer colors. For each criterion, the specific threshold velocity, as well as the threshold frequency, are given.
  5. Orientation cube: It helps with resetting the view and selecting standardized views for result comparison.

Last updated: July 5th, 2021

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