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    How to Generate Particle Traces and Slices in PWC Analysis?

    In PWC analysis, the main goal is to generate pedestrian wind comfort results. To be able to see the flow results for other regions within the domain, the user has to set “Additional results”. This article explains how to set a PWC analysis to be able to generate particle traces (streamlines) and slices.

    Please visit the documentation page to learn more about the Pedestrian Wind Comfort Analysis type.

    this picture shows the particle traces or streamlines and pressure slice around a building in pwc analysis results
    Figure 1: Particle traces (velocity) and a slice (pressure) around the main building in a PWC.


    1. Region of Interest and Flow Domain

    Region of interest (ROI) defines the area around the main building on which the pedestrian comfort should be evaluated.

     how to define the region of interest around a building in pwc analysis
    Figure 2: Setting ROI (Region of Interest) marked by a transparent blue disc.

    Once you define the ROI, you will be able to see the Flow domain (Wind tunnel) around it by clicking the eye icon.

    This picture shows how to make the flow domain visible and define the North angle
    Figure 3: Flow domain is created for every direction of wind but is shown only for the North angle.

    In PWC analysis, the LBM solver by Pacefish®\(^1\) calculates the aerodynamics effects inside the flow domain and generates a comfort map. Even though the solver calculates the velocity, pressure, and turbulence values, it does not automatically saves all the data due to the storage space concerns. The user needs to define a region and set additional results for export.

    2. Geometry Primitives

    2.1 Local Cartesian Box

    The aim of particle traces is to track the flow path in three dimensions. Therefore, we need a 3D domain to generate them.

    Create a Local cartesian box where you would like to visualize the particle traces. Change the Orientation reference to the External flow domain. This will ensure the box aligns itself automatically, with respect to different wind directions.

    Orientation Reference

    The Orientation reference setting has two different options that are available to choose from:

    • External flow domain : When this option is selected as the Orientation reference, the geometry primitive orientation is adjusted with respect to the wind direction. The center of the rotation is the ROI (region of interest) center point.
    • Geometry : When this option is chosen, the geometry primitive will stay fixed during the whole simulation, with respect to the coordinate system of the model.

    This picture shows how to create a local cartesian box around a building of interest
    Figure 4: Make sure your local cartesian box captures your building of interest well and enough as 3D results are computationally expensive.

    Saving the additional data in 3D creates a larger results file. You may run out of disk space. Therefore, we strongly recommend keeping the box as small as possible to prevent an increase in simulation time and costs.

    2.2 Local Slice

    Particle traces is not the only way to observe the flow effects. Similarly, you can create 2D planes and visualize the data on them. Create a Local slice, where you would like to place the contours. Similarly, change the Orientation reference to the External flow domain.

    This picture shows how to create a local slice inside the flow domain
    Figure 5: Creating slices will allow to visualize results in 2D which is computationally less expensive.

    3. Additional Result Export

    As we mentioned earlier, the solver does not save the flow parameters inside the whole domain. You need to save them additionally. You can either save transient results (Transient output) or steady-state results (Statistical averaging). Statistical averaging uses the results from last time steps, takes the average, and saves them for one time. Due to the storage, computation time, and cost concerns, we strongly recommend saving the results only in statistical averaging.

    Activate the local cartesian box and/or the local slice you created before. Saving these results will allow you to create particle traces and slices in PWC analysis.

    This picture shows how to save the statistical average additional data in a box and on a plane
    Figure 6: Saving additional statistical average results to a 3D box and a 2D plane.

    To learn more about the result control, please visit the following documentation page: Incompressible LBM (Lattice Boltzmann Method) Advanced Options

    3.1 Fraction From End

    Pedestrian Wind Comfort (PWC) or Incompressible (LBM) analysis are developed to capture the transient behavior of a flow. Under the Transient output and Statistical averaging are Additional result exports. If you click one of these two, you will see the following settings:

    • Fraction from end: Taking the simulation end time as a reference, how large the time frame should be?
    • Sampling interval: How often should the data be saved.
    statistical averaging result control settings of pedestrian wind comfort analysis  type in simscale
    Figure 7: Statistical averaging result control settings

    In a transient flow analysis (time-dependent), initial time steps are unsteady. During the early fluid passes, we expect to see the flow development. In other words, the flow field is still not steady. In the final fluid pass, we assume that the flow will reach a steady-state (time-independent). The time-independent result is what we need to achieve. Fraction from end defines the last fraction of the total simulation time. As an example, assigning 0.2 ensures that only the last 20% fraction of the results will be saved. The picture below shows the side view of a flow over the buildings with respect to different times.

    flow field over a city model with respect to six simulation intervals
    Figure 8: Transient behavior of the flow field over a city model

    Figure 8 clearly shows the transient behavior of the flow. During the initial phase (up to 32 seconds), the flow is being developed. Later on, we do not see much change in the flow field, which means that the flow field reached a steady-state. Keep in mind that this is only a vertical slice, therefore we do not see the status of the rest of the domain. Usually, using at least 3 fluid passes and saving the last 20% fraction of the simulation results is convenient to extract useful results.

    4. Results

    You can see the results once the simulation is finished. Additional results are always saved per wind direction. Therefore, you need to choose a wind direction and navigate to Additional results.

    this picture shows how to access the additional results in pwc analysis
    Figure 9: Access to the additional results

    4.1 Generate Particle Traces

    Once you are in Solution fields, you will see a green box. This is the Local cartesian box you defined under Geometry primitives. Velocity and pressure values are saved in this region.

    1. Select the ‘Particle Trace’ filter on the top of the page to get started.
    2. Click the ‘Pick’ button.
    3. Place the seeds on a flat surface. This surface should be on or inside the flow domain.
    4. Adjust the number of seeds, size, type, and space in between them.
    this picture shows how to create particle traces and place the seeds on flow domain
    Figure 10: Creation of particle trace and placement of the seeds. The particle traces are generated inside the fluid domain.

    After capturing a nice view, you can animate the particle traces. First, create an ‘Animation’, then define the number of steps and animation speed, and finally, click the play button.

    this picture shows how to animate the particle traces in pwc analysis
    Figure 11: Animation of particle traces. This is only possible if transient results are exported.

    4.2 Generate Contours on a Slice

    If you set a slice under the Additional Results before you created your run, then you can use it to visualize the flow parameters on it. Initially, ensure that your Local slice is visible.

    This picture shows how to visualize the local slice and create velocity contours on it
    Figure 12: Local slice with averaged velocity magnitude contours

    Apart from the velocity, you can select other parameters to be visualized, from this menu:

    clicking on the menu for the parameters for visualization on slice
    Figure 13: There are more options that can be displayed on a slice.

    Best Practices

    • Firstly, make sure the Orientation Reference for the geometry primitives selected is selected as the External flow domain.
    • Secondly, avoid large boxes due to the storage and simulation costs concerns.
    • Finally, statistical averaging is useful enough for additional results. Above all, the transient output might cause storage issues or an increase in simulation costs.
    pressure gradient on a local slice and velocity vectors in pwc analysis
    Figure 14: Pressure contours and velocity vectors on a cutting plane.


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

    Last updated: June 15th, 2022