Exporting 1.7m Pedestrian Velocity Data from Incompressible to GIS

Using SimScale Fluid Flow / CFD
1

I am conducting an urban microclimate study on the SAAO site in Cape Town using the Incompressible (OpenFOAM) solver.

Since the automated “Pedestrian Wind Comfort” (LBM) suite is not available for this analysis type, I am seeking the most robust way to harvest velocity data at the 1.7m breathing zone for external GIS analysis.

Current Plan:

  • Result Control: Point Data at specific coordinates.

  • Post-processing: Z-Slice at 1.7m to manually compare Mean Velocity (U) against Lawson/Davenport criteria.

Questions for the Team:

  1. Data Export: Is there a way to export the Slice data directly as a structured CSV or table from the workbench? I need to avoid downloading the huge VTK result if possible, as I only need the 2D plane data for QGIS/Raster interpolation.

  2. Gust Factors (G): Since this is a steady-state run and not transient LBM, are there recommended Result Control settings (e.g., using Turbulent Kinetic Energy (k)) to estimate gust peaks (Upeak​) for a more conservative comfort assessment?

  3. Surface Data vs. Map Plane: To achieve a “Raster-like” output, is it better to use a Surface Data object on a Geometry Primitive plane, or stick to post-simulation slicing?

Project Link: [saao]

2

Hi @akriger,

I checked your project setup. It looks consistent for a baseline steady-state run, but I also noticed no runs have been executed yet, so before optimizing export workflows I would first verify the external-flow domain size and near-wall treatment. Also keep in mind the Incompressible Analysis Type only accepts a single solid (the flow region) as the simulation domain, so you should exclude all solid bodies after creating the flow volume.

SimScale’s external-flow examples recommend a sufficiently large enclosure, and your wall setup should be checked against wall-function / y+ guidance and a representative incompressible external-aero tutorial.

For your 3 questions:

  1. CSV export of a post-processor slice is not possible for the Incompressible solver.

  2. For gusts, in steady-state you can only build an engineering estimate from mean flow plus turbulence quantities, not true peak gusts. Treat it as an approximation.

  3. I’d stick to the cutting plane in the post-processor, as surface data only outputs a single averaged / integrated value for all variables

Cheers,
Igor

3

I am so super grateful @igaviano . Thank you.

I know nothing and your direction is helpful. As I’m still navigating the learning curve here, sharing your knowledge and experience is appreciated.

I think my challenge is the CAD hierarchy. When I attempt to generate the mesh, I receive the following dialogue: `Please ensure that all CAD entities are assigned a material... Missing material detected for: Open CASCADE STEP translator 7.8 1.21...`

The External Flow Domain (the air) correctly ‘sees’ the model (buildings and terrain) and changes to accommodate faces intruding into its space –which is then assigned as a no-slip Wall. I understand that the External Flow Domain (the air) is the only body that should have a material (Air), but SimScale seems to still ‘see’ the original solid buildings and terrain as unassigned parts.

  • Question: Is the best practice here to manually “Delete” or “Exclude” the original solid bodies from the simulation tree after the Flow Domain is created to satisfy this requirement? Or is there a specific way to “suppress” them so the solver only sees the carved-out air volume? (an advanced setting / option?)

Regarding CSV Data Export: Since a post-processor slice can’t be exported as a CSV in the Incompressible solver, what is the best Result Control setup to get tabular data for GIS?

  • Question: Would you recommend a “Point Data” result control with a dense grid of coordinates at 1.7m, or is there a way to use a “Surface Data” object on a transparent geometry primitive (a plane) to harvest local velocities? I am trying to avoid a 15GB VTK download just to get a 2D slice of data.

Here it is now: saao

Thank you again for your patience and help!

Cheers, Adrian

6

Hi @akriger,

You’re welcome - and I checked your project setup.

The main blocker is not the solver itself, but the geometry/setup: the model is still in a non-runnable state, with the wall condition applied to 3,668 separate faces. That level of fragmentation often goes together with the kind of CAD/material issues you are seeing. The best next step is to simplify the CAD in CAD Edit and make sure you are simulating only the fluid region by using a flow volume extraction, not the solid buildings as simulation volume (to your question - yes! you should absolutely delete the solid geometries, and you can do it in CAD Edit).

I also noticed there are currently no result controls defined, so even once the mesh runs, you will not get structured outputs for GIS unless you add probes/surfaces/field calculations first.

So I would focus on these in order:

  1. clean/simplify the geometry,
  2. extract the fluid volume,
  3. reduce boundary assignments to grouped faces,
  4. add result controls for the 1.7 m data you want.

Cheers,
Igor

7

We are getting somewhere. Thank you @igaviano

The challenge was the model (building and terrain). I deleted the bodies. And only selected the External Flow volume faces (Wall no-slip).

The mesh ran successfully but I am experiencing a few challenges.

I am very grateful for your help. The new challenges have to do with the quality of the mesh and I won’t raise them here. I will open a new topic. Let me run the simulation and see what I get to understand how the result can be harvest and used in a Spatial Data Science environ.

This is where we are now : here

8

We have a successful simulation. Thank you @igaviano

the result is here

I will continue experimenting with refinement regions (I’ll select the faces of the External Flow Volume that represent the walls of buildings and set those to a fineness of 4 or 5) but would be extremely grateful if you could have a look please.

How do I visualise results like these please: here

And it is possible to download something; is it the post-processing result?

10

Hi @akriger,

I checked your project - the run itself looks good. The main limitation is that this setup writes only the final timestep, and I do not see any Surface data or probe result controls defined. That is why, for this run, there is no lightweight table export ready for GIS.

To visualize results like the city wind example, use the Integrated Post-Processor and create a cutting plane at z = 1.7 m, then color by velocity magnitude.

For downloading: if you need the full velocity field on that slice for QGIS, then yes - in this case you would typically download the simulation results and post-process them externally, e.g. via ParaView / 3rd-party post-processing.

So in short:

  • Visualize in SimScale = cutting plane in post-processor
  • Export GIS-ready field data = download results for external processing
  • For future runs = add result controls to reduce export size

Cheers,
Igor

11

I am grateful. Thank you @igaviano

Here is the result