Urban Comfort Mapping

Urban Comfort Mapping

The Urban Comfort Mapping module allows you to process cached CFD results from a Virtual Wind Tunnel calculation along with an EnergyPlus weather file and Radiance ray-tracing to get an accurate estimate of thermal sensitivity and the complete microclimate around an urban site.

There are five major steps to carrying out an Urban Comfort Mapping calculation, these are:

  1. Model Drawing and/or Importing
  2. Model Setup and Boundary Conditions
  3. Meshing of the CFD domain
  4. Running the CFD Virtual Wind Tunnel
  5. Processing the Urban Comfort Results

The sub-steps required for each of these phases of the calculation are outlined below in more detail.

Model drawing and/or importing

The urban comfort calculation is moderately sensitive to dirty geometry, it is often possible to import the geometry from other programs such as ArchiCAD or Revit in .3DS or .STL formats respectively or from Sketchup in the Collada .DAE format.

Refer to the instructions outlined in the "Getting Started" page for importing a Sketchup model into Blender.

Model Setup and Boundary Conditions

Once you have imported or drawn your geometry into Blender you must set up the CFD domain and boundary conditions for the urban comfort calculation.

  1. Ensure that you have saved the .blend file to a path with no spaces in it.
  2. Start up the ODS Studio Urban Comfort addon by going to User Preferences (Ctrl-Alt-U) and then going to “Addons” → “Architecture” and starting the Urban Comfort Module.
    • Note: Ensure you only press the checkbox to enable the addon once. Don't double-click the checkbox!
  3. Add a CFD Box around your model by going to Add → Mesh → CFD-Domain. You can tweak the size of the box using the menu system on the left of the viewport.
    • Note: It is a good idea to add the CFD Domain box to another layer. Either hold Shift and select an empty layer before adding the box OR after you have added it select the box and hold Shift to select MinX, MinY etc. etc. and then press “m” and move to another layer.
  4. Ensure your CFD Domain box is centered on x=0 and y=0 and that the z-coordinate is half the height of the box (thus the bottom of the box is at z=0). Only move the CFD Domain box by entering location, rotation and dimension in the input fields on the right-hand-side of the 3d viewport. Don't enter editmode for the CFD Domain box!
    • Note: To ensure that you step to nice round numbers in the Virtual Wind Tunnel then set the size of the CFD Domain to be a multiple of both “9” AND the Base Mesh Cell Size.
  5. In ODS Studio CFD → Mesh menu enter the Base Mesh Cell Size. Select quite a large size to get started with. Use the SnapBB button to ensure that the CFD Domain box dimensions are a multiple of the base mesh cell size. Ensure that you re-centre the CFD Domain to x=0, y=0 and z=H/2 after using SnapBB.
  6. Apply mesh levels to all the objects. A mesh level of around “3” is a good starting point generally.
  7. Set MaxY to have a boundary condition of “fixedVelocity” with a value of (0, -5, 0) and set MinY to have a boundary condition of fixedPressureOutOnly

Meshing the CFD Domain

You now have to "mesh" your CFD domain which you have set up in the previous steps

  1. In CFD → Setup set number of processors (if applicable). This is done by entering the number of divisions in the x, y and z direction under the Nxyz field. For example, setting (2,2,0) for Nxyz will create 4 pieces that will calculate across 4 processors
  2. If you have followed all the steps in the Model Setup and Boundary Conditions correctly then you should be able to simply go to CFD → Mesh → Write Case Files and then press the “Mesh” button.

Running the CFD Virtual Wind Tunnel

  1. Its a good idea to first of all try running a single CFD case (for the northerly wind direction) by going to CFD → Control → Run Case. See if it runs for 100 or so iterations. If there are any issues or divergence (ie if it starts and then fails with an error at iteration 26 or so) then there is normally a problem with either your mesh or your boundary conditions and there is no chance it will work in the Virtual Wind Tunnel. Fix your boundary conditions and then try again.
  2. Once the model is working consistently as above then go into the “foam” directory and delete all the numbered directories other than “0” and then in Blender go to the Virtual Wind Tunnel menu panel. Set the number of initial and subsequent iterations and the size of the angular step and then press “Run”.
  3. The virtual wind case should run and automatically create a results file called VWT. The U-velocity field that you want for post-processing the results should be called “Utrans”.

Processing Urban Comfort Results

Now that you have set up your domain, created a CFD mesh and run a full virtual wind tunnel calculation through 360-degrees you have the cached wind results for an urban comfort calculation

  1. Create a radiance case directory as follows: a) Expand the SunPath Menu on the right hand side panel of the 3D viewport; b) Click the “Draw” button and uncheck the “Sun Path” checkbox. The press the right arrow key until the time in the bottom-right of the viewport shows 12:00 (midday); c) Go to ODS → Radiance → Stencil and click the “Write Case Files” and then the “Generate Octree” buttons.
    • This generates a Radiance Octree in at the location rad/octrees/0101_1200.oct. This is required by the Urban Comfort scripts for doing calculation of direct solar exposure.
  2. Go to the Urban Comfort Mapping panel: a) select a preset time period from the dropdown box; b) select a scale velocity; and c) type in the Patch name for the surface that you are interested calculating the velocity above (ie the ground plane). Then press the “Run” button.
  3. Post-process the results in paraivew. The results are found in the “urbanComfort” directory.  Be aware that the results are mapped onto the patch which you entered in step 2 above, thus you only need to import this patch when starting paraview.

Additional Notes

  1. Ensure that you have run a Virtual Wind Tunnel calculation
  2. If you want to do full solar exposure ray-tracing for each hour of the year then ensure that you have carried out a simple Radiance ray-trace so that an octree for the current time exists in "rad/octrees"
  3. In the scene menu panel for Urban Comfort Mapping:
    • In the Weather File box select an EnergyPlus .epw weather file on your system which will be used for getting wind velocity and direction as well as temperature, humidity and global irradiance values
    • From the "Preset" selection box choose a time of year and day over which to do the calculation.
    • From the "Method" selection box choose either "Thermal Sensitivity" to carry out weighted summation and averaging of calculated variables or "Lawsons Criteria" for a calculation of the proportion of time the wind velocity exceeds a particular threshold value
    • Set the Scale Velocity equal to the reference input velocity that was used in the Virtual Wind Tunnel calculation. This value is used to normalise the wind velocities in a high Reynolds number flow such as an urban environment
    • Important: Enter the patch name across which to do the calculation. You can also specify an offset height in order to calculate the velocity at a height above the patch. The default offset height is 1.2 metres.
    • From the "Presets" field at the bottom of the menu select whether you would like to calculate the default which is full thermal sensitivity or only wind or only solar exposure. You can also enter your own custom weightings in the text box above.
  4. When you are finished entering the above information press the "Run" button to do the processing. A new OpenFOAM case folder called "urbanComfort" will be created in the directory where your .blend file is saved. The output times stored in this case correspond to the "Output Time" that you have set for the particular time of year and day over which to carry out the simulation (set in "b" above)