This CFD example will go through the steps required for a typical external CFD analysis:
External analyses are simpler than internal analyses in that you do not need to define any internal boundary conditions. Also they are isothermal and take account principally of only velocity and pressure.
It is possible to edit the grid and select monitor cells with external CFD as well as with internal CFD.
The model comprises a simple rectangular block with dimensions 30.0m x 20.0m x 30.0m.
Click on the Add new block tool and change the Block type to Building block and the Height to 30.0m. To place the origin of the building at the site origin, type in 0 0 and press the <Return> key. To then create the block with a 30.0m x 20.0m perimeter, type in 30 20 and press the <Return> key. A block will be created with the required dimensions. Alternatively create the block by drawing the base perimeter in the normal way.
Each DesignBuilder project can have one or more CFD analyses. Each CFD Analysis can be a different set of results for a different variation on the model. Unlike simulation data, CFD Analyses are not deleted automatically when the model is modified on the Edit screen.
Notice the rotation of the domain with respect to the building to account for the wind direction.
It is usually a good idea to display the state of one or more monitor cells in the domain so that you can ‘monitor’ the extent to which the calculations have settled down to a final converged solution. DesignBuilder automatically sets up a single monitor cell at the centre of the domain. You can add new monitor points at any time as follows.
Click on the Define CFD monitor points tool and select the Z grid axis using the grid axis selector tool. Move the cursor along the axis to select a plane a couple of grid cells above the building and then move the cursor across the selected plane and select a cell roughly above the centre of the building. The Cell monitor data panel is displayed at the bottom left of the screen. Enter the name Roof into the Monitor location name field and click on the green tick Apply changes button.
Press <Esc> to cancel the Define CFD monitor points tool.
To set up calculation data and run the CFD calculations, click on the Update calculated data button, which will bring up the CFD Calculation options dialog. The left hand side of the calculation dialog contains various calculation options.
Keep the default options except for Iterations which should be set to 5000 and under the Residual Display header, switch on the X, Y and Z-axis velocity residuals.
Click on the Start button to begin the calculations. Wait until the calculation is fully converged when the residuals have reached the specified minimum and the “Finished (Converged)” indicator is displayed at the bottom left of the dialog. Notice that the displayed velocity for the supply monitor point becomes constant.
Press the Cancel button to close the calculation dialog.
3-D contours can be displayed to show the values of a variable throughout the domain. 3-D contours can display a lot of data: