Trial / Demo
Following our recent webinar ‘Using IESVE for CFD Studies’, this article will consider an example case of MicroFlo-CFD in use. We will look through the assignment of boundary conditions from VistaPro Results, running a CFD simulation and analysing the results.
Schools within the UK must comply with B101 requirements, which include a number of comfort analytics including the carbon content of the spaces. To assist in this, more schools are incorporating different natural ventilation solutions, including roof ventilation terminals. This example covers the case of a classroom with a number of roof ventilation terminals used to ensure compliance with BB101 regulation.
Once a model has been created and a simulation is about to be run, there is one additional option that must be included to allow the export of CFD boundary conditions. This is under the output options button at the bottom of the Apache simulation window. The option to include outputs required by MicroFlo must be selected and the room which you wish to analyse should be chosen in the list. An example of this is shown in figure 1 below, where the room for analysis is named “Example”.
Figure 1
Once a simulation is run, the results are opened within VistaPro in order to export the boundary conditions for use in MicroFlo. This is done by selecting the appropriate simulation result in VistaPro, then pressing the VistaPro dropdown on the toolbar, then exporting boundary conditions, as shown in figure 2.
Figure 2
This will open this dialogue, shown in figure 3:
Figure 3
Where you will select the appropriate time to take results from, this will take all aspects applicable to MicroFlo CFD and export these results, including window/door openings and flow rates, temperature of surfaces, internal gains, among other variables. When the correct date and time are selected, press ‘OK’ and the boundary condition file will be created. Note there is no confirmation message for this.
Once the boundary condition is created as shown in figure 3, it must be assigned within the MicroFlo application in order to correctly simulate the space. This is done by moving into the MicroFlo application, then moving down to the room level of the space you wish to analyse. You will notice that the below icon will colour on the toolbar, which will be used to import the boundary conditions.
When pressed the following window will show.
Figure 4
All options for the importing of boundary conditions to the room are shown on this pop up. First, the boundary condition file is selected from any created on this project, useful when looking at different simulations as multiple files can be created and compared quickly. By selecting the opening flows option the bottom ‘Window Opening Position’ box is displayed. This will allow the user to define the entry point for the air from window openings, along with minimum opening requirements. Finally, the option to include room gains will apply any internal gain to the space. Note that if you wish to apply specific heat gains to specific areas of the zone, then components would be required, not the room gain option here.
Once the correct options are selected the ‘OK’ button is pressed to apply these to the space and create the appropriate boundary conditions, and will display as shown in figure 5.
Figure 5
In this example, the windows are set to open from a top hinge, so the air inlet and outlet are placed at the base of the windows and are shown in blue and red. Furthermore, the roof ventilation terminals are assigned opening conditions also.
More boundary conditions can be added manually if this is desired, however in this example the imported conditions are all that will be considered.
Once the boundary conditions are set up, the simulation can be run to obtain results for the space, this is done by pressing the button shown in figure 6.
Figure 6
This will open the dialogue shown in figure 7.
Figure 7
This displays the requirements for the CFD simulation to run along with displaying the maximum aspect ratio, which for the best results should remain below 20:1. The cell sizing can be altered with the CFD options if this is not reached.
When satisfied that the simulation is correctly set up and able to run, ‘OK’ is pressed and the below window opens, shown in figure 8
Figure 8
In this example, the simulation has been run to show the process of convergence. There is a number of important options to note on this pop up, such as the option to change the iteration number, which is useful when convergence is not achieved. There are options to change the variable control, the inner iterations, and much more.
Convergence is the process where the residuals of each variable reach a low value, typically aiming for 1e-5 in MicroFlo, or all variables have minimal change over the previous runs. In the figure 8 visual it can be seen that all variables have aligned low except the red line for mass which has levelled out, so this can be considered as converged in this case.
Once the simulation is complete and convergence has been reached, the simulation dialogue can be closed, then the MicroFlo Viewer is opened here:
Figure 9
This viewer is the results analysis portion of the MicroFlo tool, and can be used to gain greater insight into the behaviour of the air in the space. In this project, the consideration of draughts was the main component. With the space passing for temperature and CO2 requirements, the consideration for the occupant comfort in terms of draught is the final component.
When loaded the results viewer shows each variable down the left hand side:
Figure 10
Within the top list are the ‘Key’ options, when these are added it will show a results key along the bottom for the chosen variable. The next set of options are the different variables that can be displayed, for example the velocity vectors, or as shown here the filled velocity contours.
Figure 11
With this option selected the slice distance at the bottom can be altered to view the results at different portions of the space, or in different planes for analysis.
Within this project it can be seen that the high flow rate of air from the window in this low roof space causes a channel of air which will lead to high draughts on the students within the space. This study allows the ability to adjust the parameters of the design to avoid any unwanted air movement in the space, whether this is high flow rates, stagnant air or poor recirculation.
For more information on MicroFlo-CFD and how it can benefit your projects, explore our IESVE CFD Airflow Simulation resources and book a demo of the software. You can also contact our sales team at sales@iesve.com or your account manager to discuss your IESVE package.
