Link to post-processing state:
Step-by-step Tutorial: Post-processing for Air Change Rate
To ensure that the minimum required air change rate is obtained, a post-processing state has been created in the post-processing software Paraview, which does as much as possible to make this an easy process for the user, however certain inputs are required for accurate results. The overall process is opening the state, selecting the downloaded results, changing the clip filters to isolate certain surfaces and volumes and then read out the air change rate value (ACR) in number changes per hour (1/h).
Download and extract the results
The first step is to download and extract your results from SimScale, to do this, simply select the run in which you wish to download and select the download icon.
Figure 1: Selecting the required run and downloading using the download icon.
You will need to extract the zip folder before continuing.
Opening the State
As previously mentioned the state is there to make your job easier, you simply need to open the state, select the case, modify the volume and read out the ACR value. So to start, open Paraview (the state was created using version 5.5, other versions might produce warning messages but shouldn’t be a problem) and open the state, when prompted, load the case from file, and select the case.foam file from your extracted run.
Figure 2: open a state.
Figure 3: Choose file names of select the case.foam file in the extracted directory you have downloaded. Finally, select ok.
Modify the Internal Volume
There is a required air refresh rate for VIP Latrines of 6 refreshes per hour, to calculate this change rate, you need two parameters, volume flow rate and the volume of the reference space. Here we slice a surface across the drop holes and vent pipes and reduce the surface using clips Reduce Surface. This surface is used to evaluate the flow rate and should be checked to ensure that the vent pipe disks are not cut by the clips.
Figure 3: Disks not cut by the Reduce Surface filters.
Next, the volume needs to be set so that it represents only the volume within the walls as depicted below, this does not have to be accurate, however, should not be used to increase volume to give a bigger ACR. the top should be level with the bottom of the roof and the sides should extend to the wall fully, including about half of the wall vent volumes as shown. The bottom should extend to just include a small part of the holes.
Figure 4: Entire internal volume, up to the roof part and down to the floor, extending approximately 50% into the walls.
Reading the ACR
Finally after the adjustments have been made it is safe to take the ACR rate in the right-hand panel as the final ACR for this design and wind direction.
Figure 5: ACR value in a number of refreshes per hour to use in the cost function.
The recommendation is to ensure that the ACR is kept above 6, however, to allow an error margin 8 refreshes should be the minimum that the design achieves for all wind direction. The overall task is to minimise the cost calculated in the CAD model but to exceed this minimum of 8 ACR calculated using CFD.