I was with a new customer this week who wanted to run a thermal Flow Simulation on a PCB with multiple heat sources - all cooled by natural convection. That was straightforward enough but he then asked if we could run a simulation on the complete enclosure with a rack of 8 identical PCBs.
This prompted me to think about the 'Periodic' option in the 'Computational Domain'. I knew it was there but had never found a decent application for it - until that moment. It was then obvious that this is a great way to work if you have a row of repeating geometric and thermal conditions. It means you can simulate just one of the PCBs - but get Flow to work as if there were more - all in a row. It then enables you to see how hot the ones in the middle get as they are influenced by their neighbours.
Here is an assy I knocked up to illustrate. It is 8 PCBs in a linear pattern, spaced at 26 mm. They are then enclosed by a thin walled box with cooling slots top and bottom.
To switch on the 'Periodic' condition, rmb on the 'Computational Domain' and pick 'Edit Definition'. You can then select the drop down box on the right hand side of the 'Size and Conditions' and pick 'Periodicity'. Below I have set the Zmax and Zmin values to Periodicity. The gap between the Z planes is 26 mm - exactly the spacing of the linear pattern of parts in the rack ...
This creates a domain as below (with perspective on). The dotted lines indicate that this where the repeating zones will be ...
Once the study has been run you get results in the domain as shown below ...
Now the clever trick that Flow pulls off is to match the flow conditions on one side of the domain in the Z axis with the other. In other words, with a Periodic domain, whatever flows in/out of one side is the same as on the other side. This is different from symmetry where the flows are mirrored. The Periodic condition is the Flow equivalent of a linear pattern.
To prove that this is the case, I captured the Cut Plot shown above and using an Image Editor copied it multiple times. I made the images touch so that I could see the flow vectors. This is shown below. It illustrates that the flow coming in/out from the left is identical to what is exiting/entering the domain on the right. It is as if there were a series of PCBs in a row and they were each affecting each other.
For any Flow users running thermal simulations on banks of identical assys then this is a great technique.
By Andy Fulcher
Solid Solutions Management Ltd