I wanted to highlight a neat little feature that I came across the other day while trying to run an extremely large thermal flow simulation study. It's a feature called "flow freezing". It's buried deep in the calculation control options and isn't something that's talked about a whole lot, but in specific cases it can be extremely useful!

What flow freezing allows you to do is shut off or only run the flow calculations at certain periods of time. Basically what it does is just take the values of the flow calculations from that moment you shut them off and assume they are constant from that point forward. This is extremely helpful because the time of the solution is dependent on the slowest process that is being performed. The flow calculations are a much more complex process than heat transfer calculations so they are usually the limiting factor. In the case that I ran into, I had a model that was millions of cells and needed to run a heat transfer calculation on it. It was taking an insane amount of time to run before I could even see if the results were in the ball park or not. I was able to utilize this feature by turning off the flow calculations and letting it solve only the heat transfer calculations. This gave me an idea of whether my solution was in the ball park of what I expected without having to wait weeks for it to run the full calculation.

There are two options when selecting flow freezing. You can either completely shut off the flow calculations using the permanent option or you can stop and start the flow calculations of a period of time.

*Click to enlarge*

In the periodic options shown on the left, you specify when you want the freezing to start and then how long you want it to run with freezing on, and then how long you want it off. From that point forward it will alternate the freezing option on/off. The permanent option on the right is even simpler, you just simply tell it when you want to shut off the flow calculations. I would recommend running at least a full travel before turning on flow freezing of any kind. This will allow your flow calculations to fully develop through the system before shutting them off.

I hope this little feature will help you save some time when running those large heat transfer problems.

*By: Chris Olson, Simulation Applications Engineer*

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