[CellZone]

Good morning,

if I calculate a flow (pressure & velocity field) and also take into consideration the energy equation for temperature, why does it take longer for temperature to converge than for velocity?

If I first calculate my velocity field and freeze it, and afterwards I calculate my transient temperature increase, will this speed up my simulation? If yes, why?

Thank you!

[sbaffini]

There might be a lot of factors at play here. And clearly depends from how you are actually solving them. Are you using some commercial software or your own solver?

Also, while what you state is not a rule, it is obvious that anything influenced by the flow field can't converge faster than the flow field itself in general, if you solve them together (either coupled or segregated).

If the energy equation is decoupled from the flow ones, your best option is to first converge the flow ones. Afterwards you have just a linear equation for temperature that is very easily solved in the most common cases, and should be very fast in comparison to the flow ones.

However, in general, my first guess would be at Cp or Pr effects. So, basically, very different time and length scales for the two set of equations. If you don't leverage such knowledge in setting up a proper solver for each of them, you tipically endup using a single solver which is the most conservative for the two problems. And tipically it is the flow equations that are more stringent; so you end up using their setting to solve the energy equation less fast than possible.

[FMDenaro]

I agree, I would first think about a Reynolds number different from the Peclet number ...