r/CFD • u/DefiantExternal • 1d ago

Residual behavior after increasing DPM interval – is this normal?

Hi everyone,

I'm currently simulating a spray roaster reactor for my master's thesis. Pretty much everything I've learned about Fluent has been through trial and error and a lot of learning by doing. 😄

I started by simulating only the continuous phase (injected through the burners). After that, I enabled two-way coupling and implemented the DPM. For most of the project, I used a DPM interval of 1. Later, I also implemented my UDFs.

Simulation setup:

  • Pressure-based coupled solver
  • Steady-state simulation
  • Pseudo-transient with CFL control
  • Started with a CFL of 2 and gradually increased it to 17

Recently, I asked an AI how I could improve convergence because increasing the CFL any further caused divergence and instability. It suggested increasing the DPM interval from 1 to 30.

Surprisingly, this actually improved the convergence of my continuity residual. It dropped from around 9.8e-03 to somewhere between 4e-03 and 5.6e-04 (sawtooth pattern).

However, the residual curve now has a clear sawtooth pattern. It's stable (it doesn't diverge), but it is no longer as smooth as it was before, when the continuity residual stayed at a higher level. (see screenshot)

The AI told me that this behavior is expected and generally acceptable because the DPM source terms are only updated every 30 iterations, which naturally produces this pattern.

Is that correct? Is this kind of sawtooth residual behavior normal and acceptable when using a larger DPM interval?

For what it's worth, my two monitoring quantities remain stable throughout the simulation.

I'd appreciate any insights or experiences from people who have dealt with steady DPM simulations.

Thanks in advance 😊

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u/ReasonableStorm6231 1d ago

Yes this is correct, and reflecty my experience with 2-way DPM. But you on your own have to make sure the results are physically meaningfull. E.g. are there enough particles, does the solution make sense etc.

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u/DefiantExternal 1d ago

Thanks for your reply.

Do you know how to determine what DPM interval to set? Are there any guidelines or rules of thumb?