PIC "default" settings

MFIX-Exa Help
1

Is there a set of “default” (read: suggested starting point) PIC parameter settings for setting up a new case. I have something like this (below) but I’m not sure if this is a good default or if I’ve adjusted these out of whack for some problem years ago.

#----------------------------------------------------------------------
# PIC model settings
#----------------------------------------------------------------------
#dem pic.solve = None 
pic.solve = solid0

pic.verbose = 1

pic.max_iter = 3

pic.beta = 3.0
pic.pressure_coefficient = 100.0

pic.close_pack = 0.60
pic.parcels_per_cell_at_pack  =  30.

pic.small_number = 1.0e-8

pic.damping_factor = 0.40
pic.damping_factor_wall_tangent = 0.9
pic.damping_factor_wall_normal  = 0.7

pic.velocity_reference_frame = 0.50

pic.advance_vel_p = 0.5

pic.initial_step_type = nth_eps
2

What you posted is a great starting point for PIC settings and should work well for most simulations.

  • pic.close_pack should match whatever corresponds to the solids you are modeling.

  • pic.parcels_per_cell_at_pack can be thought of in terms of resolution with 24 being an approximate lower bound. This input controls how much effective volume each parcel represents. If you use a small value (e.g., 4), each parcel represents a lot of solids volume. Therefore, small changes in parcel positions will cause large swings in the solids volume fraction, and in turn, the solids stress.

  • I hardly ever use different values for beta an pressure_coefficient. The same can be said about the damping_factor settings.

As for the rest – I do not recommend changing them unless you have a good understanding of how they affect the PIC update algorithm. Arbitrarily setting initial_step_type, advance_vel_p, or velocity_reference_frame to different values most likely will not end well.

3

you can’t go as low as 4. you put a limit at 12. I tried to force it so I could get a stat wt of 1 on a dx = dp and got swatted at haha.

4

you can’t go as low as 4. you put a limit at 12.

I regret nothing.

5

Comparing with a “regular” MFiX PIC setup, the default values seem to differ a little bit, added in bold below. Should these be used instead, or are these just different across the two editions? I’m setting up a Geldart group A PIC case, not much different from the NETL paper, but I get quite fluctuating pressure reading, and far less bubbles than found in say TFM. As the NETL paper keeps P=100, gamma=3 and statistical weight = 1000, I’m trying to see which other parameters that should be tweaked. Could it be damping, or something in the advanced section?

pic.beta = 3.0
pic.pressure_coefficient = 100.0

pic.close_pack = 0.60
pic.parcels_per_cell_at_pack  =  30. **statistical weight?**

pic.small_number = 1.0e-8

pic.damping_factor = 0.40 **0.85**
pic.damping_factor_wall_tangent = 0.9 **1**
pic.damping_factor_wall_normal  = 0.7 **0.85**

pic.velocity_reference_frame = 0.50 

pic.advance_vel_p = 0.5 **1**

For probes inside the bed (inlet velocity increasing every 10 seconds):

image
image1674×711 37.2 KB

Similar TFM curves:

image
image1653×723 38.9 KB

6

I would start with the values I provided. parcels_per_cell_at_pack is not statistical weight, it’s how many parcels it takes to fill a fluid mesh cell up to close_pack. You have to back out what the statwt is yourself. I doubt the wall damping factors have much of an impact at all. TBH I don’t understand the reference frame nonsense, so I never adjust that value.

Since you have a TFM curve, I’m assuming that’s from MFiX classic. Do you also have a PIC model with MFiX classic? Keep in mind, this is not just a new edition; it’s an entirely different code.

7

Thanks. Yes, forgot to mention, all of this is MFiX classic. Been looking for advice on the PIC setup, and thought this post could be of help. So my bad, I didn’t know the difference was that big between the two editions. (Although still a little surprised that the default PIC parameters are not the same.)