[Saturne 6.0.6] Particles are accelerated by head loss

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Antech
Posts: 197
Joined: Wed Jun 10, 2015 10:02 am

[Saturne 6.0.6] Particles are accelerated by head loss

Post by Antech »

Hello. I guess I found the bug in Saturne 6.0.6. I'm not allowed to share the entire geometry, but, I think, it's not a matter of particular case. In this case, there is a gas duct with recirculation line intake and head loss volume zone that represent the tube bundle. Gas phase calculation was successful (tube bundle resistance is accounted for without problems), then I introduced particles on frozen gas velocity field. Although particles move by expected streamlines, when they enter volume zone with head losses, their velocity rises gradually by ~25m/s (gas velocity is around 7.5 m/s in this area). I attached picture with particles mean velocity field.
Case setup is very basic. Gas velocity in problematic area is ~7.5 m/s (aw well as particles velocity before resistance zone), gas pressure drop across resistance zone is ~250 Pa at height of 1.2 m (resistance coefficient is 16.8 m^-1). Gas density is 0.365 kg/m3, dynamic viscosity is 3.17e-05 Pa*s. There are 9 particle fractions (classes) with sizes from 2.5 to 100 mkm. I attach case setup [xml] files for gas-phase and particle runs (case for gas-phase run is marked with A suffix that means "pure aerodynamic"). Please note that, in particles case, some values for gas phase are not set.

I tried Saturne 6.3 feature version but I was unable to set resistance coefficient in this version. GUI just doesn't accept it, when you switch to another dialog and back, all 3 resistance coefficients become zeroes whatever you set.

Would anybody, please, fix this particle acceleration in resistance zone? Maybe pressure gradient force is accounted for incorrectly?

BTW, Saturne gave 11% of particles coming into recirculation line while Fluent gave 15% that is quite good agreement (this case's purpose is to ensure that Fluent doesn't give us too low particle fraction in recirculation line).
Attachments
D-G000-R000-S000A.xml
Pure aerodynamic run case
(16.81 KiB) Downloaded 129 times
D-G000-R000-S000.xml
Particle run case
(17.63 KiB) Downloaded 125 times
Particle velocity profile
Particle velocity profile
Yvan Fournier
Posts: 4070
Joined: Mon Feb 20, 2012 3:25 pm

Re: [Saturne 6.0.6] Particles are accelerated by head loss

Post by Yvan Fournier »

Hello,

I reproduce the issue for the head loss in v6.3 (in a somewhat variant form) so I'll fix this.

For the main issue, I'll suggest the Lagrangian model specialists to take a look and post here.

Best regards,

Yvan
Yvan Fournier
Posts: 4070
Joined: Mon Feb 20, 2012 3:25 pm

Re: [Saturne 6.0.6] Particles are accelerated by head loss

Post by Yvan Fournier »

I just fixed the GUI head losses input issue in the v63. GUI, so the fix will be in v6.3.1, and is already on the v6.3 branch on GitHub.

Best regards,

Yvan
Antech
Posts: 197
Joined: Wed Jun 10, 2015 10:02 am

Re: [Saturne 6.0.6] Particles are accelerated by head loss

Post by Antech »

Thanks! Willl wail for reply about particle acceleration from Lagrangian module programmers.
Martin FERRAND
Posts: 44
Joined: Wed Mar 14, 2012 10:06 am

Re: [Saturne 6.0.6] Particles are accelerated by head loss

Post by Martin FERRAND »

Dear user,

Thank you for the feedback. We are aware of this problem which is due to the fact, if I understand well, that the additional force you have on the mean fluid (with the head loss) is missing in the lagrangian module (all terms on the momentum are not automatically taken as they should).
We will try to fix that soon.
Best Regards
Martin
Antech
Posts: 197
Joined: Wed Jun 10, 2015 10:02 am

Re: [Saturne 6.0.6] Particles are accelerated by head loss

Post by Antech »

Hello, thanks for so fast reply. Sorry, but, I think, it cannot be due to a missing force on particles because, if you leave such particles with only drag and gravitational forces under case conditions (given densities and gas viscosity, constant gas velocity), they will not accelerate more than by 1.1...1.4 m/s (terminal velocities by 3 different models for largest particle class of 100 mkm). So I can only guess that you consider gas pressure gradient force on particles and there is a mistake like unit conversion in pressure gradient force. Although it may be something more deep. Hope you will fix it without problems.
Antech
Posts: 197
Joined: Wed Jun 10, 2015 10:02 am

Re: [Saturne 6.0.6] Particles are accelerated by head loss

Post by Antech »

Regarding aerodynamic resistance zone interaction with particles, in my opinion, maximum that can be done for distributed resistance approach is an optional calculation of resistance with gas-particle mixture density instead of gas density (0.5*DX*ResistanceCoef*RhoMixture*GasVel^2). If calculation is performed with frozen gas flow field, density effect from particles on head losses cannot be accounted for (no particles at this stage). If you have porosity<1, gas will accelerate and so particles, it doesn' require additional code. I think it will be as physical as possible.
Yvan Fournier
Posts: 4070
Joined: Mon Feb 20, 2012 3:25 pm

Re: [Saturne 6.0.6] Particles are accelerated by head loss

Post by Yvan Fournier »

Hello,

What Martin means is that when using head losses in code_saturne, an external force is always added, but the Lagrangian module does not take it into account in a consistent manner, which explains the issue you have. This is more a refactorization issue in the code.

If you are trying to reach a steady state and have one-way coupling, a workaround would be to use head losses in an initial Eulerian computation, then run a "frozen field computation" , removing the head losses to avoid the extra term. For a coupled computation, a fix in the code is necessary.

Best regards,

Yvan
Antech
Posts: 197
Joined: Wed Jun 10, 2015 10:02 am

Re: [Saturne 6.0.6] Particles are accelerated by head loss

Post by Antech »

Hello. May I know current progress in resolving the issue?
Regarding physics, sorry, but I cannot agree with applying additional force to particles in resistance zone. What happens physically in that region? Fluid is accelerated and decelerated when passing through construction elements like tubes or perforated plates, static pressure is not restored fully that cuses head loss and it's correct to represent it by mass force. Particles physics is different. They are carried by the fluid, if there was no fluid, just gravity, their behavior would be completely different from gas or liquid. For particles it's enough to be driven by the drag force and other forces from liquid + gravity etc in this approach (small concentrations, not KTGF \ fluidized beds). If you apply additional force to particles in resistance region, their velocities and concentrations will change that is not true. In reality, if you measure particle and gas mean axial velocities and concentrations in resistance zone referred to full section (superficial velocity, that is shown by the model when porosity is 1), they will be the same as for other parts of the straight duct with constant section.
Yvan Fournier
Posts: 4070
Joined: Mon Feb 20, 2012 3:25 pm

Re: [Saturne 6.0.6] Particles are accelerated by head loss

Post by Yvan Fournier »

Hello,

As I am not an expert in that part of the code, I was perhaps not very clear, but the issue is one of numerics, not physics:

In the Eulerian part of the code, head losses are handled by adding an "equivalent" external pressure force. The Lagrangian model uses some elements from the Eulerian part, and terms should be handled in a consistent manner. It does not mean we would be adding a "physical" force, just making the model consistent. In any case, in a head loss region, I would assume competing effects, with acceleration with the flow due to reduced porosity/flow section, and slowdown due to collisions.

There has been recent work (in the master branch) so as to handle the "complete" turbulence model for Particles in a cleaner manner: so either particles are purely fluid (tracer), or handed as solid particles, whose handling with stochastic equations is adapted/synchronized with the Eulerian part. I cannot say more because I am not familiar with the details of the theory for the Lagrangian model in code_saturne (which is based on stochastic differential equations, not a collision model).

Again, the issue here is related to an inconsistent handling of some terms between the Eulerian and Lagrangian parts. So the forces which would need to be added are "numerical" in nature, to compensate for the artificial force induced by the pressure forces added to the Eulerian model. Otherwise, you get the acceleration you observe.

In any case, head loss zones are tricky, since the concept of head loss is simple in 1D but subtle in 3D, and mainly because the obstacles which lead to head loss often have effects on porosity (which handled by code_saturne), and turbulence (so specific turbulence source/sink terms should be added, but depend on the actual geometry, so a specific finer, resolved model, or measurements would be necessary for each physical component type).

In any case, I do not know if the issue is easy to fix (just a matter of refactoring some code), or much more subtle. I'll let Martin or Mathieu comment on this, as they are more familiar with this.

In the meantime, you might try to experiment with the "full" model to see if the same effects occur.

Best regards,

Yvan
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