Learning Forum

[Lokman61]

Hallo Ansys Community,

I dont get a error from Fluent, so i will discripe my problem as good as i can.

I have an single flow gas (air) steady system for the continuous phase. Now i add a discret phase model (DPM) with unsteady Partikel tracking (two-way). My particle material ist water-liquid as Inert. Surface injection with a mass flow rate of 0.06 kg/s and an velocity of 47.4 m/s.

I need to activate unsteady paticle tracking because I would like to have wall-film activated on my wall. This option is only available if unsteady paticle tracking is selected.
Now Fluent would like to have injection times start time / stop time. I left them at zero.

My first question:
I want a steady state simulation what exactly should I enter as start and stop time?

My problem is that particles are tracked but no particles escape or remain incomplete. All values are set to 0, so it is not possible to export the particle history file.

Why are no particles injected?

 

Comand bar:

"Advancing DPM injections ....

number tracked = 36, escaped = 0, aborted = 0, trapped = 0, evaporated = 0, incomplete = 0, incomplete_parallel = 0"

 

thanks in advance.

 

[Rob]

Transient particles mean you're tracking a particle in a transient reference on top of a steady flow field: hence you need to set a start and (long) stop time. As the solution progresses you'll then see particles in your domain, and they'll update positions with their time; this also means you'll need a lot of iterations to reach an equilibrium state. 

[Lokman61]

Is there a way to calculate the maximum stop time? Is this dependent on max. number of Steps or Particle time Step Size?

I set:

Particle time steps size: 1E-06

Number of time steps: 1

Max. Number of Steps: 500

Step length Factor: 5

by 500.000 elements

 

 

[Rob]

The end time of what? The particles are injected and travel through the domain, you need to run sufficient iterations to ensure this happens. 

[Lokman61]

Ok. I will try this. I set the number of iterations at 10000 and the injection stop time at 3600 sec.

thanks a lot

[Lokman61]

Unfortunately, the problem still exists. I think my problem lies in the step size. He shows tracked particles but no escaped or spalshed one.

Iterations: 40000

My settings:

Max Number of Steps: 2700000
Step lenght Factor: 5
Particle time step size: 1E-06
Number of time steps: 1

Interaction with Continues Phase: enable
unsteady Particle tracking: enable

Step lenght factor * N (elements) = Max Number of Steps, is this assumption correct?

how can i know how high the step lenght factor has to be?

Where could the problem be that the particles don't walk through the cells?

[Rob]

Where are the particles if you plot them? 

[Lokman61]

Still a certain number of particles is only displayed on "traped", but none with "escaped" or "splashed".

I choose for the injection surface -> inlet.

did I understand your question correctly?

[Rob]

If you plot the particle tracks where are the particles?

[Lokman61]

nowhere.

they are not depicted. You can see from the residuals that they are not loaded into my model because there would be an increase of the residuals.

[Rob]

What residuals?  If you have injected particles they're in there somewhere. Try altering the graphics display (particle size) to find out where they are. 

[Lokman61]

The problem is that the export of the particles does not work in this case. The iteration step to export is not executed. Without error message.

Therefore an import is not possible and therefore no representation of the particles is possible.

 

The residuals which are ploted by calculating the case:

When simulating I get the scaled residuals (continuity, velocity's, k, e- course) which go down to 10^-5. If particles start to be injected, there would be an increase in residuals. That is not happening.

 

[Rob]

In Fluent go to Results -> Particle Tracks and display. You may need to alter the particle display (centre top of the panel). If the particles are really small then it's possible that they're not causing the residuals to spike. However, as I can't download files there's not much I can do.  

A quick test may be to turn off transient particles and track an injection. This will confirm whether particles do enter the domain. 

[Lokman61]

Yes, if I turn off "unsteady particle tracking", then the iteration works and particle export as well. Only the step "unsteady Particle tracking" doesn't work.

 

particle diameter: 0.0002 m

[Rob]

OK, so that confirms that particles can enter the domain and pass through. Now look at the particle residence time. Switch back to transient particles and run: if you display the particles where are they? Post process in Fluent for this as you just want to view them. 

[Lokman61]

OK thanks for help.

I get a solution with varying the Parameter for

Particle time step size: 0.8
Number of time steps: 30

it's not convergent, but that's another problem.

 

I'd like to ask you another question.
Could you tell me why within a 100% symmetric model the turbulent flow (free flow) is asymmetric? Which factor in the k-epsilon turbulence model causes this?  Since no environmental influences exist in a simulation, there should be no differences along the symmetry plane in a symmetric model? The literature and my simulation show that symmetric models also show asymmetric flow patterns. 

[sajjad]

Hinsorry I have a Similar problemnYou can take a look؟nplease!n

[Rob]

Staff are not permitted to open or download files, we're limited to commenting on images etc. Other members of the community can download and review the files though.n

[Rob]

sorry missed that. Have a look for coanda effect and pitchfork bifurcation (two separate phenomena). n

[sajjad]

hi. can i have your email address?.

[Rob]

No. n