[AlexanderZ]

you are using symmetry boundary condition on the symmetry plane (oy in your case)

the plane is not important, you may cut your model more (to 1/4 for example)

best regards

[ydima]

Actually I think I didn't define symmetry anywhere

Is what I did on the picture enclosed enough (defining a symmetry region on all the faces in the plane of symmetry)? Or I also have to select all the faces and create a named selection "symmetry"?

Thanks.
Yoann

[AlexanderZ]

you should create a named selection "symmetry"

best regards

[ydima2]

Hi,

Okay a short post to give you some news (I've lost the password of my first account). I need to go back to a 2D model (which is actually easier so it's great ). Everything works fine with a velocity inlet, but when I try to use a pressure inlet I have issues of reversed flow. Thus I really need to open my air domain, because I am sure it is the source of the problem. I've put pressure outlet with a Gauge Pressure of 0Pa and a Backflow Volume Fraction of water of 0 (with an Operating Pressure of 0Pa in the Operating Conditions). The issue of reversed flow is even worse when I choose an outflow instead of pressure outlet...

Do you have a solution please?

Thank you very much.
Yoann

[AlexanderZ]

from my point of view reversed flows on outlet are ok for your case

best regards

[ydima2]

Hi,

As you explained me on another topic, I've decided to slice my model in half to have an axisymmetric thing. Thus, I've used a boolean tool to subtract half of the syringe+air area , and I've defined my boundary conditions the same way as before, but now I've added the axis boundary condition. In Fluent, I've changed Planar to Axisymmetric (don't know what "Axisymmetric Swirl" is). For the rest, I've put all the same parameters.

Unfortunately when I try to initialize, I have the following error:
"Error: Divergence detected in AMG solver: ads-0
Error Object: #f"

When I change pressure-outlet to walls, I can initalize, but when I launch the simulation I have:
"Error: Divergence detected in AMG solver: x-momentum
Error Object: #f"

My mesh looks good enough (check pic enclosed), so I think that this comes from the axis thing. Did I forget something? I've never done that before...

Thanks!
Yoann

[AlexanderZ]

axis-body_fluid should be of type axis

do syringe walls have thickness? it is not clear from pictures

you have shadow wall between air and syringe wall - which is ok in case you have wall thickness,
but there is no shadow between fluid and wall, which is strange

check material properties, are they correct?

in fluent check mesh quality:
General -> check
General -> mesh quality

check scale

best regards

[ydima2]

Hi,

Sorry to keep you waiting, I've spent some days to solve the problem (and I did ).

Walls don't have thickness, and I get what you say about walls, but I think my sketching is fine.

I checked mesh and you said and indeed there was a problem. The software was actually calculating negative volumes, because I wasn't in the y positives. Also, my axis of symmetry wasn't along the x direction, which seems to be a source of problems

So I took some days to put all my structure in a better placement, and I have cool results even with a pressure-inlet (on the first two pictures you can see the end of the tip on the right, and the fluid going out to the left)

I still have issues when I display pressure. I still have reversed flow for pressure-outlet (you told me that this is fine), but also for pressure-inlet after a certain time (you can see on the last picture that the pressure is going crazy at the end, which stops the simulation from running). I don't really know what is the problem here. Moreover, some negative pressures are mentioned in the values on the left, and the pressure seems to go higher in the syringe during the simulation, I don't really know if it's a good thing.

Thank you so much for your help

Yoann

[AlexanderZ]

is you are using operating pressure = 0, then you can get negative pressure. to get the absolute pressure you should add 1 bar

show pressure field on time moments: 0, 1e-05, 1e-04, 2e-04 may be more

best regards

[ydima2]

Hi,

I'm not sure to understand I've put 1 bar in the Operating Conditions (101325 Pa). What should I put in the pressure-inlet if I want a pressure of 72000Pa? 72000Pa or 72000+101325 Pa? And if I want an opened air domain, at the pressure-outlet I should put 0Pa or 101325Pa?

Thank you very much.
Yoann

[AlexanderZ]

If you want to have 170000 Pa on inlet (which is absolute pressure), then:
1. You put 170000 Pa on inlet, if your operation pressure is 0 Pa
2. You put 070000 Pa on inlet, if your operation pressure is 101325 Pa

same for outlet

best regards

[ydima2]

Hi,

Thank you very much for all your help these weeks my friend. You've always been here during 3 months, I can't tell you how grateful I am. How can I thank you?

Yoann

[AlexanderZ]

enjoy your life

best regards

[ydima2]

Hi,

I come back to you because it is my last week of internship. I have worked a lot on the simulation and I have better results now. When I display volume fraction of water, I don't have intermediate values between 0 and 1 now, except at the interface, which is more relevant.

However, my flow (water here) is not leaking straight, it tends to go away from the axis of symmetry. Do you have an idea of why please? It is totally non-sense. Maybe it is due to the reversed flow on the pressure-outlet?

Thanks.
Yoann

[ydima2]

Moreover, when I try to define a shear-thinning fluid, by defining a viscosity following a non-Newtonian Power-Law in the Materials section, the fluid just doesn't get out of the syringe I understand that the fluid is now more viscous, but it should actually get out... I've put the parameters used in the pic enclosed.

Do someone have a solution please?

Thanks.
Yoann

[AlexanderZ]

regarding your first question:
if you are using VOF model, you may play with surface tension
did you switch on gravity?
actually, fluid may go side, because pressure there is lower

try to apply more input pressure for the second case

unfortunately, I dont have much experience in multi phase flows

best regards

[ydima2]

Hi,

Yeah I switched on gravity. I indeed really think that the fluid is going away from the axis because the pressure is lower there, but I CAN'T find why I've put a pressure-outlet with a Gauge Pressure of 0Pa so it should be fine.... (in my operating conditions, operating pressure is 101325Pa). Maybe I'm wrong in the patch thing after initialization? Do you have a solution please

Thanks.
Yoann

[AlexanderZ]

fluid moves through the domain and air "flows" around the fluid. Thats why on the sides of water drop pressure becomes lower.

surface tension should keep fluid in drop shape. This is my vision of your model

may be flow speed is too high for surface tension forces.
From my point under some conditions your pictures could exist

best regards

[ydima2]

Hi,

Can you explain me why the fluid would go where the pressure is lower please?

Thanks.
Yoann

[AlexanderZ]

fluid (or gas) flows to region with lower pressure -> this is physics base
same as you changing your route if there is traffic jam ahead

from my point of view critical is time of your flow and fluid properties

best regards