[ydima]

Hi,

I've put your parameters, but I still have the issue of reversed flow. I tried to play with the size of the time step and refine the mesh but still the same problem... People on internet advise to increase the length between inlet and outlet, but first I need to have precise dimensions for my syringe, and actually I don't have an outlet in my case...

Do you have any tips?

Thank you.
Yoann

[AlexanderZ]

you've told, that in 2D everything works well.
All boundary conditions should be the same, the logic of mesh should be the same. Compare carefully.

best regards

[ydima]

Hi,

I will compare more in detail the 2d and 3d works tomorrow and Thursday. Still, this doesn't change the issue of reversed flow... Do you know there this could come from please?

Thank you very much.
Yoann

[AlexanderZ]

if your mesh is ok (looks like ok) than reverse flow (from my point of view) means, that in domain you have the same or even higher pressure as in inlet.

Initialize your model with 0 pressure everywhere, use patch to set other pressure in the region with water (in syringe)

best regards

[ydima]

Hi,

I'm not sure to understand. I've done the Patch as on the pictures enclosed, otherwise the fluid was going up in the syringe. Is it right?

I have interesting results in STEADY (transient is not working...) as you can see on picture 3, but the fluid still tends to be blocked into the air, and I still have the issue of reversed flow, even with the patch thing. Moreover, I need to put a huge pression (500000 Pa) to make the fluid going out of the syringe, but this leads to divergence issues.

By the way, I wondered, when I launch Fluent, I have the whole mesh in white and the inlet and blue (picture 4). Is it normal?

Thank you so much for the time taken... I just can't thank you enough.
Yoann

[AlexanderZ]

By the way, I wondered, when I launch Fluent, I have the whole mesh in white and the inlet and blue (picture 4). Is it normal?

by default wall boundary has white color, inlet - blue color. So it is normal.
It is impossible, that transient doesn't work.

What is steady simulation? It means, Fluent tries to find the case when energy and mass which comes to system equals to energy/mass which goes out.
In your model there is no outlet...
So reverse flow in inlet makes sense for steady case.
But this is not what you need, or your model should be changed.

when you patch, you should define 500000 Pa in zones, which you called fluid (syringe)

best regards

[ydima]

Oh okay thank you so much for the explanation.

Okay so with transient, pressure of 100000Pa, a standard initialization instead of hybrid one, and a time step size of 1e-05, I have some pretty cool results after waiting a (very) long time. However the issue of reversed flow appeared after many iterations, and it keeps increasing

I think this is my only issue right now... Do you have an idea on how to fix it? Maybe changing the solution method/solution controls? I never touched to these and I have no idea if they have a real impact on my solution.

Yoann

[AlexanderZ]

check the average pressure in your domain, is it the same with your inlet pressure?

best regards

[ydima]

Hi,

Actually, I left the simulation running during 5 hours, and crazy things happen for the pressure (I displayed pressure and volume fraction of water). Unfortunately I can't show you the results because I have some issues for having the animation thing, it's so frustrating... Anyway, to explain you briefly, at the beginning everything seems to work fine, I have a pressure of 100000Pa in the syringe and 0 in the air, and then I can see a pressure slowly decreasing in the syringe tip (between the 100000Pa and the air area). This stays like this during a long time, and meanwhile the fluid is slowly going out of the syringe. However, at one point and for any reason, everything get reversed (pressure is decreasing in the syringe and increasing in the air area). It is where reversed flow issue happens. Thus, at the end, I have a pressure of 100000Pa in the syringe, and 110000 in the air! This is completely incomprehensible... The pressure is then slowly returning to usual, with the decreasing thing in the tip as I described before. And then again it get reversed with a bigger pressure in the air. It just does a cycle.

I'm really sorry to not have pictures, it just doesn't work right now, and I have stuff to do to the lab. I'll put pictures next week if you want something more easily understandable.

Thanks.
Yoann

[AlexanderZ]

what is the goal of your simulation?

best regards

[ydima]

Hi,

I am doing a 3 months internship in a research centre. We work in a laboratory where we try to print "materials of the future" (with new properties). The 3D printer that we are using is working with a syringe. Thus, I try to model it on Fluent, to find the high-strain zones (from the pressures measured by Fluent). I wait to have decent results with my actual model to go further; I would like to test different trajectories for my fluid going out of the syringe, and also replace water by the fluid that we are really using in the lab (the syringe sketched has the dimensions of the ones in the lab already).

Unfortunately I am the only one who has a minimum knowledge base on simulations (even if I'm new to this), so I'm working all alone. That's why it can be very frustrating to be blocked by errors that I don't really understand, and that nobody around me (except you ) can explain to me.

Yoann

[ydima]

Okay so I actually some pretty cool results (the mesh is not refined at all but the calculation is slow enough). Fluid is actually going out of the syringe, I just didn't have time to run it for a long time so we just see the beginning on the pics. Unfortunately I still have this issue of pressure going crazy all of a sudden, as you can see on the screens.

Do you have an idea of what this could be? In boundary conditions, for the pressure-inlet thing, for WATER, should I put a volume fraction of 0 or 1? I'm not sure about that.

Thanks.
Yoann

[AlexanderZ]

1. I don't understand your 5th picture, what is it? where is syringe, which I can easily recognize on 1,2,3,4 pics?

2. It seems, that mesh quality is low, you should refine it new syringe tip and down along the flow

3. In Inlet you should define water 1 and air 0, from my point of view

4. If you are focused on flow from syringe, than it is better to "open" your air part -> change boundary condition type of air part from wall to pressure outlet with boundary condition pressure = 0 (if your operating condition is 0)

5. Also I recommend you to increase the size of your air zone

6. If your are interested in pressure distribution inside syringe -> this is other story. Than you don't need to path syringe zone, use only boundary conditions of pressure inlet: define pressure and water = 1. Use small enough timestep

best regards

[ydima]

Hi,

The 5th picture is actually just a zoom on the end of the syringe, you can clearly see water going out (even if the mesh is low).

Okay so today I increased the size of the air area, and refined a lot the mesh. I didn't have time really to launch a simulation during a long time, because it is just soooo long with the new mesh. Maybe I'll lower it a little tomorrow, or find another solution.

However I just try to run quickly with the pressure-outlet thing, and I have the reversed flow issue for the outlet now... And it's way more bigger than for the inlet (for the inlet I don't have it except for some iterations, whereas here I have it immediately and for a huge amount of faces). I just replaced the boundary conditions walls_air by outlet, as you can see on the pic enclosed (you can see the parameters used in Fluent on pics 2 and 3). Do you see a problem please?

Thanks.
Yoann

[AlexanderZ]

you may get reversed flow in outlet because of your model configuration.
However, flow through outlet is not the goal of your simulation -> you may ignore it, focus on water flow from syringe

you may use more symmetry in your model -> you may use not half but 1/4 or ever smaller 1/6, 1/12 up to you, but mesh quality should be good enough

best regards

[ydima]

Hi,

Okay so I tried to run the simulation with the bigger air area and a pretty low mesh. I get very good results, but unfortunately at around 3500 iterations I have the reversed flow in the pressure-inlet issue, and then the solution diverges. This happens where the mesh starts to be bad, so I wonder: can it be linked? Can a poor mesh leads to an issue of reversed flow?

Thanks.
Yoann

[AlexanderZ]

how do you know, that the results are good?
show pressure fields and time
what is time, when you get reversed flow in inlet?

best regards

[ydima]

Hi,

Well I told you that it is working because water is going out of the syringe, and there is no issue of reversed flow until having a low mesh, so... But you're right, I should display more things to be sure. I haven't checked the time where I started to have a reversed flow. However, my computer is too slow, so these days I'm looking for a better one in the school to display more things and results for a really long time.

Moreover, I need to model a complex fluid and not only water, so I've just installed OpenFOAM to work on the modelling of this fluid.

I'll keep in touch next week if I have some news on the work. Anyway thank you so much for time taken!! You've been so helpful.

Thanks.
Yoann

[ydima]

Hi,

I wonder if the plane in which I sketched my syringe is important? Because I want to model an axisymmetric model you know, and I'm not sure if Fluent understands well that I only did the half of the syringe and that it's symmetric. For your information I sketched it along the Oy direction.

Thanks.
Yoann

[ydima]

Someone for my short question please?