[swtbkim]

Hi all.

I have some questions about imbalance, especially momentum imbalance.


1.
I heard that imbalance means the difference between what come in and what go out.
Then, where is the criteria?
I mean, where do things come into, and where do things go to?
Each grid?
I'm not good at english, and I hope you guys understand what I mean..


2.
I'm trying to simulate simple flow.
There is a fan inside pipe, and flow is induced by it. I attached the figure.

There is a momentum source, and it replace the roll of fan. Farfield is an 'Opening'.
I had a problem that CFX didn't start, so global initialization is added. (I don't know why global initialization is needed for starting CFX. It just worked.)
There is no other boundary condition.


Residual looks nice. It oscillates, but quite low.
Problem is imbalance. It does not converge.

Attached image is newly calculating one, so # of iteration is small, but things are similar when # of iteration is about 3000. It oscillates.

What can cause this??
I think the geometry is simple, and this is quite easy problem.. but I'm lost for a month..

Any help will be appreciated.

Thank you in advance.

[adunne304]

Originally Posted by swtbkim

Hi all.

I have some questions about imbalance, especially momentum imbalance.


1.
I heard that imbalance means the difference between what come in and what go out.
Then, where is the criteria?
I mean, where do things come into, and where do things go to?
Each grid?
I'm not good at english, and I hope you guys understand what I mean..


2.
I'm trying to simulate simple flow.
There is a fan inside pipe, and flow is induced by it. I attached the figure.

There is a momentum source, and it replace the roll of fan. Farfield is an 'Opening'.
I had a problem that CFX didn't start, so global initialization is added. (I don't know why global initialization is needed for starting CFX. It just worked.)
There is no other boundary condition.


Residual looks nice. It oscillates, but quite low.
Problem is imbalance. It does not converge.

Attached image is newly calculating one, so # of iteration is small, but things are similar when # of iteration is about 3000. It oscillates.

What can cause this??
I think the geometry is simple, and this is quite easy problem.. but I'm lost for a month..

Any help will be appreciated.

Thank you in advance.

Hi Sangwoo,

An imbalance means that more (or less) mass or momentum is entering your domain than leaving. This usually happens with round-off/interpolation errors associated with poorly described boundary conditions.

I think that's what's happening here. Opening BCs are not a good solution for boundaries where you're not sure what's happening. You should look at the physics of the model better and try prescribe a single inlet/outlet combination, bounded by walls.

[swtbkim]

Thank you Adrian!

But I think opening condition should be used.
Pipe and fan will be in air: an opening itself.

Do you think that larger opening can be helpful in this case?


Just FYI, details about opening BC are as follows.
Boundary details > Mass and momentum > Option > Entrainment
Relative pressure = 0Pa
Pressure option(checked)
Option > Opening pressure

[adunne304]

No problem.

I understand this. I still suggest setting your upper and lower boundaries to be free-slip walls. Set your left and right boundaries to be an inlet and an outlet. If they're far enough away, then it shouldn't have much impact on your solution; but it should solve your imbalance problem.

If you still find an issue with imbalances, then your problem could be with the specification of your fan BCs.

[swtbkim]

As you can see in attached figure, air is drawn into pipe from left boundary, and sprayed to upper and lower boundary, not right boundary. - Right boundary is actually kinda inlet
Nevertheless you suggest that wall is better than opening?


And one more,
Multi block grid can cause imbalance problem? at interface?


Thank you very much.