[newbie384]

Hi

I am trying to perform cfd on annular flow channel with rotating inner wall and stationary outer wall (please see attached image for the sketch). I would like to make use of 2d axissymmetric in fluent to save computation time and I hope that someone can enlighten me on the matters below.

1) There is axissymmetric and axissymmetric with swirl in fluent. I am abit confusing with the difference in these two and I wonder which one is appropriate for my problem?

2) However, some documentations said I should set the zone type of axis of rotation to axis. Since the axis of rotation is not coincide with the model, I set the rotating inner walls as moving wall with specified rotating speed. (I can't set zone type of axis of rotation, right?) Am I correct to do so? If not, please advice on the correct procedure.

Thank you in advance.

[delaneyluke]

axisymmetric swirl is for rotating walls, so this would be applicable in your case.
You would have to specify a rotating axis as part of your model

regards
Luke

[newbie384]

Hi

Thanks for replying. I would like to clarify one more thing.
You mentioned that I have to specify a rotating axis as part of my model.
Did you mean I have to create a line of rotating axis and define the line as axis boundary in Fluent? I thought by default Fluent assumes the rotational axis to be x axis and I wonder whether there is a need for me to specify rotating axis if the domain is not lying on x axis (above y=0). Please advice if I am wrong.

Thank you in advance.

[flotus1]

You dont have to model explicitly an axis of rotation.
All the parameters for the rotation can be set in the boundary conditions option of your rotating wall.
Fluent assumes the x-axis as axis of rotation automatically.

The axisymmetric swirl option is the right choice in your case, because there will be a velocity component perpendicular to the plane we are looking at in your sketch.

[newbie384]

Hi

Thanks for the clarifications. Now I have more confident to do my simulation in 2d axisymmetric with swirl.

I realized that we can choose 2d axisymmetric for rotating wall simulation but in that case, there is no velocity component out of the 2d plane (swirl velocity) as you mentioned. I just wonder what is the practicallity for 2d axisymmetric without swirl option for rotating wall. Is it just for simplification, i.e. for the case when the rotating speed of wall is low and we can neglect the swirl component?

[flotus1]

Actually, you cannot have a rotating wall in a plane axisymmetric case as pointed out by delaneyluke.
Well you can in fluent, but the input is ignored.

[newbie384]

Hi

Actually I ran two simulations with axisymmetric solver(no swirl). One with zero rotational speed of wall and the other one with non-zero rotational speed of wall. I found that the pressure distribution along the flow path is the same for both cases and I think this is because FLUENT ignore the effect of centrifugal force caused by rotation, even though we set the rotational speed of wall (Please correct me if I am wrong). However, when I look at the temperature distribution along the flow path, both cases gave very different results. The one with non-zero rotational speed of wall gave higher temperature. Can anyone advise on this matter? Is it because FLUENT include the effects of friction for non-zero rotational speed for wall and this friction increase the fluid temperature?

[flotus1]

Are there any heat sources in your simulation?
Maybe you could update the sketch of your setup in the initial post with a complete description of boundary conditions and all effects you are trying to simulate.
The fluid should not heat up due to frictional forces unless you activated the "viscous heating" option

Fluent DOES take into account centrifugal forces. They might just be too small to have a substantial effect on the pressure field. If you evaluate the results more accurately, you will see a difference between the two results.

[newbie384]

Hi

I tried to simulate heat transfer of fluid passing through a passage which is made of a rotating wall and a stationary wall. I would like to simulate the friction effects (something like disk friction) as well so I turned on the viscous heating option. I think with the viscous heating option on, there is a heat source.

I am wondering if Fluent ignores any input of rotational speed for axisymmetric (without swirl), why there is a different in temperature for zero and non-zero rotational speed.

Can you describe more on what did you mean by "if you evaluate the results more accurately"?

[flotus1]

Originally Posted by newbie384

Can you describe more on what did you mean by "if you evaluate the results more accurately"?

From my experience in this forum, most of the people complaining about physical effects being ignored in their simulations simply chose parameters way too small. This results in minor differences in the result that can only be noticed when the results are evaluated very thoroughly and not by looking at two contour plots with a global scaling.
In your example: increase the rotational speed and/or the fluid density several times by an order of magnitude.

I have no idea why there is a difference in the temperature field in the axisymmetric case without swirl when the rotation is applied to the wall. Might be a bug, but since this case has no physical relevance i wouldnt care too much about it.

[newbie384]

Thanks for all the good advises!!

[hitzhwan]

Originally Posted by newbie384

Hi

I am trying to perform cfd on annular flow channel with rotating inner wall and stationary outer wall (please see attached image for the sketch). I would like to make use of 2d axissymmetric in fluent to save computation time and I hope that someone can enlighten me on the matters below.

1) There is axissymmetric and axissymmetric with swirl in fluent. I am abit confusing with the difference in these two and I wonder which one is appropriate for my problem?

2) However, some documentations said I should set the zone type of axis of rotation to axis. Since the axis of rotation is not coincide with the model, I set the rotating inner walls as moving wall with specified rotating speed. (I can't set zone type of axis of rotation, right?) Am I correct to do so? If not, please advice on the correct procedure.

Thank you in advance.

Excuse me, how do you set your axis in the fluent?

[vinerm]

While creating your geometric model, you need to have a line drawn along the x-axis; not just parallel to x-axis, it must be coincident with x-axis. This line is defined as axis type in 2D axisymmetric solver.

[hitzhwan]

Originally Posted by vinerm

While creating your geometric model, you need to have a line drawn along the x-axis; not just parallel to x-axis, it must be coincident with x-axis. This line is defined as axis type in 2D axisymmetric solver.

You see this model , the axis is not connect with model ,how can I draw a line along the x-axis.

[vinerm]

Presence of an axis is not must for axisymmetric problem. You don't need to define an axis for an annular case like yours.

[hitzhwan]

Originally Posted by vinerm

Presence of an axis is not must for axisymmetric problem. You don't need to define an axis for an annular case like yours.

You mean there is no need to draw a real line for the axis？

[vinerm]

Fluent defines x-axis as axis. User never defines an axis. That's the reason user should ensure the edge coincident with the x-axis if he or she wants to use a particular boundary as axis. When user chooses a boundary as axis, he or she is only defining that there is a boundary coincident with the axis but user does not define the axis itself.

[hitzhwan]

Originally Posted by vinerm

Fluent defines x-axis as axis. User never defines an axis. That's the reason user should ensure the edge coincident with the x-axis if he or she wants to use a particular boundary as axis. When user chooses a boundary as axis, he or she is only defining that there is a boundary coincident with the axis but user does not define the axis itself.

You mean in the boundary condition setting, we do not need to use the axis condition to fit anything, we just put a virtual line (do not exist)on the x-axis.

[vinerm]

The boundary conditions should be as per the real system. If there is no domain coincident with the x-axis, then you do not use axis boundary condition for any of the domain boundary. E.g., if you wish to study flow through an annular region, there is no boundary touching the axis.

There is nothing called a virtual line or boundary in Fluent. User does not need to do anything to define the axis. X-axis is hard-coded as axis in Fluent.

[hitzhwan]

Originally Posted by vinerm

The boundary conditions should be as per the real system. If there is no domain coincident with the x-axis, then you do not use axis boundary condition for any of the domain boundary. E.g., if you wish to study flow through an annular region, there is no boundary touching the axis.

There is nothing called a virtual line or boundary in Fluent. User does not need to do anything to define the axis. X-axis is hard-coded as axis in Fluent.

Yes， thank you. Do you know the ansys designxplorer ?