Modeling 2D vs. 3D Flow in a Circular Pipe

In summary, Bone suggests that if the flow is axisymmetric, using cylindrical coordinates is appropriate. However, if the flow is not axisymmetric, neglecting the ##\theta## direction will result in loss of information.
  • #1
Mad_Jack
4
1
Hi everyone!

I have the following problem. I have a two phase flow in a circular pipe and I want to model it. I need to decide and justify whether the model should be 2D or 3D. For the moment, I can say only one thing: considering z-axis in the length direction (that it is the dominant one) I have to neglect x or y. If I do this, I suppose I would loose the circularity of the pipe because I consider x or y as the elongated direction, obtaining something more similar to the flow between two flat plates. Then, probably, I need a 3D approach.

What do you think about it? Do you have any suggestions? What other things could I loose if I choose a 2D model?

Thank you very much!
 
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  • #2
Is there any chance of even partial separation of the phases because of gravity?
 
  • #3
Are you familiar with the concept of axisymmetry?
 
  • #4
Sorry, I forgot to mention that the problem is in absence of gravity and the two phases won't be separated because the diameter is beneath the critical one.

Yes, I'm quite familiar with axisimmetry.
 
  • #5
Then I suppose the next question is, if you are familiar with axisymmetry, why are you trying to work this in Cartesian coordinates? Your question shouldn't be whether or not you can neglect ##x## or ##y##. Whether the flow is 2D or not you'll lose information doing that. The real question you should ask is whether this is axisymmetric.
 
  • #6
I'm not trying to work it in cartesian coordinates, it was just a way to explain my thoughts on what I was supposed to solve using a 2D model as I told. But then, if I consider the channel as planar, cartesian coordinates should work well.

In any case the problem is not axysimmetric.
 
  • #7
You can't consider the channel as planar, though. You said in the original post that you are dealing with a circular pipe, so treating the problem in cylindrical coordinates is appropriate. I brought up axisymmetry because the operative question regarding 2D vs. 3D flow here is therefore whether or not the ##\theta## direction can be neglected, and that depends on the situation and the level of fidelity you require.
 
  • #8
Thank you very much Bone! You helped me a lot to gain a deeper insight in my problem. In the next few days I'll see if it works! ;-)
 

1. What is the difference between 2D and 3D flow in a circular pipe?

2D flow refers to flow that occurs in a plane, meaning that the flow variables (such as velocity and pressure) vary only in two dimensions. In contrast, 3D flow occurs in three dimensions, meaning that the flow variables vary in all three dimensions.

2. What is the importance of modeling 2D vs. 3D flow in a circular pipe?

The importance of modeling 2D vs. 3D flow in a circular pipe lies in the fact that the flow behavior can differ significantly between the two cases. Understanding these differences is crucial in accurately predicting the flow behavior and making informed decisions in engineering and scientific applications.

3. How are 2D and 3D flow in a circular pipe modeled?

2D flow in a circular pipe is typically modeled using 2D Navier-Stokes equations, which are simplified versions of the full 3D Navier-Stokes equations. 3D flow in a circular pipe is typically modeled using the full 3D Navier-Stokes equations, which take into account all three dimensions of flow.

4. What are the limitations of modeling 2D vs. 3D flow in a circular pipe?

The main limitation of modeling 2D flow is that it cannot capture the effects of flow in the third dimension. This may be a significant limitation in certain applications where the third dimension plays a crucial role. Additionally, 2D flow models may not accurately capture complex flow phenomena such as turbulence. As for 3D flow models, their main limitation is their computational complexity, which may require significant computing resources and time.

5. Which situations are better suited for 2D vs. 3D flow modeling in a circular pipe?

2D flow modeling is generally suitable for situations where the flow is predominantly in two dimensions and the third dimension does not play a significant role. This can include cases such as laminar flow in straight pipes. 3D flow modeling is more appropriate for situations where the third dimension plays a crucial role, such as in turbulent flow or flow around complex geometries.

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