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Old   April 2, 2016, 13:59
Question dynamic mesh/moving mesh
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nguyen lam
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Hi every body,
I'm newbie in CFD and I have a problem.
I simulate the mixer tank, with 2/3 of fluid region is Naoh and 1.3 of fluid region is air, I also have an impeller which rotates at 110(rev/min).
I want to analyse stress of the impeller with interaction of fluid.
I used immerse solid, but it is not supported for multiphase. So I use moving mesh, but I don't know about moving mesh.
Can anyone help me? Thank you so much!
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Old   April 3, 2016, 06:30
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Glenn Horrocks
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For rotating machinery you should use rotating frames of reference where ever possible. There is no restriction on multiphase models and it is a fast, simple, accurate model.
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Old   April 9, 2016, 07:44
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Thanks a lot for your advice!

I did it. But now I have a problem:
p.JPG

v.JPG

The volume Fraction of Water, I think it's true. But the Pressure, Its distribution quite strange. Can You explain for me if it is true? Why the pressure at the top is higher than the bottom. My boundary at the top is opening and I have 2 phase: water and Air.

Thank you so much!
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Old   April 10, 2016, 06:04
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You have a fundamental error in your simulation. Please post your CCL.
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Old   April 10, 2016, 09:20
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https://drive.google.com/open?id=0B9...WZGWU9abzJFN0U

Here is my CCL. Thank you!
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Old   April 10, 2016, 18:38
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In models with buoyancy the pressure is a modified pressure without the hydrostatic head. Have a look in the documentation about the modified pressure for buoyant simulations. That is what is causing the high pressure at the top.

Also - you don't seem to have a free surface model. Is this what you intended?
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Old   April 10, 2016, 19:11
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I used an expression "if(y<2[m],1,0)" to modify the level of water. Isn't it free surface?
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Old   April 22, 2016, 07:27
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Thank you! Now I knew about free surface. And I have some problem, Can you explain for me?
Here is distribution of dynamic pressure from the impeller to the wall of tank
xpres.JPG
And this is the dynamic pressure at wall of tank from the bottom to the top
ypres.JPG
Is it right? I think at the wall of tank, the velocity will nearly equal zero so dynamic pressure is zero. Then I compare the pressure and the total pressure, these are nearly the same!
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Old   April 22, 2016, 08:05
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I do not understand your question. Can you state your question clearly?
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Old   April 22, 2016, 08:14
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Oh I just want to ask if the distribution of dynamoc pressure in 2 picture is right? Is the dynamic pressure at the wall of tank equal zero?
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Old   April 22, 2016, 08:19
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Dynamic pressure is 0.5*density*velocity^2 (https://en.wikipedia.org/wiki/Dynamic_pressure). At a normal no slip wall the velocity is zero, so the dynamic pressure is zero.

Your calculations are showing a slightly non-zero number as the calculation is based on conservative numbers, not hybrid numbers. This means the cell nearest the wall has a small non-zero velocity as its centroid is slightly off the wall.
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Old   April 22, 2016, 08:33
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Thank you! So you mean the dynamic pressure at the wall of tank is zero. Is it match with the realistic model? I mean in realistically, is the dynamic pressure equal zero?
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Old   April 23, 2016, 07:10
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Yes, it really is zero.
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Old   April 23, 2016, 08:35
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Thank you so much!!!
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