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wrong velocity value from solution initialization |
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September 26, 2014, 07:31 |
wrong velocity value from solution initialization
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#1 |
New Member
Flavio
Join Date: Oct 2013
Posts: 12
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Hello everybody,
I'm trying to make a 2D simulation, with incopressible flow (water) with energy equation solving too, on Fluent. I use mass flow inlet and outflow as boundary conditions at the inlet at the outlet. I don't know what's happening, but when I initialize the solution, from the inlet, the axial speed value calculated is completely wrong. It returns approximately 390m/s instead of 0.5m/s . I have checked the values settings for the cross-section , the density and the flow and they are all correct. Could anyone help me please Thanks in advance |
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September 26, 2014, 07:37 |
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#2 |
Senior Member
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Can you show the calculation why you think it should be 0.5 m/s?
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September 26, 2014, 08:15 |
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#3 |
New Member
Flavio
Join Date: Oct 2013
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Hello,
here are my calculations: mass flow=0.15 kg/s Diameter=0.02m -> Section=3,14*10^-4 m^2 density=1000kg/m^3 velocity=mass flow/(section*density)=0,4777 thanks a lot |
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September 26, 2014, 08:28 |
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#4 |
Senior Member
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It could be that your axis of rotation is wrong...
Can you let Fluent calculate the mass flow rate through the inlet after initialization? (Reports->Flux reports) |
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September 26, 2014, 08:35 |
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#5 |
New Member
Flavio
Join Date: Oct 2013
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I tried to make this check after initializing and it returns the correct mass flow rate: 0.15 kg/s at the inlet.
After the calculation and convergence it returns the correct mass flow rate: 0.15Kg/s at the inlet and -0.15kg/s at the outlet. So the flow field is completly wrong. I'm desperate! |
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September 26, 2014, 08:40 |
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#6 |
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Rick
Join Date: Oct 2010
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Did you check if you need to scale the mesh?
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September 26, 2014, 08:45 |
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#7 |
Senior Member
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Your model is axisymmetrical. How sure are you that Fluent uses the axis of rotation that you intended?
What is the axis of rotation in your model? And which axis does Fluent use? |
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September 26, 2014, 09:00 |
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#8 |
New Member
Flavio
Join Date: Oct 2013
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I'm sure Fluent uses the correct pipe's axis, because it's the once in the boundary conditions. I checked the cross section and it's correct. In addiction I tried to rotate my model around the axis...
For ghost82: what do you mean for scaling mesh? I'm sorry |
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September 26, 2014, 09:04 |
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#9 |
Senior Member
Rick
Join Date: Oct 2010
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Edit: sorry, wrong calculation.
0,15 kg/s at the inlet are 0,47 m/s in velocity, so what's wrong? The -0,15 kg/s means fluid is exiting your domain. Can you attach a picture of the velocity contour? |
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September 26, 2014, 09:12 |
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#10 |
Senior Member
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Don't be so sure. By default, Fluent will assume that the axis boundary is on the y=0 line. (Fluent User's Guide, 6.3.17.) Is that true in your case?
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September 26, 2014, 09:25 |
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#11 |
Senior Member
Rick
Join Date: Oct 2010
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Hi pakk,
sorry for ot, I read what fluent's user guide reports about axisymmetric problems and y axis; however I always set the x axis in the preprocessor as the axis of rotation (and domain in +y, +x region). Any other axis, y or z, gives divergence at first iteration. Do you know more about this? http://www.cfd-online.com/Forums/flu...-momentum.html Error in the user guide?Or different axis for different fluent versions? http://aerojet.engr.ucdavis.edu/flue...g/node1217.htm "For axisymmetric problems, in which the rotation axis must be the x axis,..." EDIT: ok, y=0 means x is the axis of rotation |
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September 26, 2014, 09:52 |
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#12 |
New Member
Flavio
Join Date: Oct 2013
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SOLVED!
I did not know that , by selecting a different material than the default , would create a mix ! I noticed when I created the new geometry to the attachment. Thank you very much. Sorry if I wasted your time ! |
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September 26, 2014, 09:57 |
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#13 |
Senior Member
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If it is a simply pipe, you should only model half of a cross-section.
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September 26, 2014, 12:52 |
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#14 |
New Member
Flavio
Join Date: Oct 2013
Posts: 12
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Sure, I usually model only a half of the cross section and after the convergence I rotate the model around it's axis.
Thanks a lot everybody for your availability. |
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