# Mesh Compressible Flow around an Airfoil

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 July 19, 2010, 12:28 Mesh Compressible Flow around an Airfoil #1 New Member   Hector Antonio Alcalde Ludeña Join Date: Oct 2009 Location: Lima-Peru Posts: 10 Rep Power: 9 In these days I learn how to analyse the flow around an airfoil, but the model was inviscid. after that, I try to model a quite 'complex' model; I put K-epsilon turbulence model (I Don't change anything); Velocity equal to 300 m/s (5º Angle of attack), I activate Energy Ecuation, in material I define air as an ideal gas. But when I try to run the model; I obtain this message: Error: Divergence detected in AMG Solver: Temperature Please try to help me with that. Regards,

 July 19, 2010, 15:03 #2 Senior Member   Chris Join Date: Jul 2009 Location: Ohio, USA Posts: 169 Rep Power: 10 How many iterations does it run before diverging?

July 19, 2010, 23:58
#3
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Hector Antonio Alcalde Ludeña
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Quote:
 Originally Posted by Chris D How many iterations does it run before diverging?
Mmm I don't remember exactly but it happens iteration number 30 or 50; in this interval, after that appears the message

 July 21, 2010, 09:23 #4 Senior Member   Chris Join Date: Jul 2009 Location: Ohio, USA Posts: 169 Rep Power: 10 How good is the grid in the region where the solution is diverging? How are you initializing the solution? Have you tried reducing the courant number?

July 21, 2010, 13:08
#5
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Hector Antonio Alcalde Ludeña
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Quote:
 Originally Posted by Chris D How good is the grid in the region where the solution is diverging? How are you initializing the solution? Have you tried reducing the courant number?
Hello Chris

In the file forward I show my grid; I Inizializate my solution in the zone called Farfield1, is in the front of the airfoil.

In the pictures, you can see how I make the analysis process in Fluent.

Attached Images
 alabe.jpg (95.3 KB, 35 views) Model Solver.JPG (29.1 KB, 22 views) Residual.JPG (37.8 KB, 23 views) Sol. Control.JPG (42.4 KB, 21 views) Viscus Model.JPG (54.2 KB, 20 views)

Last edited by hector; July 21, 2010 at 14:07.

July 21, 2010, 14:13
#6
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Chris
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Quote:
 Originally Posted by hector Hello Chris In the file forward I show my grid; I Inizializate my solution in the zone called Farfield1, is in the front of the airfoil. In the pictures, you can see how I make the analysis process in Fluent. Thanks for your answers!
grid.jpg

It looks like you're grid has vary large changes in cell volume, which definitely cause some stability problems. You want to make the grid such that the cell volume doesn't change too much from one cell to the next.

Also, you might think about switching to either the density-based solver or the pressure-based coupled solver for compressible flow.

Regarding the initialization, using fmg initialization gives a much better initial flowfield than initializing with a constant value. This reduces the chance that your solution will go unstable due to initial transients.

edit: I just noticed that you might be using an older version, so ignore what I said about density based and pressure based solvers.

July 24, 2010, 00:59
#7
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Hector Antonio Alcalde Ludeña
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Quote:
 Originally Posted by Chris D Attachment 4153 It looks like you're grid has vary large changes in cell volume, which definitely cause some stability problems. You want to make the grid such that the cell volume doesn't change too much from one cell to the next. Also, you might think about switching to either the density-based solver or the pressure-based coupled solver for compressible flow. Regarding the initialization, using fmg initialization gives a much better initial flowfield than initializing with a constant value. This reduces the chance that your solution will go unstable due to initial transients. edit: I just noticed that you might be using an older version, so ignore what I said about density based and pressure based solvers.
Hello Chris

Sorry to be late in ask you in the picture forward is the correction that I made, Is much better like this?

I have another question, why, when you analyze airfoils specially, you have to build this kind of mesh (as in the picture); why is not possible to mesh with a rectangle or another geometry?

Attached Images
 Alabe Corregido.jpg (49.6 KB, 16 views)

Last edited by hector; July 24, 2010 at 12:19. Reason: Forgot to attached files

 July 26, 2010, 11:56 #8 Senior Member   Chris Join Date: Jul 2009 Location: Ohio, USA Posts: 169 Rep Power: 10 You can use other topologies to mesh the airfoil. See this link for a description of O and C grids. Also, you can use an O-H type topology, which I crudely sketched here.

July 30, 2010, 13:59
#9
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Hector Antonio Alcalde Ludeña
Join Date: Oct 2009
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Quote:
 Originally Posted by Chris D You can use other topologies to mesh the airfoil. See this link for a description of O and C grids. Also, you can use an O-H type topology, which I crudely sketched here.
Hello Chris

I can see your mesh styles alternatives, are interesting, I think you must have a better solution control in these ways.

I have another question, if I would like to model an unsteady flow event? (Transient event) such a Waterhammer? or maybe a wave propagation in a chanel, I heard that is possible to model in Ansys, but how I can enter the boundary conditions and also the equations to describe this transient event?

 Tags airfoil2d, compressible flow, fluent, mesh

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