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-   -   Mesh Compressible Flow around an Airfoil (https://www.cfd-online.com/Forums/fluent/78353-mesh-compressible-flow-around-airfoil.html)

 hector July 19, 2010 12:28

Mesh Compressible Flow around an Airfoil

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,

 Chris D July 19, 2010 15:03

How many iterations does it run before diverging?

 hector July 19, 2010 23:58

Quote:
 Originally Posted by Chris D (Post 268068) 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

 Chris D July 21, 2010 09:23

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?

 hector July 21, 2010 13:08

5 Attachment(s)
Quote:
 Originally Posted by Chris D (Post 268334) 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.

 Chris D July 21, 2010 14:13

1 Attachment(s)
Quote:
 Originally Posted by hector (Post 268370) 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!
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.

 hector July 24, 2010 00:59

1 Attachment(s)
Quote:
 Originally Posted by Chris D (Post 268379) 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?

thanks for your help, was really helpfull for me.

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?

Thanks for your time again

 Chris D July 26, 2010 11:56

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.

 hector July 30, 2010 13:59

Quote:
 Originally Posted by Chris D (Post 268915) 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?

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