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-   -   Residuals flow around cylinder (http://www.cfd-online.com/Forums/fluent/111595-residuals-flow-around-cylinder.html)

pedroxramos January 10, 2013 11:49

Residuals flow around cylinder
 
Hello there!

What you think about this residuals for the flow simulation around a cylinder?

Q=0.02 m3/s h=0.10 m diameter of cylinder=48mm

transient and LES turbulence model

http://i45.tinypic.com/352je61.jpg

RodriguezFatz January 11, 2013 10:56

Can you show us the grid?

pedroxramos January 11, 2013 11:12

Of course, thanks.

http://i50.tinypic.com/2ho8jrb.png

RodriguezFatz January 11, 2013 11:27

Hey,
i) Is this 2d?
ii) Did you try any other turbulence model / stationary case and got better convergence?
iii) Did you try to improve the grid? It looks like you have several "jumps" in your mesh size... although I wouldn't expect the residuals to behave like this due to these jumps...

pedroxramos January 11, 2013 11:30

Hello, thanks for your reply!

Do you think that the residuals are too bad?

This is 3d, i show you the plant of the grid.

I'm obligued to use the LES-model...

Only the continuity value is bad don t you think?

RodriguezFatz January 11, 2013 11:37

I wouldn't really trust a result if "just one" residual looks unusual. Can you localize the point of bad contiunity?
Could you post a picture of what happens during just a few timesteps. I can not really see something in the residual picture.
I guess you did not change fluent's way of normalizing the residuals. You just use the default way, right?

pedroxramos January 11, 2013 11:43

Ya... i'm new in CFD...

There is a problem wich concerns me... the velocities are too high in the initial section next to channel's bed. Like this contour of shear stress in the bed shows

http://img23.imageshack.us/img23/6396/captureauk.jpg

I defined the mass flow rate in the inlet with the flow (20 l/s).

Thanks for your help!

RodriguezFatz January 11, 2013 11:51

I meant the residuals. Can you post a picture of the residuals where you can see just a few time steps? I want to see how each time step converges...

pedroxramos January 11, 2013 11:56

I'm sorry :) Like this: ?

http://img651.imageshack.us/img651/6492/aaavoy.jpg

RodriguezFatz January 11, 2013 14:07

Great... Is each of the peaks the beginning of a new time step?

pedroxramos January 11, 2013 14:54

No, sir... I think that each of the peaks represents one iteration. I defined 20 iterations for time-step...

Do you recommend any example or tutorial for the simulation of the turbulent flow around a body (cylinder)?

Thank you, sir.

RodriguezFatz January 11, 2013 15:01

I don't think you are right. It looks like you have about 10 peaks each 100 iterations, thus 10 iterations per timestep. I guess you did not switch off the convergence critertion. Go to monitors -> residual monitor and set the convergence criterion to "none". You have the default case of "1.0e-3", which means that fluent will stop iterations each time-step, when all residuals are below 1.0e-3. If you switch that off and maybe increase the maxminum number of iterations per time step -> tadaaa.

pedroxramos January 11, 2013 16:41

Thank you for your great advise!

The results improved and, in the other hand, the simulation is too slow now... but i'm patient :)

http://img22.imageshack.us/img22/2226/sssmn.jpg
I think it is possible improve the residuals. Do you know the meaning of "extrapolate variables?"

RodriguezFatz January 14, 2013 03:38

Fluent Help says, that "extrapolate variables" uses a Taylor series expansion to estimate an initial guess for the next time step. If it is faster, I don't see any drawback...


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