# Residuals flow around cylinder

 Register Blogs Members List Search Today's Posts Mark Forums Read

 January 10, 2013, 11:49 Residuals flow around cylinder #1 Member   Pedro Ramos Join Date: Mar 2012 Location: Portugal Posts: 63 Rep Power: 7 Sponsored Links 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

 January 11, 2013, 10:56 #2 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 20 Can you show us the grid? __________________ The skeleton ran out of shampoo in the shower.

 January 11, 2013, 11:12 #3 Member   Pedro Ramos Join Date: Mar 2012 Location: Portugal Posts: 63 Rep Power: 7 Of course, thanks.

 January 11, 2013, 11:27 #4 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 20 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... __________________ The skeleton ran out of shampoo in the shower.

 January 11, 2013, 11:30 #5 Member   Pedro Ramos Join Date: Mar 2012 Location: Portugal Posts: 63 Rep Power: 7 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?

 January 11, 2013, 11:37 #6 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 20 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? __________________ The skeleton ran out of shampoo in the shower.

 January 11, 2013, 11:43 #7 Member   Pedro Ramos Join Date: Mar 2012 Location: Portugal Posts: 63 Rep Power: 7 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 I defined the mass flow rate in the inlet with the flow (20 l/s). Thanks for your help!

 January 11, 2013, 11:51 #8 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 20 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... __________________ The skeleton ran out of shampoo in the shower.

 January 11, 2013, 11:56 #9 Member   Pedro Ramos Join Date: Mar 2012 Location: Portugal Posts: 63 Rep Power: 7 I'm sorry Like this: ?

 January 11, 2013, 14:07 #10 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 20 Great... Is each of the peaks the beginning of a new time step? __________________ The skeleton ran out of shampoo in the shower.

 January 11, 2013, 14:54 #11 Member   Pedro Ramos Join Date: Mar 2012 Location: Portugal Posts: 63 Rep Power: 7 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.

 January 11, 2013, 15:01 #12 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 20 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 likes this. __________________ The skeleton ran out of shampoo in the shower.

 January 11, 2013, 16:41 #13 Member   Pedro Ramos Join Date: Mar 2012 Location: Portugal Posts: 63 Rep Power: 7 Thank you for your great advise! The results improved and, in the other hand, the simulation is too slow now... but i'm patient I think it is possible improve the residuals. Do you know the meaning of "extrapolate variables?"

 January 14, 2013, 03:38 #14 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 20 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... __________________ The skeleton ran out of shampoo in the shower.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post goodegg Main CFD Forum 12 January 22, 2013 12:47 vmlxb6 CFX 17 May 16, 2011 02:29 butch85 Main CFD Forum 3 January 31, 2011 17:10 cfdxue Main CFD Forum 0 November 27, 2007 00:26 Anna Main CFD Forum 9 March 24, 2006 15:32