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April 13, 2006, 11:30 
Hi!
I still try to calculat

#1 
Member
Andreas Hauffe
Join Date: Mar 2009
Location: Dresden, Germany
Posts: 36
Rep Power: 9 
Hi!
I still try to calculate the flow arround an airfoil and have a convergence problem. If I use another scheme then upwind all residuals decrease quite well but suddenly they increase to a certain level and stay there. The values of the solution are great (cl,cd,pressure distribution), but the residuals won't decrease. I think the aspect ratio of the elements behind the airfoil is really bad, so it acts nearly like a wall. This could be a problem. Is there an other solution than use much more elements behind the profil which is uninteressting for me? Or could it be another problem? The values of the solution are really great, but I need the convergence for an automatic calculation. Thanks Andreas 

April 13, 2006, 11:35 
Hi!
Sorry, I forgot to say,

#2 
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Andreas Hauffe
Join Date: Mar 2009
Location: Dresden, Germany
Posts: 36
Rep Power: 9 
Hi!
Sorry, I forgot to say, I'm using a cgrid. That's the reason for the thin elements. Andreas 

April 27, 2006, 03:56 
Hi!
Sorry, for starting thi

#3 
Member
Andreas Hauffe
Join Date: Mar 2009
Location: Dresden, Germany
Posts: 36
Rep Power: 9 
Hi!
Sorry, for starting this thread new, but I'm still having the same problems. I reduced the mesh resolution so that there are no bad elements, but the convergence isn't better. The same behavior of the residuals. I tried other turbulence models to check if this is a problem. The only scheme for div(phi,U), which converge, is upwind. Does anyone have an idea? Thanks Andreas 

April 28, 2006, 17:13 
I've pasted some possible expl

#4 
Senior Member
Srinath Madhavan (a.k.a pUl)
Join Date: Mar 2009
Location: Edmonton, AB, Canada
Posts: 703
Rep Power: 13 
I've pasted some possible explanations for the residual behavior you've observed. These are from the Fluent manual. Assuming your problem setup is correct, here are some possible reasons for the obseved behavior:
[1] If you have provided a very good initial guess, the residuals may not drop three orders of magnitude. [2] If the governing equation contains nonlinear source terms which are zero at the beginning of the calculation and build up slowly during computation, the residuals may not drop three orders of magnitude. [3] If the variable of interest is nearly zero everywhere, the residuals may not drop three orders of magnitude. In fullydeveloped flow in a pipe, for example, the crosssectional velocities are zero. If these velocities have been initialized to zero, initial (and final) residuals are both close to zero, and a threeorder drop cannot be expected. In such cases, it is wise to monitor integrated quantities, such as drag or overall heat transfer coefficient, before concluding that the solution has converged. It may also be useful to examine the unnormalized unscaled residual, and determine if the residual is small compared to some appropriate scale. Hope this helps! 

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