# Slow convergence for Boundary Layer flow

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

 November 11, 2004, 14:25 Slow convergence for Boundary Layer flow #1 Biga Guest   Posts: n/a Hy there, I use a time-explicit (5-stage Runge-Kutta) finite-volume code for unstructured grids for my PhD thesis. This is a compressible code, using a 2nd-order Roe scheme for spatial discretization. This code is presenting slow convergence of velocity profiles, say an incompressible flat plate boundary layer flow. What I would like to ask if this is usual for explicit codes. Also, there would be any work-around for that behaviour, that is, for accelerating convergence? I already have a "geometric" multigrid... what else could be done: implicit residual smoothing, preconditioning... ? Any suggestion? Thanx a lot! Biga

 November 14, 2004, 07:26 Re: Slow convergence for Boundary Layer flow #2 Rami Guest   Posts: n/a Biga, Most compressible methods are notorious of slow convergence (if at all) for incompressible problems since the matrix becomes ill-conditioned. There are some remedies in the literature (usually by using preconditioners). Alternatively, stick to solution of compressible flows (Mach > 0.3).

 November 18, 2004, 17:51 Re: Slow convergence for Boundary Layer flow #3 Mani Guest   Posts: n/a Decoupling between the continuity and momentum equations, and stiffness, due to incompressibility is part of the problem, and can be helped by preconditioning, as was mentioned. What Mach number are you using? If it's too low, not only is your convergence bad, but the code may not even converge to the correct incompressible solution! I am assuming you apply local time stepping, for steady-state problems? The definition of your local time steps may have a strong influence on convergence. Residual smoothing certainly will boost the stability of your scheme and allows you to increase the CFL number for faster convergence. I am not sure how easy it is to implement with unstructured grids. A Gauss-Seidel method might be useful. Also make sure to use appropriate cell geometries (hexahedrons) in regions where velocity gradients are predominant in a certain direction, e.g. in boundary layers.

 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 Blasius_Pohlhausen_Crocco Main CFD Forum 12 September 30, 2013 17:35 Luk CFX 3 February 27, 2009 04:22 rakesh CFX 3 September 18, 2006 10:13 SN CD-adapco 0 July 19, 2006 09:12 Jesper CFX 1 July 7, 2004 16:59

All times are GMT -4. The time now is 19:16.