On Apr 18, 2013, at 9:49 AM, Roy Stogner <[email protected]> wrote:
> On Thu, 18 Apr 2013, Manav Bhatia wrote: > >> I use a constant time step of 0.05 sec with a backwards Euler >> implicit time scheme. Of course, the local CFL increases to >> large numbers as the mesh is refined, since I do not change the >> time step. >> >> Do you typically change the time step post refinement? Or use >> local time stepping? > > I don't know how well Ben's and my experiences will carry over to your > code - we're using SUPG rather than GLS, so we ought to be getting a > better conditioned matrix at the cost of more finicky stability. > Don't GLS matrix condition numbers grow like h^-4 instead of h^-2? Well you did say inviscid, correct? Then I would think the discretizations would be equivalent - modulo your choice of tau. Which is what exactly? > But with that in mind: I definitely reduce the time step > post-refinement, and not just by a factor of 2. Otherwise it gets too > hard for our scheme to handle sudden sharpening of shocks in > hypersonic real gas flows. Usually our adaptive time stepper ramps up > the time step afterwards pretty aggressively, though, successfully. > Also, problems with no shock or with no sensitive chemistry are more > robust. If you have shocks, this is very important. The problem is that for inviscid flows shocks are self-similar under refinement - if you smear the shock over three cells on the coarse grid, you expect it will be smeared over three cells on the fine grid too. When you interpolate the coarse->fine, at the first time step your shock will be spread across 3->6 cells, and a large disturbance will develop there that has to be dealt with. This is why we often reduce the time step immediately after refinement. -Ben ------------------------------------------------------------------------------ Precog is a next-generation analytics platform capable of advanced analytics on semi-structured data. The platform includes APIs for building apps and a phenomenal toolset for data science. Developers can use our toolset for easy data analysis & visualization. Get a free account! http://www2.precog.com/precogplatform/slashdotnewsletter _______________________________________________ Libmesh-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/libmesh-users
