[petsc-users] Modified Taylor-Hood elements with piece-wise constant pressure for Stokes equation

Jed Brown jed at jedbrown.org
Tue Jun 2 15:00:11 CDT 2015

Justin Chang <jychang48 at gmail.com> writes:

> I originally solve that example problem using LU. 

I'd like to learn why LU didn't notice that the system is singular.
(The checks are not reliable, but this case should be pretty obviously

> But when I solve this one:
> http://fenicsproject.org/documentation/dolfin/1.5.0/python/demo/documented/stokes-iterative/python/documentation.html
> By simply running their code as is for TH and adding the one like I
> mentioned for MTH, I get the following outputs when I pass in -ksp_monitor
> -ksp_view and -log_summary

Thanks, Justin.  Could you please run with

 -ksp_type gmres -ksp_gmres_restart 1000 -ksp_monitor_true_residual -ksp_monitor_singular_value

Also, can you run a tiny problem size (like less than 1000 dofs) with

  -pc_type svd -pc_svd_monitor

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