[Nek5000-users] Increase DT in Usrchk based on CFL

[nek5000-users at lists.mcs.anl.gov]

On Fri, 9 Jul 2010, nek5000-users at lists.mcs.anl.gov wrote:

> Hi,
>
> I would like to increase my DT dynamically by checking the current CFL
> at each step.  For example:
>
>
> if C < 0.15 then  param(12) = param(12) + 1E-05 
>
> I know I can adjust the time step in usrchk by setting param(12), but
> can I access C in the same manner by param(26)? Or is this only the
> parameter I set in the rea, and it is accessed by another variable?
>
> This issue occurs after a few 100 steps where my C drops and starts to
> settle around 0.1-0.15.  The problem is I'm not sure that it always
> occurs around the same time steps, and there for would like to adjust
> it based on the actual value. I know nek will adjust DT if my C
> becomes too large, but when C becomes too small is the issue DT is
> not adjusted so long as it meets what I set in the rea .
>

Michael,

I use the following routine, which I call w/no arguments from userchk.

It takes your initial dt and sets it to dt/(scale^2), then ramps back
to your target dt value.

Paul


c-----------------------------------------------------------------------
       subroutine usr_dt

c     Reduce initial dt by scale*scale. 
c     Then ramp up by 5% each step to dt/scale.
c     Then resume ramping after Step 120 till dt = dtmax.

       include 'SIZE'
       include 'TOTAL'

       real dtmx1,dtmax,p93,p94,p95,p103
       save dtmx1,dtmax,p93,p94,p95,p103

       if (istep.eq.0) then
          scale = 10.
          dtmax = abs(param(12))
          dtmx1 = dtmax/scale
          dt    = dtmx1/scale
          p93   = param(93)
          p94   = param(94)
          p95   = param(95)
          p103  = param(103)
       endif
       dt = dt*1.05
       dt = min(dtmx1,dt)
       param(12) = -dt

       param(93) = p93   ! turn off projection till dt is fixed
       param(94) = p94
       param(95) = p95
       if (dt.lt.dtmx1) then
          param(93) = 0
          param(94) = 0
          param(95) = 0
          p95       = istep
       endif
       if (istep.gt.120) dtmx1 = dtmax

       if (nid.eq.0.and.istep.le.20) write(6,*) istep,dt,dtmax,' dtmax'

       fexp       = 0.85   ! filter strength doubled for first few steps
       param(103) = p103*(dt/dtmax)**fexp   ! adjust filter weight
       if (nid.eq.0.and.istep.le.110)
      $   write(6,*) istep,param(103),p103,' p103'

       param(103) = p103   ! std filter weight

       return
       end
c-----------------------------------------------------------------------