[Nek5000-users] stokes flow with time-dependent boundary
nek5000-users at lists.mcs.anl.gov
nek5000-users at lists.mcs.anl.gov
Sat Mar 3 16:03:56 CST 2012
Hi, Paul,
thank you for your confirmation. so now we can come back to my initial
attempt, .ie.
a series of Stokes problems linked by a time-dependent boundary
condition.
i am not sure if nek5000 can directly solve such a problem yet, as i
find that in connect2.f file,
IF (.NOT.IFTRAN) THEN
PARAM(11) = 1.0
PARAM(12) = 1.0
PARAM(19) = 0.0
ENDIF
it seems to me that PARAM(11) will be ignored and reset to 1 by nek5000,
am i right?
are there any possible ideas to continue on with nek?
many thanks in advance.
lailai
On Thu, 2012-03-01 at 14:46 -0600, nek5000-users at lists.mcs.anl.gov
wrote:
> Lailai,
>
> This appears to be working fine.
>
> The code iterates until a reasonably small L2-norm of the
> velocity divergence is obtained.
>
> For plane Poiseuille flow there will be no (or at most 1)
> pressure iterations.
>
> Paul
>
>
> On Thu, 1 Mar 2012, nek5000-users at lists.mcs.anl.gov wrote:
>
> > Hello, Paul,
> >
> > thank you for you reply. After making these changes, i still did not get
> > convergent results. however, i find that if i change NMXH to 1000 in
> > driver2.f, then at least, ' **ERROR**: Failed in HMHOLTZ' will
> > disappear, but 'Divergence' message printed by the subroutine uzawa
> > remains. Another good news is that the resultant flow field looks
> > reasonable compared to the analytical results, btw, i am calculating a
> > 2d poiseuille flow with 'v O w w' BC.
> >
> > the divergence message is like
> > '20 2.58595E-06 2.05697E-06 2.58595E-01 7.95442E-06 1 Divergence'
> >
> > i guess the 'Divergence' message is printed by the uzawa subroutine, its
> > iteration number NMXH is set to 1000 in driver2.f file.
> >
> > any ideas? thanks in advance.
> >
> > lailai
> >
> >
> >> Hi Lailai
> >>
> >> It's not clear that the simulation has broken down (nor that it hasn't).
> >> In particular, your divergence on the first iteration is relatively small,
> >> which is good. My guess is that this will ultimately converge.
> >>
> >>> 1 1.00000E-08 2.67182E-01 2.67182E-01 1.00000E+00 1 Divergence
> >>
> >> I would suggest to set the "divergence" and "helmholtz" parameters in
> >> your .rea file to be 0, and set tolrel and tolabs to be 0.01.
> >>
> >> Nek will then try to optimize the tolerance for the iterative solvers
> >> to give you a good solution with minimal computational overhead.
> >>
> >> Paul
> >>
> >>
> >>
> >>
> >>
> >> On Tue, 28 Feb 2012, nek5000-users at lists.mcs.anl.gov wrote:
> >>
> >>> Hello, Paul,
> >>>
> >>> thank you for your reply. By following you suggestion, after switching
> >>> the flag IFNAN to F, i can run unsteady stokes flow, however, if i want
> >>> to run a single steady stokes flow, i switch the flag IFTRAN to F(i
> >>> guess i am right here), but the simulation will break down. the first
> >>> recorded error in the log file looks like
> >>>
> >>> 1 100 **ERROR**: Failed in HMHOLTZ: VELX 6.2080E-01 1.7717E+02
> >>> 1.0000E-08
> >>> 1.0000000000000000E-008 p22 1 1
> >>> 1 1 Helmholtz VELY F: 0.0000E+00 1.0000E-08 1.0000E+00
> >>> 0.0000E+00
> >>> 1 Hmholtz VELY: 0 0.0000E+00 0.0000E+00
> >>> 1.0000E-08
> >>> 1 1.00000E-08 2.67182E-01 2.67182E-01 1.00000E+00 1 Divergence
> >>> 1.0000000000000000E-008 p22 1 1
> >>> New CG1-tolerance (RINIT*epsm) = 5.4238943644626018E-014
> >>>
> >>> any ideas on this? thanks in advance.
> >>>
> >>> lailai
> >>>
> >>>
> >>>
> >>>
> >>> On Wed, 2011-12-21 at 15:17 -0600, nek5000-users at lists.mcs.anl.gov
> >>> wrote:
> >>>> Hi Lailai,
> >>>>
> >>>> I suggest initially starting with a single run, using steady Stokes.
> >>>> (Note, steady Stokes works only for Pn-Pn-2, so set lx2,ly2 = lx1-2, etc.)
> >>>>
> >>>> Paul
> >>>>
> >>>>
> >>>> On Wed, 21 Dec 2011, nek5000-users at lists.mcs.anl.gov wrote:
> >>>>
> >>>>> thank you for your tip, Paul.
> >>>>>
> >>>>> i think we are solving a series of Stokes problems linked by a time-dependent
> >>>>> boundary condition.
> >>>>> The time-derivative term is zero and we are not solving an unsteady Stokes
> >>>>> problem. I think we are not clear how to solve a series of Stokes problems.
> >>>>>
> >>>>> lailai
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>> On 12/21/2011 06:47 PM, nek5000-users at lists.mcs.anl.gov wrote:
> >>>>>> Hi Lailai,
> >>>>>>
> >>>>>> To switch to unsteady Stokes, you simply set the flag
> >>>>>> IFNAV to "F" in the .rea file, which turns off the convective
> >>>>>> term and simultaneously eliminates the CFL timestep constraint.
> >>>>>>
> >>>>>> [ Set:
> >>>>>>
> >>>>>> T F F F F F F F F F F IFNAV & IFADVC (convection in P.S. fields)
> >>>>>>
> >>>>>> to
> >>>>>>
> >>>>>> F F F F F F F F F F F IFNAV & IFADVC (convection in P.S. fields)
> >>>>>>
> >>>>>> .]
> >>>>>>
> >>>>>> It's still not clear to me if you are solving an unsteady
> >>>>>> Stokes problem, or a series of steady Stokes problems.
> >>>>>> (There is a difference...)
> >>>>>>
> >>>>>> Nek can handle either case with equal ease.
> >>>>>>
> >>>>>> I hope this helps.
> >>>>>>
> >>>>>> Paul
> >>>>>>
> >>>>>>
> >>>>>> On Wed, 21 Dec 2011, nek5000-users at lists.mcs.anl.gov wrote:
> >>>>>>
> >>>>>>> On 12/19/2011 06:17 PM, nek5000-users at lists.mcs.anl.gov wrote:
> >>>>>>>>
> >>>>>>>> Hi Lailai,
> >>>>>>>>
> >>>>>>>> I have used the approach you proposed for solving multiple
> >>>>>>>> steady stokes problems... you use an artificially large
> >>>>>>>> timestep. That works.
> >>>>>>>>
> >>>>>>>
> >>>>>>> thank for your reply, if understand correctly, here you are talking about
> >>>>>>> the second approach i proposed. For each time step, we solve a transient
> >>>>>>> problem with very large internal timestep to quickly get to the
> >>>>>>> steady-state solution.
> >>>>>>> since i am very new to nek5000, thus i am not sure how to implement this
> >>>>>>> method which seems not trivial.
> >>>>>>>
> >>>>>>> On the other hand, i started from the first example of the Kovasznay
> >>>>>>> problem. I remove the time-derivative and convection term by setting the
> >>>>>>> density in .rea file to zero, the numerical results agree very well with
> >>>>>>> the analytical solution with zero Re number. I guess here the solver is
> >>>>>>> just inverting a matrix which seems feasible for a 2D problem but might be
> >>>>>>> too expensive or inefficient for a 3D problem.
> >>>>>>>
> >>>>>>>
> >>>>>>>> If you really have a time-dependent boundary condition, there
> >>>>>>>> is no reason you can't just use the unsteady Stokes solver
> >>>>>>>> with whatever timestep is required to accurately resolve your
> >>>>>>>> temporal bcs. Note that, in this case, you would indeed have
> >>>>>>>> the inertial term rho du/dt present in the physics.
> >>>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>>> Paul
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> On Mon, 19 Dec 2011, nek5000-users at lists.mcs.anl.gov wrote:
> >>>>>>>>
> >>>>>>>>> Hello, guys,
> >>>>>>>>>
> >>>>>>>>> i am a new guy to Nek5000, i saw the manual of nek5000 that it can solve
> >>>>>>>>> the steady stokes flow.
> >>>>>>>>> i guess when i solve such a flow, do i need to set it as a transient
> >>>>>>>>> simulation with time-derivative term included to get a steady-state
> >>>>>>>>> solution? or, i can solve it by a direct solver method to get the
> >>>>>>>>> solution immediately?
> >>>>>>>>>
> >>>>>>>>> since i want to add some time-dependent boundary condition for the
> >>>>>>>>> steady stokes flow, so it will be pretty nice if i can solve it using
> >>>>>>>>> the direct solver, for each time step, i solve one stokes flow; if
> >>>>>>>>> nek5000 cannot solve it in such a way, i guess i have to use the first
> >>>>>>>>> way; then for each time step i have to solve a transient problem to
> >>>>>>>>> approach the steady state with some artificial time step used.
> >>>>>>>>>
> >>>>>>>>> i am not sure if i have stated my problem clearly. hopefully you guys
> >>>>>>>>> have some experience on the feasibility of the two ways mentioned above.
> >>>>>>>>> Thank you in advance.
> >>>>>>>>>
> >>>>>>>>> lailai
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> _______________________________________________
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