[Nek5000-users] stokes flow with time-dependent boundary

nek5000-users at lists.mcs.anl.gov nek5000-users at lists.mcs.anl.gov
Tue Feb 28 13:55:36 CST 2012


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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> >>>>> 
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