Re: [Nektar-users] problem with changing time-step

28 Jul 2023

      Alexandra,

I will do as you suggest. However I don't get what you mean by the error. I
don't see any in the log files. Would you clarify that for me?

Kind regards
syavash

On Fri, Jul 28, 2023, 13:32 Alexandra Liosi <liosi.alex@gmail.com> wrote:
...
Hi Syavash,
Thank you for sharing your log files with me. There are 2 things that I
noticed:
1. In both cases, the element with ID 31453 has the largest CFL value, I
would check how the mesh looks around this element.
2. I would recommend to run with verbosity activated to see the error from
the Conjugate Gradient solver. To activate the verbosity, you need to use
the -v option after your solver. For example, the command would look like
this:
IncNavierStokesSolver mesh.xml session.xml -v > results
Could repeat these tests with verbosity on to see what is happening with
the conjugate gradient solver ?
Kind regards,
Alexandra
On Fri, 28 Jul 2023 at 12:46, Alexandra Liosi <liosi.alex@gmail.com>
wrote:
...
---------- Forwarded message ---------
From: Ehsan Asgari <eh.asgari@gmail.com>
Date: Fri, 28 Jul 2023 at 09:49
Subject: Re: [Nektar-users] problem with changing time-step
To: Alexandra Liosi <liosi.alex@gmail.com>
Hi Alexandra
Following your comments, I made an attempt to compare two cases with
different time steps, one with dt=0.0005 and the other with dt=0.00025.
Both runs started from the same initial condition.
I have attached the log files of both simulations. It is very strange, as
the case with lower time-step is taking significantly more time to
converge!!
I really have no clue how this can be explained. I run simulations on a
pure hex mesh with wall-resolved boundary layer.
Do you have any suggestions?
Kind regards
syavash
On Wed, Jul 26, 2023 at 10:10 PM Alexandra Liosi <liosi.alex@gmail.com>
wrote:
...
Hi Syavash,
Thank you for the explanation!
Is there a reason why you choose to start from a higher CFL and then
reduce it? Also, where do you notice the oscillations? Would it be possible
for your to rerun the case even for a couple of time-steps to print the
error of the CG iterations? In my simulations using the incompressible
solver, I have noticed that the forces were oscillating because the
pressure and velocity tolerances for the conjugate gradient(CG) solver were
very relaxed. There could be other reasons as well, hence my list of
questions.
Kind regards,
Alexandra
On Wed, 26 Jul 2023 at 11:40, Ehsan Asgari <eh.asgari@gmail.com> wrote:
...
Hi Alexandra
Thanks for your kind reply.
I let the simulation run for a while with CFL=0.4 for instance. Then I
modify the time-step size so that I have CFL=0.2 and restart the simulation
from the latest field file.
I use the following session file:
<NEKTAR>
...
<EXPANSIONS>
    <E COMPOSITE="C[30011]" NUMMODES="6" FIELDS="u,v,w"
TYPE="MODIFIED" />
    <E COMPOSITE="C[30011]" NUMMODES="6" FIELDS="p" TYPE="MODIFIED" />
  </EXPANSIONS>
  <CONDITIONS>
        <FUNCTION NAME="InitialConditions">
            <F VAR="u,v,w,p" FILE="step98_Cubic7_restartFieldHdf5.fld"
/>
        </FUNCTION>
    <SOLVERINFO>
      <I PROPERTY="SolverType"
 VALUE="VelocityCorrectionScheme"/>
      <I PROPERTY="EqType"
 VALUE="UnsteadyNavierStokes"    />
      <I PROPERTY="Projection"              VALUE="Continuous"
     />
      <I PROPERTY="EvolutionOperator"       VALUE="Nonlinear"
      />
      <I PROPERTY="TimeIntegrationMethod"   VALUE="IMEXOrder2"
     />
      <I PROPERTY="SpectralVanishingViscosity" VALUE="True"/>
      <I PROPERTY="GlobalSysSoln"
VALUE="IterativeStaticCond"     />
      <I PROPERTY="Driver"                  VALUE="Standard"
     />
  </SOLVERINFO>
    <GLOBALSYSSOLNINFO>
          <V VAR="u,v,w">
            <I PROPERTY="GlobalSysSoln"
VALUE="IterativeStaticCond" />
            <I PROPERTY="Preconditioner"       VALUE="LowEnergyBlock"/>
            <I PROPERTY="IterativeSolverTolerance"    VALUE="1e-7"/>
          </V>
          <V VAR="p">
            <I PROPERTY="GlobalSysSoln"
VALUE="IterativeStaticCond" />
            <I PROPERTY="Preconditioner"
 VALUE="FullLinearSpaceWithLowEnergyBlock"/>
            <I PROPERTY="IterativeSolverTolerance"    VALUE="1e-6"/>
          </V>
       </GLOBALSYSSOLNINFO>
I don't have the log file as it was overwritten.
Kind regards
syavash
On Wed, Jul 26, 2023 at 12:00 PM Alexandra Liosi <liosi.alex@gmail.com>
wrote:
...
Hi Syavash,
Thank you for your email. Are you using as an initialisation field the
one that you received with the higher time-step? How large are your
starting CFL and the CFL using the new time-step? Could you provide the log
file and the session file that you use?
Kind regards,
Alexandra
On Wed, 26 Jul 2023 at 05:25, Ehsan Asgari <eh.asgari@gmail.com>
wrote:
...
This email from eh.asgari@gmail.com originates from outside
Imperial. Do not click on links and attachments unless you recognise the
sender. If you trust the sender, add them to your safe senders list
<https://spam.ic.ac.uk/SpamConsole/Senders.aspx> to disable email
stamping for this address.
Dear All,
When I change the time-step size to reduce the CFL and restart the
simulation, I get oscillations in both pressure and velocity that won't go
away.
Is there any explanation for this?
Kind regards
syavash
_______________________________________________
Nektar-users mailing list
Nektar-users@imperial.ac.uk
https://mailman.ic.ac.uk/mailman/listinfo/nektar-users

Ehsan Asgari

tags

participants (1)