[Nek5000-users] Corner elements giving large values

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

Hi Swarandeep,

Is 0/1 a realistic BC for T ?

Best,

Paul

________________________________________
From: nek5000-users-bounces at lists.mcs.anl.gov [nek5000-users-bounces at lists.mcs.anl.gov] on behalf of nek5000-users at lists.mcs.anl.gov [nek5000-users at lists.mcs.anl.gov]
Sent: Thursday, June 01, 2017 12:53 PM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] Corner elements giving large values

 Hi Nek Forum, Paul

       I am having temp = 1 and temp = 0 at the inner and outer
boundaries. Gravity is pointing in the -ve z-direction along the axis
of the
     annulus.  The top and bottom planes have zero heat flux conditions.
       I have Coriolis force added as a body force.
       The Rayleigh number is around 10000. The resolution is 8, 25,
24 elements in the r, phi and z directions respectively with order 9.
       My initial condition is very low arbitrary velocity
perturbation (~10^(-8)). The time resolution is set to 10^(-5) for
param12. The CFL is
set to 0.5. Please indicate how to control it.

      As you have rightly mentioned, I start the simulation with a
very low value of the Rayleigh number and gradually increase it. Yet,
the corner values only get large. Other regions of the domain are
fine.

Thank you
Swarandeep

On Thu, Jun 1, 2017 at 7:38 PM,  <nek5000-users at lists.mcs.anl.gov> wrote:
>
> Hi Swarandeep,
>
> How large is your Rayleigh number and what is your resolution?
> What are your thermal bcs?  What are your ICs?
>
> There should be no particular issue with your configuration - but you do need to
> control spatial and temporal resolution.
>
>
> For RB, you often have to start at low Ra and/or small dt because the nonlinear
> response can often drive the velocities to be quite high very rapidly before the
> CFL constraint kicks in.   RB flows are one of the rare instances where I use
> variable time step size (param 12 > 0), but even then you have to keep the target
> CFL quite small until the flow is established.
>
> hth,
>
> Paul
>
> ________________________________________
> From: nek5000-users-bounces at lists.mcs.anl.gov [nek5000-users-bounces at lists.mcs.anl.gov] on behalf of nek5000-users at lists.mcs.anl.gov [nek5000-users at lists.mcs.anl.gov]
> Sent: Thursday, June 01, 2017 7:27 AM
> To: nek5000-users at lists.mcs.anl.gov
> Subject: [Nek5000-users] Corner elements giving large values
>
> Dear Nek forum,
>
>       I am simulating Rayleigh Benard flow in a 3D finite cylindrical annulus.
>       I am getting spurious large values of velocity at the corner regions
>       of the annulus i.e. the edges where the inner cylinder joins the
> top and bottom
>       planes. The boundary condition is no-slip and no penetration for velocity
>       at these edges. Can someone point out a plausible reason.
>
> Thank you
> Swarandeep
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