[Nek5000-users] Corner elements giving large values

Fri Jun 2 01:04:08 CDT 2017

Hi Nek Forum, Paul

     The top and bottom BC are zero heat flux. Please let me n=know
how to control the CFL. It is 0.5 in the logfile. I am having no
particular difficulty except these corner elements at the junction of
inner cylinder and the top/bottom planes. Is there a better way to
treat edge boundary elements. Also is it possible to have different
order of elements at different locations.

Thank you
Swarandeep

On Fri, Jun 2, 2017 at 12:50 AM,  <nek5000-users at lists.mcs.anl.gov> wrote:
>
> Hi Swarandeep,
>
> Sorry - I misunderstood.  I guess my question was more about the
> temperature bcs at the bottom and top of the domain?
>
> In general, however, there should be no difficulty.  What happens
> when you reduce your dt ?
>
> You need to keep the CFL <  ~0.5 in general.  CFL is printed in the logfile,
>
> grep tep logfile
>
> and you'll see it  "C= ....."
>
> 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 1:57 PM
> To: nek5000-users at lists.mcs.anl.gov
> Subject: Re: [Nek5000-users] Corner elements giving large values
>
> Hi NekForum, Paul
>
>       I intent to have isothermal conditions at the inner and outer
> cylinders. The differential heating is achieved by imposing a
> temperature difference across the two cylinders. I have used T = 1 and
> 0 values for non-dimensional temperature. I think it should be fine.
> Do you see any problem here.
>
> Thank you
> Swarandeep
>
> On Thu, Jun 1, 2017 at 11:28 PM,  <nek5000-users at lists.mcs.anl.gov> wrote:
>>
>> 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
>>> _______________________________________________
>>> Nek5000-users mailing list
>>> Nek5000-users at lists.mcs.anl.gov
>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>> _______________________________________________
>>> Nek5000-users mailing list
>>> Nek5000-users at lists.mcs.anl.gov
>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>> _______________________________________________
>> Nek5000-users mailing list
>> Nek5000-users at lists.mcs.anl.gov
>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>> _______________________________________________
>> Nek5000-users mailing list
>> Nek5000-users at lists.mcs.anl.gov
>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
> _______________________________________________
> Nek5000-users mailing list
> Nek5000-users at lists.mcs.anl.gov
> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
> _______________________________________________
> Nek5000-users mailing list
> Nek5000-users at lists.mcs.anl.gov
> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users