[Nek5000-users] Corner elements giving large values

Fri Jun 2 10:31:22 CDT 2017

Dear Nek Forum, Paul

       Yes, I have user defined grid such that the locations of the
elements are distributed to lie on the chebyshev grid  at the top and
bottom and near the side walls.
       Also please tell which param sets the CFL.

Thank you
Swarandeep


On Fri, Jun 2, 2017 at 8:00 PM,  <nek5000-users at lists.mcs.anl.gov> wrote:
>
> Dear Swarandeep,
>
> Did you refine your element size at the top and bottom and near the side walls?
>
> Please set your CFL to 0.05 in order to get the flow started.
>
> 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: Friday, June 02, 2017 1:04 AM
> To: nek5000-users at lists.mcs.anl.gov
> Subject: Re: [Nek5000-users] Corner elements giving large values
>
> 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
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