[Nek5000-users] Corner elements giving large values

Sun Jun 4 14:42:39 CDT 2017

Hi Nek Forum, Paul

     Thanks for the suggestions. I implemented them. Waiting for the
simulation to be over.

Another issue I have is that the BC for T at the inner cylinder wall
does not stay at 0 as I have given in userbc. It becomes 0.2 over some
time.

Thank you
Swarandeep

On Fri, Jun 2, 2017 at 10:28 PM,  <nek5000-users at lists.mcs.anl.gov> wrote:
>
> Hi Swarandeep,
>
> Set p26 in the .rea file to be 0.1 and make certain IFCHAR is F (a few lines below);
>
> Make certain your dt is positive (param 12)
>
> 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 10:31 AM
> To: nek5000-users at lists.mcs.anl.gov
> Subject: Re: [Nek5000-users] Corner elements giving large values
>
> 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
>>>>> _______________________________________________
>>>>> Nek5000-users mailing list
>>>>> Nek5000-users at lists.mcs.anl.gov
>>>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
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