[Nek5000-users] Stress formulation

Tue Jan 5 08:32:17 CST 2016

Hi Paul,

No, I have run my own example with my grid, but now I'm running "axi" case
in examples and after 20000 steps I don't get the parabolic profile. In
addition, the solution does not change after 1000 steps.

Best, Andrew

2015-12-18 17:19 GMT+01:00 <nek5000-users at lists.mcs.anl.gov>:

>
>
> Hi Andrew
>
> Are you running the "axi" case in the examples?
>
> This case started with u=0 as an initial condition.
>
> A quick glance at the output it produces in f00004, after 200 steps, shows
> plots
> similar to yours.
>
> If you run longer (change nsteps in the .rea file), the viscous boundary
> layer
> will diffuse inward from the walls and you should see a parabolic
> profile.  This
> is what I observed just now.
>
> Please let me know if this resolves the issue --- if not, I'm happy to
> check
> further.
>
> 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, December 17, 2015 7:07 PM
>
> *To:* nek5000-users at lists.mcs.anl.gov
> *Subject:* Re: [Nek5000-users] Stress formulation
>
> Hi Paul,
>
> I think I have misspoken. The thing is that I haven't got the profile corresponding
> to the inlet one anywhere in pipe.
> I add some clarifications. You can find  attached file with velocity
> curves for my simulaltions.
> I have done the simulations of Poiseuille flow in the cylindrical pipe.
> With IFAXIS = true, IFSTRS = false, param (30) = 0  the input profile
> doesn't chage. With IFAXIS = true, IFSTSR = true, param (30) = 0 the input
> velocity  profile changes considerably along the pipe  and on the axis there
> is strange pressure peak, the nature of which I can not understand. Can  I
> have clarification on this question?
>
> Thanks,
> Andrew
>
>
> 2015-12-14 0:55 GMT+01:00 <nek5000-users at lists.mcs.anl.gov>:
>
>>
>> Hi Andrew,
>>
>> In my view, you are already getting the correct flow physics.
>>
>> What happens at outflow, especially at moderate Reynolds number, is not
>> necessarily parallel flow.
>>
>> The stress formulation is giving the stress-free result.
>>
>> 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:* Sunday, December 13, 2015 5:19 PM
>> *To:* nek5000-users at lists.mcs.anl.gov
>> *Subject:* Re: [Nek5000-users] Stress formulation
>>
>> Hi Paul,
>>
>> Thank you for your reply.
>> Yes, if i increase Reynolds number, the outlet profile become much close
>> to the inlet one. But in my simulations the Reynolds number is between 300
>> and 1200.
>> Can you recommend me what kind of boundary conditions i can utilise for
>> ensure for ensure the flow physics ? I would implement these one in
>> code.
>>
>> Thanks,
>> Andrew
>>
>>
>> 2015-12-05 3:37 GMT+01:00 <nek5000-users at lists.mcs.anl.gov>:
>>
>>>
>>> Hi Andrew,
>>>
>>> I checked into this... I think what is happening is that the 'O  ' bc
>>> for the stress formulation
>>> means stress-free, which is not guaranteed to yield  a parabolic profile
>>> at the outlet.
>>>
>>> If you increase your Reynolds number I'm guessing that you'll recover
>>> the parabolic
>>> profile because the viscous stresses will diminish -- this is what I
>>> observed.
>>>
>>> From my perspective, the outlet boundary is not a region where I would
>>> count on accurate
>>> physics --- it is, after all, a truncated domain, so I don't generally
>>> worry too much about
>>> the behavior there.
>>>
>>> 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:* Tuesday, December 01, 2015 2:44 AM
>>> *To:* nek5000-users at lists.mcs.anl.gov
>>> *Subject:* [Nek5000-users] Stress formulation
>>>
>>> Hi Neks,
>>>
>>>     I have one question about stress formulation in NEK5000.
>>>     I tried to make a simulation for Poiseuil flow with stress
>>> formulation. In inlet boundary condition use was made of parabolic profile.
>>> If IFSTRS=false the outlet profile is exactly the same as inlet one but in
>>> case of IFSTRS=true the outlet profile changes significantly (both the
>>> maximum value and profile shape). Could you tell me how I can get the
>>> parabolic profile at outlet, please?
>>>      The same thing occurs with pressure values. The maximum pressure
>>> value with IFSTRS=true is three times higher then the one with IFSTRS=false.
>>>
>>>      I found that the difference comes from subroutines where the
>>> stiffness matrix is calculated (axehlm for IFSTRS = false and axhmsf for
>>> IFSTRS = true) but I don't undestand what is happening exactly.
>>>
>>> Thanks in advance for your help,
>>> Best regards,
>>> Andrew
>>>
>>> _______________________________________________
>>> Nek5000-users mailing list
>>> Nek5000-users at lists.mcs.anl.gov
>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>>
>>>
>>
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