[Nek5000-users] Stress formulation

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

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
>>
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>> Nek5000-users at lists.mcs.anl.gov
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>>
>>
>
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