[Nek5000-users] Stress formulation

Fri Dec 18 10:19:59 CST 2015

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<mailto: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<mailto:nek5000-users-bounces at lists.mcs.anl.gov> [nek5000-users-bounces at lists.mcs.anl.gov<mailto:nek5000-users-bounces at lists.mcs.anl.gov>] on behalf of nek5000-users at lists.mcs.anl.gov<mailto:nek5000-users at lists.mcs.anl.gov> [nek5000-users at lists.mcs.anl.gov<mailto:nek5000-users at lists.mcs.anl.gov>]
Sent: Sunday, December 13, 2015 5:19 PM
To: nek5000-users at lists.mcs.anl.gov<mailto: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<mailto: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<mailto:nek5000-users-bounces at lists.mcs.anl.gov> [nek5000-users-bounces at lists.mcs.anl.gov<mailto:nek5000-users-bounces at lists.mcs.anl.gov>] on behalf of nek5000-users at lists.mcs.anl.gov<mailto:nek5000-users at lists.mcs.anl.gov> [nek5000-users at lists.mcs.anl.gov<mailto:nek5000-users at lists.mcs.anl.gov>]
Sent: Tuesday, December 01, 2015 2:44 AM
To: nek5000-users at lists.mcs.anl.gov<mailto: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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