[Nek5000-users] Pipe example - Strange velocity/pressure signals

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

Hi Outi,

Generally, the incompressibility constraint will enforce the flow to be
nonzero after the first step, i.e., when you start to push water through
a pipe it comes out the other end immediately with the incompressible
model.

Is this not what you're expecting for this case?

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: Monday, March 03, 2014 3:29 AM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] Pipe example - Strange velocity/pressure signals

Hi Paul,

As far as I can see, the userbc is set only at the inlet. The first
probe point is at z=3, that is, 6 diameters downstream of the inlet
(after transformation in usrdat2 from -3 to 10). What concerns me is
that the z-velocity goes up to 0.48e-1 at z=3 at the first time step,
when the inflow hasn't reached near this point yet. When I output the
whole velocity field at the first time step (as .f00001) and look at
it in paraview, the z-velocity has oscillations near the throat. Maybe
at the end this is some transient phenomenon which doesn't matter, but
I thought it was safest to check with you.

Best,
Outi

Quoting nek5000-users at lists.mcs.anl.gov:

> Hi Outi,
>
> This looks ok to me... I see in your userbc file:
>
>       xp  = x/0.5
>       yp  = y/0.5
>       rr  = xp*xp + yp*yp
>
>       ux  = 0.0
>       uy  = 0.0
>       uz  = 1 - rr   ! Hagen-Poiseiulle flow, pipe diamter = 1.
>
> which would give a nonzero velocity after Step 1 ?
>
> Best, Paul
>
>
>
>
>
>
> On Sun, 2 Mar 2014, nek5000-users at lists.mcs.anl.gov wrote:
>
>> Dear Paul,
>>
>> Thank you very much for looking into this. My parameters are as in
>> the repository example, I didn't change them:
>> lx1=6,
>> lxd=10,
>> lx2=lx1-2
>> lx3=lx1
>>
>> The output after the first time step (dt=2.5e-2) of hpts.out is
>> attached below. Please note the third column (z-velocity).
>>
>> 2.5000000E-02 -4.7626817E-05 -4.4872405E-05  4.8233404E-01  1.3486090E+02
>> 2.5000000E-02 -6.0842050E-05 -1.8870027E-02  5.9476094E-01  1.7381205E+02
>> 2.5000000E-02  2.0295515E-07 -1.5249089E-02  4.8242873E-01  1.3487912E+02
>> 2.5000000E-02 -1.5349154E-02 -1.1545977E-05  4.8241771E-01  1.3487899E+02
>> 2.5000000E-02  9.8682769E-03 -1.7779070E-02  5.7201214E-01  1.7367640E+02
>>
>> Best regards,
>> Outi
>>
>>
>>
>> On Mar 2, 2014, at 9:02 PM, nek5000-users at lists.mcs.anl.gov wrote:
>>
>>>
>>> Dear Outi,
>>>
>>> What value of lx1 and lx2 were you using?
>>>
>>> I'm not observing the phenomenon you report.
>>> I have:
>>>
>>> lx1=6
>>> lxd=9
>>> lx2=lx1-2
>>> lx3=lx2
>>>
>>> ...
>>>
>>> Once I know your SIZE parameters I can retry.
>>>
>>> Best, Paul
>>>
>>>
>>> On Sun, 2 Mar 2014, nek5000-users at lists.mcs.anl.gov wrote:
>>>
>>>> Dear Neks,
>>>> We are interested to do a smoothly deformed axisymmetric geometry, much
>>>> like the pipe/stenosis example in the repository.
>>>> So, we added hpts probes in the pipe example, and modified nothing else,
>>>> but this turns out to give strange values initially. The first probe sits
>>>> 3 diameters downstream of the stenosis throat (z=3), and gives U=0.48 at
>>>> the first time step, despite zero initial condition. The pressure in this
>>>> position shoots up to very high values too, so maybe a spurious pressure
>>>> peak is the cause?
>>>> Could you please help us to understand/prevent this phenomenon? The .usr,
>>>> hpts.in and essential parts of .rea are attached. We are using revision
>>>> 1008.
>>>> This would be important for us as we aim to use Nek5000 to study
>>>> transient as well as steady state dynamics of similar flows.
>>>> Thank you in advance!
>>>> Outi
>>>> _______________________________________________
>>>> Nek5000-users mailing list
>>>> Nek5000-users at lists.mcs.anl.gov
>>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>>>
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>>
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