[Nek5000-users] Constant mass flow rate in simulation

Mon Sep 1 14:02:19 CDT 2014

Dear Paul,

Thank you very much for the tips. I've tested my simulation case based on your instruction and it worked perfectly for keeping constant mass flow rate. However, I noticed that in the end of subroutine 'vol_flow', the velocity and pressure filed are forced to achieve constant flow rate. By doing this, would it be like changing the resolved flow field to get fixed flow rate, in which case the flow field is scaled on purpose rather than being solved directly from N-S equations. I don't know whether my understanding of this procedure is right or not. Please point out if I was wrong.

Another thing is, in turbulent channel flow, in theory the constant mass flow rate can be achieved by balancing the force (mean pressure gradient) with mean wall shear stress, however this didn't work in my case. Is it because this method is not compatible with spectral method or is there any systematic issue?

It would be great if you can give me some clues for my confusions. I really appreciate it. Thank you very much in advance.

Best regards,
Tony

________________________________________
From: nek5000-users-bounces at lists.mcs.anl.gov <nek5000-users-bounces at lists.mcs.anl.gov> on behalf of nek5000-users-request at lists.mcs.anl.gov <nek5000-users-request at lists.mcs.anl.gov>
Sent: 01 September 2014 18:00
To: nek5000-users at lists.mcs.anl.gov
Subject: Nek5000-users Digest, Vol 67, Issue 1

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Today's Topics:

   1. Re: Outpost question (nek5000-users at lists.mcs.anl.gov)
   2. Constant mass flow rate in simulation
      (nek5000-users at lists.mcs.anl.gov)
   3. Re: Constant mass flow rate in simulation
      (nek5000-users at lists.mcs.anl.gov)

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Message: 1
Date: Sun, 31 Aug 2014 20:17:18 +0200
From: nek5000-users at lists.mcs.anl.gov
To: Nek 5000 <nek5000-users at lists.mcs.anl.gov>
Subject: Re: [Nek5000-users] Outpost question
Message-ID:
        <mailman.4827.1409509062.3424.nek5000-users at lists.mcs.anl.gov>
Content-Type: text/plain; charset="utf-8"

Hi Katherine,

I was wondering if you had time to check on the parameter to be reset so
that I can overwrite previous files?

Bonne soir?e,
JC

2014-08-15 16:57 GMT+02:00 <nek5000-users at lists.mcs.anl.gov>:

> Hi Katherine,
>
> Thanks for the answer. I knew about the parameter 15 and was not sure
> about the 64 bits precision. This is now more clear.
> Regarding the overwriting, I had guessed that prepost has a parameter that
> needs to be reset, but I haven't been able to figure out which one so far.
> That would be very helpful if you can take a quick look at it.
>
> Thanks a lot and enjoy the end of the summer
> ++
>
> JC
>
>
> 2014-08-15 16:53 GMT+02:00 <nek5000-users at lists.mcs.anl.gov>:
>
>> Hi JC,
>>
>> You should be able to control the outpost frequency and precision using
>> the parameters parameter 15 (IOSTEP) and parameter 63, respectively.
>>
>> So, if you wanted the first 100 steps to output every step, you would set
>> something like:
>>
>> if(istep.le.100)  param(15)=1
>>
>>  To set the precision to 64bit, set param(63)=8, as well.  Setting
>> param(63) to anything else should default it to 8 byte precision.
>>
>> As far as overwriting previous fld files, you should be able to reset a
>> parameter used in prepost.f that increments the file counter.  I haven't
>> had a chance to test this, but I can take a look at it last this weekend.
>>
>> hth.
>> Katherine
>>
>>
>> On Fri, Aug 15, 2014 at 3:34 AM, <nek5000-users at lists.mcs.anl.gov> wrote:
>>
>>> Howdy Nek's,
>>>
>>> I have a couple questions regarding the ouput of files:
>>>
>>>    1. How can force, within the code (say in usrchk), some files to be
>>>    output in 64 bits for high-accuracy checkpointing and other in regular 8
>>>    bits for simple visualisation?
>>>    2. Say, during the first 1000 iterations, the code outpost a file
>>>    every single iteration so that I have blah0.f00001 to blah0.f01000. How can
>>>    I then force the code to outpost my new files (from istep = 1001 to 2000
>>>    say) rewriting on blah0.f00001 to blah0.f01000?
>>>
>>> Thanks a lot.
>>> Regards,
>>>
>>> JC
>>>
>>> --
>>> Jean-Christophe Loiseau
>>> Homepage <https://sites.google.com/site/loiseaujc/>
>>>
>>> _______________________________________________
>>> Nek5000-users mailing list
>>> Nek5000-users at lists.mcs.anl.gov
>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>>
>>>
>>
>> _______________________________________________
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>> Nek5000-users at lists.mcs.anl.gov
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>>
>>
>
>
> --
> Jean-Christophe Loiseau
> Homepage <https://sites.google.com/site/loiseaujc/>
>
> _______________________________________________
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> Nek5000-users at lists.mcs.anl.gov
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>

--
Jean-Christophe Loiseau
Homepage <https://sites.google.com/site/loiseaujc/>
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Message: 2
Date: Sun, 31 Aug 2014 22:57:27 +0000
From: nek5000-users at lists.mcs.anl.gov
To: "nek5000-users at lists.mcs.anl.gov"
        <nek5000-users at lists.mcs.anl.gov>
Subject: [Nek5000-users] Constant mass flow rate in simulation
Message-ID:
        <mailman.4832.1409525852.3424.nek5000-users at lists.mcs.anl.gov>
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Dear Neks,

Recently I've been struggling with constant mass flow rate in my turbulent channel flow simulation. Based on the userchk subroutine in 'turbChannel' tutorial case, I also used torque_calc(1.0,x0,.false.,.false.) to calculate the wall shear and then got the mean wall shear. After that I set mean pressure gradient 'ffx' equal to the mean wall shear in order to keep constant mass flow rate. In this case, the flow rate started from 1 (nondimensionalised case, bulk mean equal to 1) and kept increasing. Although the increment was very small at each time step, it never stopped and the bulk mean velocity has increased to 1.002.

Is it appropriate to set constant mass flow rate by updating mean pressure gradient based on mean wall shear in Nek5000 or am I using the wrong calculation? Since the bulk mean is still increasing, the simulation is like acceleration channel flow.

Does anyone has any clue about how to set constant mass flow rate for turbulent channel flow in Nek5000? Any idea is appreciated and many thanks in advance.

Thanks and regards,
Tony

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Message: 3
Date: Sun, 31 Aug 2014 23:58:32 +0000
From: nek5000-users at lists.mcs.anl.gov
To: "nek5000-users at lists.mcs.anl.gov"
        <nek5000-users at lists.mcs.anl.gov>
Subject: Re: [Nek5000-users] Constant mass flow rate in simulation
Message-ID:
        <mailman.4833.1409529514.3424.nek5000-users at lists.mcs.anl.gov>
Content-Type: text/plain; charset="iso-8859-1"

Hi Tony,

I would just set p55=1.0; p54=-1.0

This will fix < u_x > == 1.0

If you are using variable viscosity, this might be problematic, but for the
constant viscosity case this is the best way.

Be certain to also set ffx=ffy=ffz=0 in the .usr file, the the p54/p55 combination
sets the x-body force as needed to ensure the prescribed mass flux is realized.

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, August 31, 2014 5:57 PM
To: nek5000-users at lists.mcs.anl.gov
Subject: [Nek5000-users] Constant mass flow rate in simulation

Dear Neks,

Recently I've been struggling with constant mass flow rate in my turbulent channel flow simulation. Based on the userchk subroutine in 'turbChannel' tutorial case, I also used torque_calc(1.0,x0,.false.,.false.) to calculate the wall shear and then got the mean wall shear. After that I set mean pressure gradient 'ffx' equal to the mean wall shear in order to keep constant mass flow rate. In this case, the flow rate started from 1 (nondimensionalised case, bulk mean equal to 1) and kept increasing. Although the increment was very small at each time step, it never stopped and the bulk mean velocity has increased to 1.002.

Is it appropriate to set constant mass flow rate by updating mean pressure gradient based on mean wall shear in Nek5000 or am I using the wrong calculation? Since the bulk mean is still increasing, the simulation is like acceleration channel flow.

Does anyone has any clue about how to set constant mass flow rate for turbulent channel flow in Nek5000? Any idea is appreciated and many thanks in advance.

Thanks and regards,
Tony

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