[Nek5000-users] Constant mass flow rate in simulation
nek5000-users at lists.mcs.anl.gov
nek5000-users at lists.mcs.anl.gov
Tue Sep 2 17:42:00 CDT 2014
Dear Paul,
Thank you so much for the very detailed and patient explanation.
Just a few things want to make clear since my understanding is not good. In the procedure you explained, does the linear combination of (2) and (3) form the flow field at next time step? If so, what's the boundary condition for U' and p'? In the process, actually we are not providing the body force, by working out the alpha value based on current flow field and resolved (U',p') we achieve the target mass flow rate, is that right?
By calculating the mean wall shear stress and update the mean pressure gradient (actually the body force) every time step we should be able to fix the rate, I'm just wondering why it doesn't work here?
I'm just trying to understand this and I apologise if those questions are actually stupid. I really appreciate your help. Thank you very much.
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 23:50
To: nek5000-users at lists.mcs.anl.gov
Subject: Nek5000-users Digest, Vol 67, Issue 2
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Today's Topics:
1. Re: Constant mass flow rate in simulation
(nek5000-users at lists.mcs.anl.gov)
2. Re: 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)
----------------------------------------------------------------------
Message: 1
Date: Mon, 1 Sep 2014 19:02:19 +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.4865.1409598165.3424.nek5000-users at lists.mcs.anl.gov>
Content-Type: text/plain; charset="iso-8859-1"
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
Send Nek5000-users mailing list submissions to
nek5000-users at lists.mcs.anl.gov
To subscribe or unsubscribe via the World Wide Web, visit
https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
or, via email, send a message with subject or body 'help' to
nek5000-users-request at lists.mcs.anl.gov
You can reach the person managing the list at
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When replying, please edit your Subject line so it is more specific
than "Re: Contents of Nek5000-users digest..."
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)
----------------------------------------------------------------------
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
>>>
>>>
>>
>> _______________________________________________
>> Nek5000-users mailing list
>> Nek5000-users at lists.mcs.anl.gov
>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>
>>
>
>
> --
> 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
>
>
--
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>
Content-Type: text/plain; charset="iso-8859-1"
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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------------------------------
Message: 2
Date: Mon, 1 Sep 2014 16:14:40 -0500 (CDT)
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.4866.1409606082.3424.nek5000-users at lists.mcs.anl.gov>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
Hi Tony,
Nek uses a semi-implicit formulation in which the nonlinear term
is treated explicitly in time.
The update is of the form:
(1) L(U,p) = NL(u) + BC + F(t)
where, L is the linear unsteady Stokes operator, NL() accounts for
nonlinear terms and BC accounts for boundary data and F(t) accounts
for body force. A fixed flow rate implies that F(t) adjusts to
ensure the constant flow rate.
Because L is linear, we can split the time advancement into two
parts using superposition:
(2) L(U',p') = NL(u) + BC
(3) L(U0,p0) = F0 + BC(0), where BC(0) := homogeneous boundary
conditions
(4) U = U' + a*U0,
(5) p = p' + a*p
where a is a constant
For fixed flow rate, you need
(6) \int U.x_hat dV = constant
which implies
(7) a = [ constant - \int U'.x_hat dV ] / [ \int U0.x_hat dV ]
Note that (2) is independent of U and time and therefore can
be solved in a preprocessing step (assuming dt is fixed and viscosity
is time-invariant).
That is what the vol_flow routines do.
The scheme inherits the full temporal and spatial accuracy of the
constant pressure-drop case.
hth,
Paul
On Mon, 1 Sep 2014, nek5000-users at lists.mcs.anl.gov wrote:
> 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
>
> Send Nek5000-users mailing list submissions to
> nek5000-users at lists.mcs.anl.gov
>
> To subscribe or unsubscribe via the World Wide Web, visit
> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users or, via email, send
> a message with subject or body 'help' to
> nek5000-users-request at lists.mcs.anl.gov
>
> You can reach the person managing the list at
> nek5000-users-owner at lists.mcs.anl.gov
>
> When replying, please edit your Subject line so it is more specific than "Re:
> Contents of Nek5000-users digest..."
>
>
> 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)
>
>
> ----------------------------------------------------------------------
>
> 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
>>>>
>>>>
>>>
>>> _______________________________________________ Nek5000-users mailing list
>>> Nek5000-users at lists.mcs.anl.gov
>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>>
>>>
>>
>>
>> -- 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
>>
>>
>
>
> -- Jean-Christophe Loiseau Homepage
> <https://sites.google.com/site/loiseaujc/> -------------- next part
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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> Content-Type:
> text/plain; charset="iso-8859-1"
>
> 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
>
> -------------- next part -------------- An HTML attachment was scrubbed...
> URL:
> <http://lists.mcs.anl.gov/pipermail/nek5000-users/attachments/20140831/65501e25/attachment-0001.html>
>
> ------------------------------
>
> 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
>
> -------------- next part -------------- An HTML attachment was scrubbed...
> URL:
> <http://lists.mcs.anl.gov/pipermail/nek5000-users/attachments/20140831/0fc955b0/attachment-0001.html>
>
> ------------------------------
>
> _______________________________________________ Nek5000-users mailing list
> Nek5000-users at lists.mcs.anl.gov
> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>
>
> End of Nek5000-users Digest, Vol 67, Issue 1
> ********************************************
> _______________________________________________ Nek5000-users mailing list
> Nek5000-users at lists.mcs.anl.gov
> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>
------------------------------
Message: 3
Date: Mon, 1 Sep 2014 17:50:52 -0500 (CDT)
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.4871.1409611853.3424.nek5000-users at lists.mcs.anl.gov>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
Hi Tony,
[ A slightly revised response to address a couple of typos..]
Nek uses a semi-implicit formulation in which the nonlinear term
is treated explicitly in time.
The update is of the form:
(1) L(U,p) = NL(u) + BC + F(t)
where, L is the linear unsteady Stokes operator, NL() accounts for
nonlinear terms and BC accounts for boundary data and F(t) accounts
for body force. A fixed flow rate implies that F(t) adjusts to
ensure the constant flow rate.
Because L is linear, we can split the time advancement into two
parts using superposition:
(2) L(U',p') = NL(u) + BC
(3) L(U0,p0) = F0 + BC(0),
(3.5) F = alpha*F0
(4) U = U' + alpha*U0,
(5) p = p' + alpha*p
where BC(0) := homogeneous boundary conditions and
alpha is a free paramter. For fixed flow rate, you need
(6) \int U.x_hat dV = constant
= \int ( U' + alpha U0 ) . x_hat dV
which implies
(7) alpha = [ constant - \int U'.x_hat dV ] / [ \int U0.x_hat dV ]
Note that (2) is independent of U and time and therefore can
be solved in a preprocessing step (assuming dt is fixed and viscosity
is time-invariant).
The vol_flow routines implement this two-part scheme, which
retains the full temporal and spatial accuracy of the constant
pressure-drop case.
hth,
Paul
>
>
> On Mon, 1 Sep 2014, nek5000-users at lists.mcs.anl.gov wrote:
>
>> 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
>>
>> Send Nek5000-users mailing list submissions to
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>> Contents of Nek5000-users digest..."
>>
>>
>> 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)
>>
>>
>> ----------------------------------------------------------------------
>>
>> 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
>>>>>
>>>>>
>>>>
>>>> _______________________________________________ Nek5000-users mailing
>>>> list
>>>> Nek5000-users at lists.mcs.anl.gov
>>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>>>
>>>>
>>>
>>>
>>> -- 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
>>>
>>>
>>
>>
>> -- Jean-Christophe Loiseau Homepage
>> <https://sites.google.com/site/loiseaujc/> -------------- next part
>> -------------- An HTML attachment was scrubbed... URL:
>> <http://lists.mcs.anl.gov/pipermail/nek5000-users/attachments/20140831/13dce213/attachment-0001.html>
>>
>> ------------------------------
>>
>> 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>
>> Content-Type:
>> text/plain; charset="iso-8859-1"
>>
>> 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
>>
>> -------------- next part -------------- An HTML attachment was scrubbed...
>> URL:
>> <http://lists.mcs.anl.gov/pipermail/nek5000-users/attachments/20140831/65501e25/attachment-0001.html>
>>
>> ------------------------------
>>
>> 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
>>
>> -------------- next part -------------- An HTML attachment was scrubbed...
>> URL:
>> <http://lists.mcs.anl.gov/pipermail/nek5000-users/attachments/20140831/0fc955b0/attachment-0001.html>
>>
>> ------------------------------
>>
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>> Nek5000-users at lists.mcs.anl.gov
>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>
>>
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>>
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