[Nek5000-users] Further to the issue of ffx, ffy and ffz

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

If the question is how to use .par file with the fixed mass flow rate I would  do it with one or two userparam there and then prescribing p54 & p55 in say userchk through the corresponding uparam as I recently tried for an avg_all call


Aleks

________________________________
From: Nek5000-users <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, February 26, 2018 9:59:47 AM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] Further to the issue of ffx, ffy and ffz



For a geometry with periodic boundary conditions in the flow direction (e.g., pipe flow with periodic bcs), p54 and p55 control the mass flow rate through the domain.


p54 determines the flow direction to be controlled (which, right now, can only be x, y, or z; but this restriction is readily lifted if there was an application requiring such).


So if |p54| = 1, 2, or 3, the flow rate will be fixed in the x, y, or z direction.


If p54 < 0, then it means that p55 will be the mean velocity in the given direction.


If p54 > 0, then it means that p55 will be the volumetric flow rate: Q= \int u dA.


The flow rate is fixed according to the following algorithm.


The Navier-Stokes equations are advanced in a semi-implicit way, using the following form:


      S(U,p) = NL(u) + f


where NL includes all data that should be on the rhs of the equations and S is a linear Stokes problem to ensure div.U=0, and f is the forcing that corresponds to the mean pressure gradient.


Because it is linear, the Stokes problem can be split into two parts:


    S(U',p') = NL(u)


    S(U0,p0) = 1


Then at every step, set


      U=U' + alpha U0   p=p' + alpha p0


such that  \int U.nhat dA = desired flow rate, where nhat is the unit vector

in the x,y, or z direction.


hth,


Paul


________________________________
From: Nek5000-users <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, February 25, 2018 12:48:25 PM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] Further to the issue of ffx, ffy and ffz

Hi,

The documentation says:
P054: fixed flow rate dir: |p54|=1,2,3=x,y,z
P055: vol.flow rate (p54>0) or Ubar (p54<0)

So for instance if you want to run a pipe with bulk flow of unity, you set:
p54: -3 (z)
p55: 1

How this is done in the latest versions I do not know, as we do not use
those for pipe production runs.

Philipp

On 2018-02-25 16:06, nek5000-users at lists.mcs.anl.gov wrote:
> Hi,
>
> May I push the following question further? Is there anyone know what are
> the exact meaning for para(54) and (55) in rea file? Such as, what's the
> difference between: 0,0 and -3 1 for (54) and (55) respectively? what's
> meaning for sign (+) or (-)?
>
> Kind regards,
>
> Zhai
>
>
> Hi Philipp,
>
> Yes, they are shear velocity and bulk velocity. Can I say if the ffx, ffy
> or ffz equal to (u_t/u_b)^2, the flow is specified as a fixed pressure
> gradient in the channel, and parameter(54) and para(55) in rea file are
> fixed as 0. Otherwise, if the all ffx, ffy and ffz are 0, the flow is fixed
> at flow rate rather than pressure gradient?
>
> Cheers,
>
> Zhai
>
>
>
>
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> Nek5000-users mailing list
> Nek5000-users at lists.mcs.anl.gov
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