[Nek5000-users] Compute the derivatives

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

Hi Fred,

It looks like the delta is coming from the forcing function, ffz=.035.
For periodic flows, pressure is not periodic, but grad p is a periodic
function.   The true pressure for the computation is:

        p = pr - ffz*z - ffy*y -ffx*x

where pr is the quantity computed by Nek.  For your analysis below,
use p as defined above and all should be ok.    Note that computation
of drag over an object must acount for this --- I've done this in the
past and some of the infrastructure is in place in navier5.f (dpdx_mean,
etc.) --- but there are difficulties if the object is cut by the
periodic plane and so it does not generalize to every possible case.

Hope this helps.

Paul




>
> On Thu, 8 Jul 2010, nek5000-users at lists.mcs.anl.gov wrote:
>
>> Thanks a lot for you answer. But if I compute all my derivates in this way, 
>> I have deviation in my momentum equations...
>> In the stagnation point of my sphere (u=v=w=0) I get the following:
>> 
>> -1/rho dp/dx = 0.4125
>> nu (d2u/dx2 + d2u/dy2 + d2u/dz2) = 0.3775
>> 
>> --> delta = 3.5e-2
>> 
>> Do you have an idea from where this delta might result??
>> 
>> Best, Fred
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