[Nek5000-users] Drag calculation and mass flow rate with angled meshes

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

Hi Pardyumna,

Nek supports two fixed flow-rate modes:

   .prescribed flow rate
   .prescribed average velocity.

You toggle between these modes according to the sign of parameter 54.

   If p54 < 0, you are prescribing mean velocity

   If p54 > 0, you are prescribing flow rate.

The flow rate may not work in your case because of the fact that
your domain "length"  ( := xmax - xmin) is not the same as your periodic
pitch.

The mean velocity is computed is:

              \int u dV / \int dV

and I believe that should work OK.

Flow rate needs an estimate of the length of the domain

            Q(x) = \int_A u dA = constant (by mass conservation)

            Q := (1/Lx) \int_0^Lx  Q(x) dx = (1/Lx) * \int_Omega u dV

In your case, Lx is the periodic pitch, and not equal to (xmax-xmin).

However, Nek does not know how to find the periodic pitch.  It is using
Lx = xmax-xmin, which is incorrect for your geometry.

Which one are your using?

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: Tuesday, September 20, 2016 7:24 AM
To: nek5000-users at lists.mcs.anl.gov
Subject: [Nek5000-users] Drag calculation and mass flow rate with angled meshes

Hi Neks,

I am trying to run a simulation case similar to the turbulent channel flow case. I initially ran a simulation on the normal turbulent channel flow domain (with the geometry like the fig on the left). I used the parameter p55 in the rea file to ensure that the bulk velocity of the flow is 1 in the  domain, and used the parameter p02 to set the turbulent Reynolds number, Re_tau to my required value of 180. I used the sub-routine drag_calc to find the mean shear on the wall, from which I calculated the shear stress and then the Re_tau and u_tau values. However, recently I was trying the same thing on a slightly different domain (fig to the right).
---------------------------------------------------------------------                                                                             ---------------------------------------------------------------------
|                                                                                                              |                                                                            /                                                                                                               /                              ^ Y
|                                                                                                              |                                                                          /                                                                                                               /                                |
|                                                                                                              |                                                                        /                                                                                                               /                                  |_____>  X
|                                                                                                              |                                                                      /                                                                                                               /
|                                                                                                              |                                                                    /                                                                                                               /
---------------------------------------------------------------------                                                                  ---------------------------------------------------------------------

This domain is almost the same as the first except now there is an angle of 30 degrees in the mesh grid about the z-axis. Please note that the perpendicular distance between the walls normal to the y-direction remains the same for both the domains. In this domain if I use the same p55 and p02 parameters that I use for the orthogonal domain, then Re_tau, calculated as described above, is increasing to higher values (~220). If I reduce the parameter p55 to lesser values, then the Re_tau value is restored to 180. I could not figure out why exactly this was happening, and whether this difference is now because my way of calculating the Re_tau is not valid anymore in the angled domain. Or is the change because the mass flow rate is actually higher in the angled domain? If the case is latter, then why is the mass flow rate increasing just due to the angling of the mesh?

I went through the sub-routines drag_calc, vol_flow and set_obj (to see if the integral object needs to be changed) in moderate detail, but could not figure out the reasons for this. Any help in this might be appreciated, as I want to establish the reason, so that I can trust the accuracy of the results of this simulation.

Thanks and regards,

Pradyumna M K Prasad

TU Delft,
Student: Masters in Aerospace Engineering



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