# Trying to understand BC's following elbow tutorial

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February 4, 2013, 06:16
Trying to understand BC's following elbow tutorial
#1
New Member

nicolas
Join Date: Oct 2009
Posts: 24
Rep Power: 8
Hi,

I am a new user of OpenFOAM and I am trying to understand how boundary conditions works. I have run the elbow tutorial for incompressible flows, as is without changing anything, and I was looking at the results using Tecplot (Tec360 2012).

At the left of each of the attached figures, one can see contour plots of Ux, for the main zone (at a z=0 slice), and one of the inlet zones (zone names in Tecplot are "Step 0 Incr 0" and "velocity-inlet-5 Step 0 Incr 0" respectively). At the right, a close up is shown at the intersection of the two zones. I have attached 3 figures showing three different time steps: t=0, 1 and 10 sec.

At that inlet boundary, the streamwise velocity, Ux, should be fixed and equal to 1 according to the boundary condition (from file 0/U):

velocity-inlet-5
{
type fixedValue;
value uniform (1 0 0);
}

What I don't understand is why:
(1) at the first step of the simulation (t=0 sec) the velocity at the inlet surface is equal to zero, while at the main zone is around Ux=0.24?
(2) at the following time steps (t=1 - 10 sec) the velocity at the inlet (main zone) never becomes equal to 1? In fact it seems to be half the inlet value (0.5 instead of 1).
(3) the velocity at the inlet zone surface, seems to be consistent with the boundary condition (more or less), only a few seconds into the simulation, but not from the beginning (t=1 sec)? Even at the end of the simulation (t=10 sec), the velocity in not uniform along the inlet surface.

I was expecting the velocity to be Ux=1 at both the inlet surface, and very close to the inlet (main zone). What am I missing here?

Thanks,
Nicolas
Attached Images
 elbow_time0.jpg (41.7 KB, 25 views) elbow_time1.jpg (52.5 KB, 24 views) elbow_time10.jpg (49.1 KB, 19 views)

 February 18, 2013, 07:38 Figured it out #2 New Member   nicolas Join Date: Oct 2009 Posts: 24 Rep Power: 8 Just wanted to share with the rest, that the problem was in the way I was presenting results using Tecplot. As I said I am using the OpenFOAM loader, that comes with the newer versions of Tecplot (i am using Tec360 2012R1). Long story short, to show correctly the values one has to set the Contour Type to "Primary Value Flood". However, this way Tecplot shows one value for each control volume. In order for Tecplot to show smoothed interpolated values using cell center values, you have to use the Slice tool, extract "Current Slice" and then select Contour Type of "Flood" for the extracted slice. Nicolas

 July 31, 2013, 11:15 #3 Member   Join Date: Mar 2013 Posts: 92 Rep Power: 5 Hi to all, I have a simple question about how Bcs are treated by tecplot. When I load the simulation Tecplot doesn't load the Bcs and the value in the last domain cells is wrong. Someone knows the reason of this strange behaviour? Thanks to all

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