# Problem with Sampling plane

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December 14, 2011, 14:48
Problem with Sampling plane
#1
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Hi,
I observe that for a given plane, mass flow through that plane calculated using massFlow function is different (sometimes very sigificantly) if the plane is of Slice or of Sample type. I have taken care to have same location and bounds for the plane in both the types.

Still massflow for a Slice type of plane is correct as per my simulation and the same for Sample type is wrong.

The reason I why I want to use Sample plane is because I want to calculate mass flow through planar, non-overlapping stripes of equal size made normal to flow direction so as to have mass flow distribution along coordinate normal to flow direction. Refer attached figure which shows the stripes along coordinate y while the flow is along x-direction. Now the massflow calculated through these stripes is wrong because it does not addup to the total mass flow over the entire y extent at that location.
I cannot use Slice plane to make these stripes it does not result in stripes of equal y sizes as my mesh is non-uniform in y-direction.
How can I solve this problem ?
Thanks for your inputs
Attached Images
 massflow.jpg (31.7 KB, 56 views)

 December 14, 2011, 14:53 #2 Senior Member   Join Date: Oct 2010 Location: Zurich Posts: 176 Rep Power: 6 Just to claify, flow is along x-direction and I have made equal stripes along y-direction using Sample plane i.e. normal to the flow. Total mass flow through these stripes is 1.233e-3 kg/s while the correct value is 6.24e-3 kg/s

 December 14, 2011, 17:44 #3 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 494 Rep Power: 11 Try increasing the number of points in the sample plane. It is probably too coarse.

 December 14, 2011, 19:13 #4 Senior Member   Join Date: Oct 2010 Location: Zurich Posts: 176 Rep Power: 6 I have used the maximum allowed sample points on each stripe.

 February 16, 2012, 14:15 #5 Senior Member   Join Date: Oct 2010 Location: Zurich Posts: 176 Rep Power: 6 Hello everyone, Sorry to revive this thread after 1 month or so. I have still not solved this problem and Ansys support has not been of much help on this issue. I just want to find the mass flow distribution along the y direction in the figure attached above. Any ideas on how to get that will be extremely helpful Best regards,

 February 16, 2012, 18:02 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,825 Rep Power: 85 I suspect Edmund is on the right track, the sampling poitn density is not high enough. If you have already maxxed it out then you might have to start looking for nasty work-arounds - maybe chop the surface into lots of little ones, and add up the contribution from each surface. If the answer is heading towards the correct value you know you are on the right track.

 February 16, 2012, 20:55 #7 Senior Member   Join Date: Oct 2010 Location: Zurich Posts: 176 Rep Power: 6 Hello Glen, Yes, I am already maxed out on the number of points. By chopping the surface, do u mean increasing the number of sampling planes? One more thing. I also tried with the same number of sampling planes but on the surface marked as inlet (refer the figure above) instead of the position shown in figure (i.e. the sampling planes were x-y planes on the inlet surface rather than the y-z planes as in figure). If I sum the mass flow rate through all the planes in the second case (i.e. using x-y sampling planes on inlet surface), I get total fow rate close to the actual value. But as I already stated previously, I get a very different and wrong value in case of the y-z planes shown in the figure. Why should this happen? The total flow rate should come out to be same in both the cases.

 February 17, 2012, 05:22 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,825 Rep Power: 85 Yes, I mean multiple sampling planes. The point Edmund and I are trying to make is that the sampling approach is not fine enough to accurately resolve the flow, but the slice option uses the elemental data and is accurate. How you slice and dice is up to you but the point Edmund and I are trying to make remains the same - insufficient density of sampling points.

February 20, 2012, 08:22
#9
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Well, I noticed something new.

Please refer to the new attached figure
As I show in this figure, fluid is supplied to the inlet plane in the older figure from a fluid duct. The fluid duct is a fluid domain and the blue colored portion is porous domain.
Now when I stated the following in my previous post:
Quote:
 One more thing. I also tried with the same number of sampling planes but on the surface marked as inlet (refer the figure above) instead of the position shown in figure (i.e. the sampling planes were x-y planes on the inlet surface rather than the y-z planes as in figure). If I sum the mass flow rate through all the planes in the second case (i.e. using x-y sampling planes on inlet surface), I get total fow rate close to the actual value. But as I already stated previously, I get a very different and wrong value in case of the y-z planes shown in the figure. Why should this happen? The total flow rate should come out to be same in both the cases
the x-y sampling planes near the surface marked as inlet were located inside fluid domain. But the y-z sampling planes were in porous domain. As I already stated, sum(mass flow rate on different sampling planes) was close to actual mass flow rate on x-y planes but not on y-z planes.

Now I found out that if I take the following sum on y-z planes:
sum over all i ( mass flow rate through ith plane * masflowAve(volume porosity) on ith plane)
then this sum comes close (within 2%) to the actual mass flow rate !

In essence while calculating the mass flow rate over each y-z sampling plane, I had to weight the mass flow rate with the corresponding average volume porosity at that location !
I did not have to use this weighting when the sampling planes were located inside fluid domain (and in fluid domain, value of volume porosity would anyway be 1 everywhere if one were to use the weighting).
Remember that I cannot use a series of slice planes instead of sampling planes as that leads to planes of unequal size in y-direction due to mesh non-uniformity.

I also tested the above conclusion over a y-z plane across the entire y extent of the porous domain. at the same x location as before. I observe that:
a) If that plane is of slice type, then massflow@ that plane gives the correct mass flow rate value
b) If that plane if of sample type, then
massflow@plane * massflowAve(Volume porosity) @plane comes close to the actual massflow rate.

So the question is that why should there be a need to weigh with porosity while calculating massflow rate through a sampling plane (and not while using a slice plane) located inside a porous medium?
Attached Images

 February 20, 2012, 17:51 #10 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,825 Rep Power: 85 I see, things are clearer now. Try areaAve(massflow*Volume Porosity)@plane or something like that, that might be a little more accurate again.

 February 20, 2012, 17:57 #11 Senior Member   Join Date: Oct 2010 Location: Zurich Posts: 176 Rep Power: 6 I have already tried the areaAve and that gives a value similar to massflowAve i.e. within 2% of the correct value. But the question remains that while finding mass flow through a plane inside porous domain, why does slice plane give the correct value directly while one needs to multiply with averaged porosity to get the same if one uses a sampling plane instead of the slice plane.

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