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April 19, 2005, 04:44 
Face/Surface area export (urgent)

#1 
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I am trying to export face and surface areas of a slice through a 3D geometry (pipe) and corresponding velocities in an ascii file but I am getting some strange results. Some of the exported areas just have zero values. If I sum face areas together I dont get correct value for the total surface area of the slice, neither mass flow rate. That only works for inlet and outlet boudaries. If I take surface integral on the same slice in Fluent or in Tecplot I get correct values for the surface area of that slice and for mass flow rate. I can also nicely see faces with velocity vectors in cell centers in Fluent. What am I doing wrong? Thanks for any answer.


April 19, 2005, 07:33 
Re: Face/Surface area export (urgent)

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Are you sure you're exporting the correct areavalue?!? areas in Fluent are a vector...So you have to get the magnitude of that vector with the macro N_VMAG().Luca


April 19, 2005, 08:47 
Re: Face/Surface area export (urgent)

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Luca, thanks for the answer. But I am still confused...
First I tried to export data for inlet boundary which is perpendicular to zaxis and I got all x and yareas zero and zareas different then zero. The sumation of absolute values of zarea vectors gave correct total area. A=sum (sqrt(x*x+y*y+z*z)) for my case A=sum(sqrt(z*z))=sum(abs(z)). So basically I calculated magnitude (in Execel from exported data.) When I exported areas from other slices I got for some cells zero values for all three vectors(!). I thought that if you cut a slice in zplane Fluent creates a 2D surface which consists of triangular 2D cells with velocities stored in the centre. Like one sees it when a slice is displayed with grid and velocity vectors. And when you export cell values, you should therefore get x and y area vectors zero, and zarea vector different then zero. Like in the boundary case. But obviously that is not the case. I also calculated magnitudes from vectors and then I summed them up, but the result was still far from correct (to small area). Could be that I dont understand the thing correctly and my approach is wrong... I really dont know. David 

April 19, 2005, 08:53 
Re: Face/Surface area export (urgent)

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How can you recover data from slices? Luca


April 19, 2005, 09:11 
Re: Face/Surface area export (urgent)

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First I create a plane surface (slice was not a good term) in the Surface/Plane menu and then I export it in an ascii file in the File/Export menu (I chose the wanted surface and for location cell center). I get the set of data for x,y pairs and with constant z. David


April 19, 2005, 09:20 
Re: Face/Surface area export (urgent)

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Ah!!!ok, i thougth you were doing that with a UDF! I think that Fluent tries to build that area value...in the boundary case that value is stored, or anyway Fluent exactly builds that value from node coords. In a slice case, maybe there's a kind of interpolation. Maybe the null value you get is because Fluent gives you the area value from a cell which has a node rigth on your slide...it's just an hypothesis...sorry


April 19, 2005, 09:34 
Re: Face/Surface area export (urgent)

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Could be. I thought that fluent calculates new nodes for for points where cell edges go through the slice, because a set of new points is created. And that areas are then calculated for those new 2D cells. Nevertheless, what is interesting, is that integration on the same slice gives correct values. In Fluent and in Tecplot CFD Analyzer. Thank you anyway. Any info is helpful. David


June 24, 2014, 10:49 

#8 
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Andrew
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Was there a solution to this thread? I would like to calculate the mass flow in each cell cut by the plane surface which either requires to be able to report this directly (I dont know how to) or know the face areas of the cells (then use the velocity and density in the export option to calculate the mass flow). I presume it is possible to report the mass flux in a cell cut by a plane since the surface integral is capable of giving a total mass flux through a specified surface.


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