We need a little clarification on what you want. Do you want the pointwise across the surface or do you want the average over each surface? For the former you can compare the mass flux through the plane to the surface integral of the velocity magnitude and density (is it constant density)? If instead you wanted to create local distribution on each plane for many planes, I would recommend creating a Perl script. There is a Perl script in the ANSYS solution cases that creates planes perpendicular to the streamlines. You should be able to adapt that to your problem.
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