How does Flow Simulation calculate the average velocity on the surface of porous media? If I look at a contour or isoline plot of velocity on the surface, the result values make sense. If I ask for the average velocity on the same surface, the average is an order of magnitude higher (even higher than the maximum the contour and isoline plots claim). Is it using a volume average (using the cells inside the porous media) as opposed to an area average on the surface? I noticed that on a vector plot, the velocities are higher inside the porous media (which numerically I can explain due to the small mathematical pores inside the porous media.

This disconnect is very annoying.

The velocity average should scale with the porosity anywhere on or in the porous media. So if your porosity is say less than 10% then you would see an order of magnitude increase in velocity due to the reduction in area to maintain continuity across the region assuming the flow is constrained to move through the porous media.