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The upstream MAFS screen is there to reduce turbulence (both axial and radial) in order to provide a more uniform, aligned flow passing over the hot film sensor element, producing more accurate mass flow readings. The screen itself does cause a slight amount of turbulence, but the overall effect is a reduction.The downstream screen again conditions the flow before it passes on to the rest of the intake system. The primary reason for this being that the sensor support is a somewhat large, irregular flow obstruction: 
If you want to learn more about flow conditioning using screens and/or honeycomb sections, see if you can find this book or other books on wind tunnel design at the library. On a related note, I am currently working on designing the flow conditioning section of a cascade wind tunnel that runs at Mach 0.85 in the test section and about 25 m/s in the flow conditioning section, which will contain 4-5 screens and a 2"-thick section of aluminum honeycomb.
 "The more "F" and the less "m" you have, the more "a" you can get." -- Brian Beckman, The Physics of Racing |
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MAFS screens (what they do)... - 
