Quote:
Originally Posted by Vman455
I found myself thinking about this the other day, and I'm not sure that the conclusion that airflow through the holes would help fill the wake is correct. The Subaru figures indicate a low-pressure area behind the spare tire well which causes turbulence. If holes are added in the bumper, for the air to begin flowing it would need to be higher pressure than the low-pressure wake behind the bumper. Do we know this is the case? Seems to me the first thing to do here would be run a manometer to in front of a stock bumper cover and behind it and see what the pressure differential is, if there is one, and thus if air would actually flow into the car's wake or not.
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We know from diffuser research that the air cannot follow more than a 4-degree up-slope angle before drag increases beyond that of the 2.8-degree-to-4-degree diffuser,due to the flow separation and that's predicated upon a 'surface' to flow over.
If you cut a hole in the bumper,the air could actually flow forwards from the higher pressure wake,through the hole,to the lower pressured,more 'kinetic' turbulent air in front of the bumper.
Remember,the air in front of the bumper is in train behind all the underbody bits and pieces.It's all turbulence and has zero dynamic 'ram' pressure.The bumper cannot possibly function as a parachute.
You WANT the turbulence sequestered in front of the bumper,creating a pool of 'dead' air underneath the car,with the active flow skipping off it below.
Only a belly pan,or a belly pan with diffuser can lower drag.
If you see something on a 'track' car,avoid it like the plague.