Quote:
Originally Posted by wdb
The reason leaving the tailgate up on a pickup is better than leaving it down is that the swirl created aft of the cab improves aero, and that effect is lost when the gate is down because the air does swirl. I don't see how that translates cleanly to holes in rear bumpers.
I see the modern rear bumper -- which is effectively a big, empty, sealed plastic shell --having much more of a parachute effect than a swirl effect. I'm sure some air gets dumped out of it and into the low pressure area behind the car, so there may be some swirl taking place, but I would think that the drag generated by it trapping the air in the first place would overcome any such advantage. Besides which, cutting holes in the bumper will also dump air into the low pressure area behind the car.
In short I'm still looking for something substantive.
There is a lot of 'swirl'  in the drag racing / time attack ranks around the topic, in parallel with the 'parachute effect' of driving a closed cockpit vehicle with the window open; is it better to let it fill with air and remain a high pressure area, or to vent it and have a continuous flow through the cockpit/cabin of the vehicle? There does seem to be some consensus in those communities that if you're going to vent the air, you should do it in particular places, and not just drill holes willy-nilly. |
SAE Paper # 860216
Aerodynamic Charactersistics of Subaru XT
Teruhisi Tsukada,Joichi Sakagami,Yasuhiko Arai,and Hiroyuki Takahara
Subaru Engineering Division
Fuji Heavy Industries,Ltd
Page 65
Figure 19 "Flatness of Rear Underfloor"
*If the bottom of the car does not have a belly section preceding the rear bumper,it can mean a 10% drag increase.
*If the floor of the trunk is lowered to create a spare tire well,more level with with the belly of the car,a 10% drag reduction takes place.
*If a 'parachute' rear bumper is lowered,even with the belly of the spare tire well,a 2% drag REDUCTION is realized.
*The exception proves the rule.