Speed Holes for MPG?

[Frank Lee]

jeff, that is a great illustration of the "speed holes" effect; I imagine most of us here know that "Airgate" is aero garbage and why, and these holes in the bumper are the same thing.

[twinair]

Originally Posted by viio

Forgive me if I don't rush out and grab a drill - can't see much of a difference.

Hey this is impressive.
This is off topic: Have you ever modeled what lowering does?
(E.g. is it just helping because of reduced frontal area, is it possibly also helping because the wheels sit more in the wheel wells (although this may not produce useful results without spinning wheels), is extreme lowering possibly increasing drag, because increased shear forces underneath the car?)

[wdb]

Originally Posted by Frank Lee

jeff, that is a great illustration of the "speed holes" effect; I imagine most of us here know that "Airgate" is aero garbage and why, and these holes in the bumper are the same thing.

I'm familiar with the pickup truck tailgate vs. none discussion, but I haven't seen anything definitive relating to rear bumpers trapping air and ostensibly acting like parachutes vs. holes in same vs. a proper diffuser. If you have a cite please share it.

[Frank Lee]

If you understand why "Airgates" or "Netgates" are worse aero than nothing, that transfers over to these "speed holes" as well.

[freebeard]

So how about speed holes front and rear?



I think the best aft underbody treatment I've [personally] seen is this:



A divergent duct that forces the turbulent flow behind rear wheels out where it will be sheared off by the overall flow instead of contributing to the wake.

[wdb]

Originally Posted by Frank Lee

If you understand why "Airgates" or "Netgates" are worse aero than nothing, that transfers over to these "speed holes" as well.

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.

[chumly]

Look up the W.W.2 Dauntless dive bomber, The dive brakes were flaps on bottom and top of the wings with holes in them so the airplane could dive straight down and control it's speed while aiming at its target. Some jet fighters have dive brakes and they have holes in them, I imagine they were subject to much wind tunnel testing to find out what worked best.

[aerohead]

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.

I recommend you get a copy of Subaru's SAE Paper from 1986 on the development of their Cd 0.29 XT.They explain fully,the peculiar airflow with respect to rear bumpers and the importance of depth of the bumper to achieve lower drag.

[Superfuelgero]

more time attack. Don't really agree with the area aft the tires, but interesting none the less.



A good discussion and pic at end
http://www.eng-tips.com/viewthread.cfm?qid=182684


This is more the direction I'm leaning (vent for air trapped between belly pan)

[Otto]

Parachute effect: If the rear bumper cover is effectively a parachute, drilling holes in it might make it less a parachute, reducing drag somewhat and thereby attenuating a gross design flaw. Better to not have a parachute in the first place, by sealing the underbody with full undertray, etc.