Speed Holes for MPG?

[freebeard]

Originally Posted by xntrx

Don't really agree with the area aft the tires

Rear tires, both cars? Maybe the class rules dictate vanes are longitudinal. Some of the competitors at Bonneville said that's the case; but that 'Vette passed inspection there, and the outer vanes swept out the width of the tire.

The UFE III puts me in mind of Basjoos' Civic, with the double walled filler between the front and rear tires.

wdb -- Still looking for something substantive? Go here: http://ecomodder.com/forum/showthrea...tml#post296613

Read these:

• Session 0-1 - Mike Dickison, Programme Manager Coventry University LCVTP

  An Introduction to the Low Carbon Vehicle Technology Programme.

• Session 0-2 - Peter White, Professor of ThermofluidDynamics Coventry University, Chairman Automobile Division IMechE

  Reducing carbon emissions does aerodynamics matter?

• Session 0-3 - Chris Johnson, director, design project work coventry university

  Aesthetics and aerodynamics

• Session 0-4 - Trevor Haynes, Coventry University

  LCVTP Workstream 12: Aerodynamics Technical Introduction

• Session 0-5 - Trevor Haynes, Coventry University

  Aerodynamics and Electric Vehicles

• Session 1-1 - Scott Porteous, Development & Simulation Ricardo UK Ltd

  LCVTP Workstream 12 - Aerodynamics Efficiency - Sensitivity Analysis of aerodynamic drag over various drive cycles

• Session 1-2 - Gianluca Orso Fiet, MIRA

  LCVTP Aerodynamics Process Development

• Session 1-3 - Rishi Gupta, Mechanical and Automotive Engineering Dept. Coventry University

  LCVTP WS 12 Aerodynamics Support Activities at Coventry University

• Session 2-1 - Remus Cîrstea, Coventry University

  Effect of rear body taper on a simple body

• Session 2-2 - Jeff Howell, Tata Motors European Technical Centre

  WS12 Aerodynamic Performance University Research Projects Base Pressure Recovery

• Session 2-3 - Nick Sabrazat, MIRA

  LCVTP Aerodynamics Reference car finding:Lessons learned

• Session 2-4 - Adrian Gaylard, Technical Specialist Aerodynamics Jaguar Land Rover

  Base Pressure Recovery using Rear Cavity and Base Plates

• Session 3-1 - Jeff Howell, Tata Motors European Technical Centre

  WS12 Aerodynamic Performance University Research Projects Wheel/Wheelarch Drag

• Session 3-2 - Adrian Gaylard, Technical Specialist, Aerodynamics, Jaguar

  Land Rover Drag Reduction Through Wheel Design

• Session 3-3 - Nick Sabrazat, MIRA

  Wheel Arch Flow Investigation

• Session 4-1 - Hussain Ali, Tata Motors European Technical Centre

  Drag reduction on a production vehicle

• Session 4-2 - David Plummer, Design Institute Coventry University

  Aesthetic Design

• Session 4-3 - Adrian Gaylard, Technical Specialist, Aerodynamics, Jaguar Land Rover

  Further Opportunities

[wmjinman]

Don't most parachutes work better (create MORE drag) with a spill hole in the middle than without? Similar to the increase in drag when ventilating the pickup tailgate or the back bumper of a car? My White Gnat has one of those massive "scoop looking" rear bumpers. Dunno how easy it would be to remove, but if it's easy, a simple (but careful) A-B-A test with it on & off should show something. But instead, I intend to do the belly pan instead....

[some_other_dave]

I believe that the hole in the middle of a parachute is there to keep it from cork-screwing as it falls through the air, due to air spilling out of the side at one point.

-soD

[wmjinman]

OK yes, I remember that too, but I still think there's something to "venting" it with the spill hole. If the 'chute is solid (no spill hole), it will "fill with air", and the rest of the air will find its way around the 'chute, streamiling around the "bubble" that's created by the filled 'chute. It's my understanding that a pickup tailgate is much the same - - venting it will let air pass through it, creating more drag than if you allow the "bubble" to form, causing the rest of the air to "streamline" around the bubble.

Similar with the comments about the airplane speed brakes with holes, etc. I actually suspect the "parachute bumper" may be better than no bumper because of the air bubble it might create, causing the rest of the underbody air to "streamline" around it AND other dirty clutter under there, like the rear axle, etc.

But this is just out of my mind - no documentation or anything.

However I do remember reading in a car magazine when the new Camaros & Firebirds came out in the early '80s that the Camaro actually had lower drag, even with the open headlight buckets. And they concluded it was the same thing - an air "bubble" would form in those buckets that the rest of the air could streamline around easier than the cracks & seams in the closed headlight doors of the Firebird......

[Smokeduv]

I think they look awful when they are just a hole cut in the bumper, and as many of you, I think that they don't really work, but I have doubts. As I'm in no way an expert in aerodynamics, I see two trends here, one is having the rear bumper as low as possible and with a complete undertray to have the lowest drag, which seems correct as there is data to support it, and the other is having some kind of rear diffuser to lower air speed and fill the low pressure area behind the car, which can have many advantages but needs major understanding. I think that these holes could be in this last trend, but they are awful and don't have an specific study for every car (if for any).

I can recall an interesting example (at least for me), which is the 2010 F1 season where the winner car was the Brawn GP, which was controversial for having a "double-decker" rear diffuser, so it could have more area (the rules for 2010 shortened the diffuser length, so having a way to increase area within the limits was key) and with no major impact on drag. Can these two-layer thing be applied to a car who has a low bumper, but can fill the wake also?

Another thing I can remember is that the wheels makes big turbulence in the wheel arches, so they make holes to lower drag (see LeMans Cars with a big opening at the top or many sports cars with an opening behind the arches), so, for the rear wheels, a way to manage this air to the back could be beneficial. I don't know, maybe I'm just saying incoherent things, but is what I have seen (as in the examples) and with my very limited understanding about aerodynamics.

[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.

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.

[wmjinman]

I believe it! Same thing I was guessing. (guess I get lucky sometimes!)

[freebeard]

Parachutes have a hole for structural integrity. Fan-inflatable structures (bouncy castles are a likely example) have an exit flap so the air moves through them.

[botsapper]

The ricer gen just saw what their grand-dads and grand-uncles did when they were younger, if it made them faster...

http://i76.photobucket.com/albums/j3...er/32holes.jpg

http://i206.photobucket.com/albums/b...g/scan0001.jpg

http://i206.photobucket.com/albums/b...log/holes2.jpg

http://i206.photobucket.com/albums/b...log/holes1.jpg

...their Gramps flew in WW2 aircraft where they saw every structural metal frames were full of 'lightening holes'. Post-war, they did the same for their rods, on structural frames and even their body panels.

http://2.bp.blogspot.com/-skfk4nomX8...600/vacuum.jpg

Engineers really call it, 'belled'. A hole is made and then a belling die flares the inside edges. Properly located and flared it 'strengthens' that frame. Colin Chapman was a religious believer in aircraft fabrication techniques.

http://users.lmi.net/~ryoung/Sonerai...ging_Dies.html

[freebeard]

Bigger the hole, the deeper the required flange, according to my 'stiffness' tests. Quarter inch flange does fine up to about 1.5" diameter. Big holes, 5" and larger, need a full half-inch. Everything else does fine with 3/8ths

Neat!