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Old 03-15-2008, 05:32 PM   #1 (permalink)
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Drag reduction - A new theory?

To cut a long story short, I used to fly gliders (sailplanes). They have very low coefficients of drag, in fact it is one of the points that most glider pilots get obsessive about

One of the 'interesting' designs on the market a few years ago was a system that eliminated the seperation bubble by tripping the laminar flow prior to the transition to turbulent flow. Nothing new there, except it used high pressure air.

The leading edge had drillings that chanelled high pressure air to the point in the upper wing surface where a series of small holes (approx 1mm) along the length of the wing let the air out just prior to the seperation point.

This had two effects. Firstly to trip the flow, and secondly to add energy to it so it reattached sooner.

This has got me thinking of an idea for hatchbacks.

Why not channel high pressure air from the front of the vehicle to the upper roof line where the air exits from a slot just below the transition point from horizontal to vertical. Also a series of holes in the roof could be used if there was seperation prior to the end of the roof line.

This would entrain the current laminar flow, or turbulent flow, add energy and possibly reduce the low pressure drag effect caused by the rear of the vehicle???

I could do a diagram if required

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Old 03-15-2008, 05:53 PM   #2 (permalink)
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Two of the popular ways are blowing (like you suggest) and sucking...either speed up the slow stuff or get rid of it.

The biggest problem will be reducing drag enough that the pump driving the system doesn't use more fuel then is saved. Sucking will also get dust and debris stuck in the pores. If the sheet you are drilling into is structural, you may significatly decrease its strength

Another weird, albeit impractical one I have seen uses flush rollers at the end of an abrupt cutoff to encourage flow into an area of separation.

Quote:
Originally Posted by AXMonster
To cut a long story short, I used to fly gliders (sailplanes). They have very low coefficients of drag, in fact it is one of the points that most glider pilots get obsessive about
I envy... That is a sport I'd love to get into.

One cheap technology that could be crossed over to cars is gap seal tape.

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Old 03-15-2008, 06:03 PM   #3 (permalink)
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Got the mylar gap seal tape aspect covered Soon to be fitted to the AX

How about the idea of incorporating a rear slot just after the roof to rear window transition point to direct the flow coming off the roof (in an attempt to keep it moving down the rear screen) & reduce the low pressure area behind the car? Could it work?
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Old 03-16-2008, 04:18 AM   #4 (permalink)
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Quote:
Originally Posted by AXMonster View Post
How about the idea of incorporating a rear slot just after the roof to rear window transition point to direct the flow coming off the roof (in an attempt to keep it moving down the rear screen) & reduce the low pressure area behind the car? Could it work?
Yes, but how well? My concern is the volume of air that is needed to actually reenergize the boundary layer. I can't even begin to imagine what is necessary, but it is probably fairly high.

There are three sources of air that I can think up of:
1.) Exhaust
2.) Ducting high pressure air from the front of the car to the rear
3.) Electric pump

The exhaust will not provide enough flow in the average economy car. I know some F1 cars use the techique during acceleration to create a type of "blown flap" for down force. Every bit counts, though.

Ducting has been tried in the past, but from what I've read the benefits are extremely minimal or non-existant.

The electric pump shows the greatest hope, but it is going to be an energy hog. Jets that used blown flaps (the same concept) used compressor bleed air from their engine. Supposedly the engines lost the majority of their power when that was done, but it wasn't a concern since they were only used for landing.

I believe sailplanes that are trying to use the technology are doing feasibility tests to see what the system weight is going to be. Something like 500 watts of solar panels need to be added, in addition to a pump. 500 watts seems minimal to a car, but a sailplane is much less draggy and therefore needs to reenergize the BL much less. A hatchback will probably need much more power.

Lastly, if you do undertake this challenge, make sure that the intake air to the blower is used for effect...might as well evacuate some of the high pressure air on the front of the car.

In closing, I would say the idea is feasible but it rates high on the cost/benefit ratio list. It would be cheaper to build a boattail, but a hell of a lot less cool/innovative.

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Old 03-16-2008, 05:50 AM   #5 (permalink)
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Good post LC. I see you have a lot of knowledge in all things aero Thanks for the advice.

One area I could envisage being used is a source of energy normally wasted; the exhaust. Not in the normal snese, but to drive a free running turbocharger that supplies compressed air to the blown slots. The intake for the compressor could be a section of the front of the vehicle in a high pressure area, making use of the high pressure and compounding it with 'wasted' energy from the exhaust.

I have a lot of spare turbochargers lying around. Selection will ultimately depend on providing a flow and pressure ratio that is achieveable without causing undue back pressure and reducing FE. A VNT turbo set to a couple of psi would self regulate and hopefully not overspeed... although they can handle up to 250k rpm these days.

I guess running insulated pipework from the engine bay up the A pillars and to the C pillars wouldn't be too arduous.
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Old 03-16-2008, 08:58 AM   #6 (permalink)
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AX, you will probably want to read this
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Old 03-16-2008, 01:34 PM   #7 (permalink)
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Wow, thanks! Just what I was looking for
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Old 03-16-2008, 04:38 PM   #8 (permalink)
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Hi All,

An interesting thing in that article is that if they turned on the blowers on the top and bottom Coanda slots only, the drag went up. Not until all four slots (top, bottom, left and right) were turned on did the drag go down. So, the airflow has to be tripped symetrically all the way around the rear of the vehicle.

Another intersting thing is it only took 1/2 PSI to have a good effect.
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Old 03-16-2008, 10:29 PM   #9 (permalink)
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Quote:
Originally Posted by donee View Post

Another intersting thing is it only took 1/2 PSI to have a good effect.
I wouldn't call that "only" half a psi.... That's a LOT of air for a large surface area - especially when you're pressurizing an open container (atmosphere)
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Old 03-17-2008, 12:38 PM   #10 (permalink)
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new theory

It's the "open-pipe" theory that floats about.The challenge is to get enough air "through" the vehicle,without increasing frontal area,or imposing on occupants or cargo.The ducting must be very complex,and wall friction eats away at kinetic energy available for the wake.The ducting also affects crumple zones,occupant safety in collisions,weight,and is also effected in non-zero-yaw relative-wind conditions.In addition,its a fair-weather system,as,the pressure available at the forward stagnation point of the vehicle finds it difficult to purge or displace rainwater pouring into the discharge perforations.In the wing,its a straight-line path to the trailing edge and the pilots probably don't fly into foul weather.Its a fascinating concept,however plagued with challenges.

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