Drag reduction question

[pengilly]

What effect on drag would this test have?

If the highest pressure surface area was perforated with thousands of small holes, say 1/32", that discharged air or some gas from an onboard supply, at high velocity, into the oncoming air stream. Would this increase drag or decrease drag. If done correctly, the oncoming air stream would not touch the surface of the vehical. I can build a test unit but before doing so I would like some feedback on this therory.
Thank you
Jeff LeDoux

 

[pengilly]

The machine we are talking about is traveling at 200 mph FYI.

 

[MintJulep]

What you are describing sounds similar to boundary layer energization - which is a pretty well known and understood thing.

 

[rb1957]

practicality ... it looks like a land vehicle ... dirt will tend to mess with the idea.

i think the idea is to create a blanket of air (or "gas") around the vehicle. if this is the case i think the gas you're pushing out into the flow will "mess" in a severe way with the airflow round the vehicle, creating a huge turbluent "mess". for airplanes, boundary layer suction has been used in the past (refer Buccanneer (sp?) in-service with the RAF '60s - '80s).

maybe if you contained the gas within an elastic membrane, it'd prevent mixing with the airflow ??

 

[IRstuff]

Look up supercavitation for an alternate approach:

http://en.wikipedia.org/wiki/Supercavitation

However, it may be exceedingly difficult to do that in air.

As for the OP, the amount of air, and the velocity of the air generated must be powered by your fuel source, so, you won't be able devote as much power to propulsion, leading to a lower speed, anyway.

One would thinkg that running air at high speeds through those little holes probably burns a bunch of power as well.

TTFN

FAQ731-376

 

[pengilly]

No dirt involved and the air or gas will come from a on board tank and only be required for a 2-3 seconds.

 

[pengilly]

IRstuff,
You nailed it, thats exactly what Im trying to achieve only through air not water.

 

[pengilly]

Mintjulep,
I googled Boundry layer Energization and that looks interesting as well. Now I just need to see how it pertains to our project. It would be interesting to talk to some of the people who did the experiment.
Thank you

 

[btrueblood]

The drag-racing snowmobile fairing you show is fairly short, and would have little skin friction drag, at least relative to the pressure drag created behind it. There is little chance of having the boundary layer "trip" before the abrupt trailing edge, and so blowing along the aft outer face of the fairing also will likely do little good compared to the weight of the bottle you'd have to carry. Blowing out the front does no good, you are just creating more "dead" air for the slipstream of 200mph air to have to shove aside. You could blow (a lot of) air behind the fairing to try and reduce the base vacuum (think like a rocket nozzle), but the tank would be huge, even for a 2-second run.

Do this: try and do a better job of rounding the corners near the front, if at all possible. I see what look like square corners there. Chuck some bondo onto your thermoforming mold, and blend some fillets over those corners, and suck it again.

Also, before you get too wrapped up in the concept, do a simple spreadsheet calculation, and calculate what improvement in time you will get for a reduction in Cd from 1.0 (roughly what that bluff-ended fairing does) to 0.3 (like you might get from a more completely enclosed cockpit). Study bobsled designs, those get a lot of wind tunnel time.

 

[rb1957]

pengilly, why would you do this only for a few seconds? btue has nicely summed it up, particularly the last para ... what's going to give you the most "bang for the buck" ? increasing traction ? decreasing surface friction ? increasing surface friction ?? (this works in small sail-boats) downforce generators ?

 

[KENAT]

pengilly, I think the above have pretty much given what you need.

I think you may have missed RB1957 point on the dirt. It's not just the air passing through the holes that generate dirt. Mud, spray, bugs etc in the air through which the vehicle is passing will tend to impact on the surface and potentially clog the holes.

As several mention the idea of suction on a wing or potentially fuselage/nassle has been experimented with off & on for some time. This is to remove the sluggish boundary layer and retain laminar flow and prevent the airflow going turbulent which increases drag.

As regards the Blackburn Buccaneer, were you thinking of the flaps RB? These were blown flaps more about increasing thrust than decreasing drag.

http://en.wikipedia.org/wiki/Blown_flap

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[KENAT]

Sorry, increasing lift not thust, noticed just as I hit submit.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[rb1957]

could well be ... i wasn't sure of the precise application (only associated the words "boundary layer suction" with buccanneer) ...

thrust from flaps, indeed ! lift is more like it !!

 

[pengilly]

btrueblood,
Nice summary Thanks for helping with my tunnel vision!
RB1957,
If this concept actually could be made to work, I was only going to apply it during the period of highest pressure,The last 3 seconds of the run, thinking it would help the most at that period and help make the idea more practical. Im just trying to think outside the box.Traction issues are pretty straight forward. Tryng to make everything as effiecent as possible. Cant really get much lighter and at this point Im not sure more power will help, could actually hurt. I think a trip to a wind tunnel is in our future.
Thanks for ALL input
Jeff LeDoux

 

[rb1957]

ok, the end of the run is when you're going fastest (so drag losses are highest), but how much advantage will you see in a few seconds ? run some calc with 50% less drag for the last few seconds of the run, I think you'll see only about 1/2 a gnat's difference (and 50% is incredibly optimistic) ... what would be the difference if you could reduce Cd (for the entire run) by 1% ? i think a trip to the wind tunnel will help enormously.

good luck

 

[debodine]

rb1957 could you supply your hand calcs on that 1/2 gnat difference? I can't find the formula in my reference books...

)

 

[KENAT]

1/2 gnats is approximately equal to a midgie, which is an order of magnitude less than a mossie.

I thought the American equivalent was a pi$$ ant but I may be getting my metric/Imperial/US customary units mixed up.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[debodine]

I know I am much better informed after your post, Kenat. Thanks! )