Virtual boat-tailing

[Big time]

Originally Posted by aerohead

Hucho says nothing has worked except boat tails and box-cavities.

It would be interesting to read Flugel and Frey studies to see how those deflector wings worked.


By some reason they used multiple deflectors.
Maybe they must work like multi element airfoils to be cost effective


Those look quite similar to multi element turning vanes used in wind tunnels


If they work then I think they can be added to the rear outer corners

Their performance could be increased even further by ducting exhaust air thru them.

And/Or these multiple element deflectors could be used in a high angle diffuser.
While the high angle of the diffuser keeps overall rear area to a minimum, such high angle may cause increased drag and even flow separation.
So these multi-element deflectors should ease air turn in while reducing drag.

[Madact]

Originally Posted by freebeard

Madact -- Like this?

Kind of similar in construction, but I was thinking a bit further back and at a much smaller angle - if you can imagine the "wing" being set at an angle parallel to "the template" (k-form profile).

The Frey and Flugel wings seem to be trying to fill the wake very quickly, directly behind the vehicle, hence the high camber and multi-element design.. I was thinking more of something closer to a kammback style effect, where the wing *shapes* the wake but doesn't try and divert the airflow nearly as much as the Frey and Flugel illustrations...

[Hypermiler1995]

I agree with Madact, as too steep an angle would increase drag, that is what jets use as brakes in mid-air.

[Madact]

Here's what I was thinking, in a bit more detail:
- A: Normal "box" tail - long wake, recirculation bubble helps drag
- B: The 'rear window cleaner' - adds drag but doesn't alter wake much - might cause the recirculation bubble to be unstable, as the 'window cleaning' flow is counter to the bubble rotation.
- C: The Frey and Flugel wings appear to be trying to fill the wake directly, with a small or no recirculation bubble
- D: What I was thinking - use the wing to stabilise / control the recirculation bubble, shortening it and bringing it closer to the 'ideal' / k-form shape

[freebeard]

Any idea what Cw is? All I see is increased frontal area and interference drag.

Gerrop OTOH looks like Coanda nozzles. That could work.

[Madact]

Originally Posted by freebeard

Any idea what Cw is? All I see is increased frontal area and interference drag.

Gerrop OTOH looks like Coanda nozzles. That could work.

Unsure about Cw, but they do indeed seem to be Coanda nozzles. See: Patent DE3837729A1 - Verfahren und vorrichtung zum reduzieren des stroemungswiderstandes im ... - Google Patents
Patent DE3837729C2 - Google Patents

Google image search for DE3837729A1 and DE3837729C2 should yield some figures...

[aerohead]

Originally Posted by Big time

It would be interesting to read Flugel and Frey studies to see how those deflector wings worked.


By some reason they used multiple deflectors.
Maybe they must work like multi element airfoils to be cost effective


Those look quite similar to multi element turning vanes used in wind tunnels


If they work then I think they can be added to the rear outer corners

Their performance could be increased even further by ducting exhaust air thru them.

And/Or these multiple element deflectors could be used in a high angle diffuser.
While the high angle of the diffuser keeps overall rear area to a minimum, such high angle may cause increased drag and even flow separation.
So these multi-element deflectors should ease air turn in while reducing drag.

*All these deflectors were tested and failed to produce a drag reduction.Some increased drag.
*The blown slot will work if you have a perpetual-motion device to produce the air volumes necessary.
*Ducts have been tested and they failed also.
What may work in an aeronautical application doesn't necessarily apply to a bluff body in ground proximity.And you need to be about 12-ft off the road surface to get out of ground effect.

[aerohead]

Originally Posted by freebeard

Any idea what Cw is? All I see is increased frontal area and interference drag.

Gerrop OTOH looks like Coanda nozzles. That could work.

*The truncated wing is Cd 0.58
*The truncated wing with turning vanes is Cd 0.28
*The unmutilated wing is Cd 0.006

[Madact]

Originally Posted by aerohead

*All these deflectors were tested and failed to produce a drag reduction.Some increased drag.
*The blown slot will work if you have a perpetual-motion device to produce the air volumes necessary.
*Ducts have been tested and they failed also.
What may work in an aeronautical application doesn't necessarily apply to a bluff body in ground proximity.And you need to be about 12-ft off the road surface to get out of ground effect.

*The truncated wing is Cd 0.58
*The truncated wing with turning vanes is Cd 0.28
*The unmutilated wing is Cd 0.006

Yeah, I don't think the 'wake-filling' approach is the way to go here at all...

It is interesting that the turning vanes are an improvement over the truncated wing though. Perhaps in the case of a wing, the plain truncated case can't set up a stable recirculation behind it as you find with a ground vehicle?

OTOH an increase in drag compared to an unmutilated wing but a decrease in drag compared to a truncated wing does sound promising if you aren't considering a full boat-tail though if the figures are indicative, it sounds like a kammback would work better. Extrapolating numbers from an airfoil in a free stream to a bluff body in ground effect is a bit sketchy though...

[aerohead]

Originally Posted by Madact

Yeah, I don't think the 'wake-filling' approach is the way to go here at all...

It is interesting that the turning vanes are an improvement over the truncated wing though. Perhaps in the case of a wing, the plain truncated case can't set up a stable recirculation behind it as you find with a ground vehicle?

OTOH an increase in drag compared to an unmutilated wing but a decrease in drag compared to a truncated wing does sound promising if you aren't considering a full boat-tail though if the figures are indicative, it sounds like a kammback would work better. Extrapolating numbers from an airfoil in a free stream to a bluff body in ground effect is a bit sketchy though...

Yes,there exists a jet pumping action in free flight which doesn't occur in ground effect.The axi-assymetrical flow kills off the performance of many tricks which DO work up in free flow.
Kamm recommended the 'K-form' because it DID work in ground proximity.And its a mutilated half-body.