Drag per aft-body length/roof angle

[freebeard]

I posted that to show how much my thinking has changed since hanging around here (but didn't say so, sorry). Water might make a rooster tail but air not so much.

How much divergence is too much? A trumpet bell runs out to 90° axially. I thought they couple to the ambient air most efficiently.

[aerohead]

Originally Posted by freebeard

I posted that to show how much my thinking has changed since hanging around here (but didn't say so, sorry). Water might make a rooster tail but air not so much.

How much divergence is too much? A trumpet bell runs out to 90° axially. I thought they couple to the ambient air most efficiently.

For pipes and ducts the optimum angle for a conical diverging section depends on the k-factor (smoothness) of the boundary walls.But the drag minimums are at the 6-8 degrees range.
For a diffuser,there is a overall drag reduction with angles up to 11-degrees,but the minimums are at 2.8-degrees,to 4-degrees,depending on where the up-sweep begins.
Laterally,against the sidewall of the wheel fairing,I suspect that you'd mirror that angle,otherwise you'd have an asymmetrical pressure rise leading to vortice formation.
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On the drive here this morning,the Denton High Marching Band was crossing the street in front of me.I was watching the horn bells.I'm not an acoustics guy,but my thoughts went to a reversed-ear.
Our ears concentrate sound into the ear passage.
The bell on the horns may reverse this to 'broadcast' the sound pressure waves.Purely an acoustic,sonic wave function,rather than aerodynamic function.

[aerohead]

Here's Hucho's drag table from his 2nd-Edition,1987.
I've highlighted the teardrop I used for the aft-body of the 'Template.'

[aerohead]

Here's a Chevy Malibu LTZ advertisement which helps with the concept

[freebeard]

Julian Edgar likes bellmouths that are completely rolled. On an inlet.

So you think a flat diffuser with diverging sides or a curved diffuser with flat sides will form vortexes? What if instead of a rectangular section it's a half circle or flattened oval?

It seems like the thing would be to match the velocity of the air from under the car with the air over the car. even if there's a flat plate in between. Or a concave/convex plate with the tearing edges.

[aerohead]

Originally Posted by freebeard

Julian Edgar likes bellmouths that are completely rolled. On an inlet.

So you think a flat diffuser with diverging sides or a curved diffuser with flat sides will form vortexes? What if instead of a rectangular section it's a half circle or flattened oval?

It seems like the thing would be to match the velocity of the air from under the car with the air over the car. even if there's a flat plate in between. Or a concave/convex plate with the tearing edges.

*Yes,for a scoop,you wouldn't ignore the outer rim or you'd have separation right there.
*On the diffuser thing,some wheel fairings are showing up which are longitudinally flat-walled on the inside and contoured on the outside.
There's virtually zero info on this so far,but if it's chosen over inner-contoured fairings,then there might be a scientific reason for it.
*And as far as flat vs curved diffuser,we're stuck the same way.No data.
*From M.I.T.'s solar work,it looks like whatever we do,the side wall of the diffuser/wheel fairings should be blended to mitigate hook-vortice formation-induced intersection drag.
Hucho says that we're to be looking at the pressure profile,which is going to be affected by the velocity profile,and so this is going to determine 'flatness' or contour viability.Again,we're over-reaching empirical studies of CFD modeling.
VW's current XL is employing them

Since MIT and others are using 'double-curvature' fairings,until I hear otherwise,I'm okay with them.

Does this adequately muddy the waters?

[freebeard]

I like this :



Maybe because it looks like it would pass road debris.

[gone-ot]

Aerodynamics: the 'art & science' of parting the air (front) and then recombining it back together again (rear) so that the air doesn't even realize anything happened.

[aerohead]

Here is the graphic for a diverging nozzle.
If you look at the lower half,you'll see a notchback roofline with separated flow following the 'template' at the 'top'.
If the inner walls of the nozzle are more a K-form (Koenig-Fachsenfeld/Kamm),there'll be re-attached flow on the 'boot.'

In a perfume sprayer/carburetor,the 'template' is swung,360-degrees inside the inner wall of each barrel to create the venturi which creates the Daniel Bournoulli low pressure which will lift the perfume,gasoline through the metering jet from the bowl

[sendler]

Originally Posted by aerohead

On the drive here this morning,the Denton High Marching Band was crossing the street in front of me.I was watching the horn bells.

Bell termination of musical horns being truncated at 90* is a convenience thing since it is close enough to being complete for the expanding waveform. But there is still some sharp reflection at the edge of the termination which forms standing wave distortion back into the throat.
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A LeCleach horn profile is more complete, efficient, and lower in distortion. But is obviously much larger in diameter.
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http://www.goodsoundclub.com/Site_Im...e%20Cleach.gif
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Jean Michel on LeCleac'h horns - diyAudio
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