Hi all from Kiwi in central Canada (pickup aero cap / topper - pictures)

[rumdog]

Ok thanks aerohead, very interesting indeed. I have often wondered about the shape of the 'cut'... and its influence on the eddies behind it.
i drew a little sketch, then read your post again... "no angles in the shape..." sort of answers my questions. So a continuous lip would be similar? - like the mercedes low drag concept?
then treat the rear surface with a porous material?
Man i want some windtunnel time! Thanks for the input.
oh one more thing, when you say ring vorticies, does that describe the eddies in the smoke tail? Or something else?

[aerohead]

Originally Posted by rumdog

Ok thanks aerohead, very interesting indeed. I have often wondered about the shape of the 'cut'... and its influence on the eddies behind it.
i drew a little sketch, then read your post again... "no angles in the shape..." sort of answers my questions. So a continuous lip would be similar? - like the mercedes low drag concept?
then treat the rear surface with a porous material?
Man i want some windtunnel time! Thanks for the input.
oh one more thing, when you say ring vorticies, does that describe the eddies in the smoke tail? Or something else?

*the lip on the Cd 0.19 Mercedes probably satisfies Morelli's conditions.
*the flow is streamlined (velocities and pressures similar)as it approaches the separation line.
*the separation line is perfectly transverse to the flow direction.
*the separation line is at,or very close to the trailing edge/lip.
*when viewed from the rear,the lip is elliptical in shape.
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As to the ring vortex itself,Morelli is not specific in this short (11-page) paper.
In NACA's (NASA) papers on airships,some of the aft-bodies were intentionally blunted.As the 'ring' of turbulent boundary layer sloughed off the rear of the airship hull,it filled in some of the void behind the blunt tail,to create a more streamlined phantom (or what Morelli is calling a 'fluid') tail,
made of this turbulence.
On the Mitsubishi Evo Lancer,the boundary layer thickness where the VGs were attached was up to 25mm thick.On our pickups it would be thicker because of their greater length,and this would be spilling into the wake.
There is also a jet pumping action which can take place behind truncated boat-tailed bodies which Morelli may be inferring.
With a healthy diffuser,this phenomena would be more likely to occur on a road vehicle.
Wheel fairings would make it even more likely.

[freebeard]

So any discontinuity in the approach or the edge will cause the uniform ring to slough sideways and feed an unwanted vortex?

The search term for 'jet pumping action' would be 'box cavity'?

Originally Posted by rumdog

then treat the rear surface with a porous material?

I'm curious about what you refer to. Perforated base plate?

[rumdog]

I like that term! Also need a healthy airdam. But i get what you're saying, in near perfect conditions, clean airflow, even pressures and velocities the air behaves like the boat tail was there. COOL. my truck is a long way from here but very interesting.

Freebeard, i threw the porous material thing in there cause i had heard that a porous material on the rear end of semi trailer helped with mpg... read it somewhere, think it was NASA, but could be wrong. I pictured the 'sponge' slowing and damping the eddies in the draft. Will see if i can find a reference.

[freebeard]

Lower left. More at http://ecomodder.com/forum/showthrea...tml#post476993

Porous could mean a semipermeable membrane, a woven mesh or a perforated plate. My bet is on the latter.

[aerohead]

Originally Posted by freebeard

So any discontinuity in the approach or the edge will cause the uniform ring to slough sideways and feed an unwanted vortex?

The search term for 'jet pumping action' would be 'box cavity'?



I'm curious about what you refer to. Perforated base plate?

*On a streamline body,the velocity and pressure measured transversley,anywhere along the body is the same.
*On a streamline half-body,in ground proximity,this condition can also be the same if particular attention is paid to the underbody.
*Hucho emphasizes that bodies generate only gentle pressure gradients along the body pathway,with no 'kinks.'
*If the truncation is not perfectly transverse,the entire base pressure of the wake will be determined by the first point of separation and it's associated static pressure.
*There is essentially zero static pressure near the windshield,it's all been converted to velocity pressure.
*The most forward point of separation has the lowest pressure,and all the air will attempt to flow towards this location,shearing the flow up into eddies,vortices,then full-blown turbulence.
*So we want gentle contours,then a sharp cut off

This is what Kamm/Koenig-Fachsenfeld advocated so strongly

Here, Renault has done it with their Vesta II concept

Here,Daihatsu uses it with their UFE-II

But only Morelli provides the elliptical rear transom which is capable of generating the 'fluid' tail
.
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The jet pumping action is covered in Sighard Hoerner's book,AERODYNAMIC DRAG,from 1951,in the section on boat-tailed projectiles in free flight.The air attempts to pump air into the 'buttock' (as Lanchester referred to the )void behind the bullet or fuselage machine-gun turret.As the air curves around and impacts the transom,it imparts some forward momentum.Morelli depicts a similar phenomena with the fluid tail technique (FTT).Dornier aircraft works also experienced this phenomena with respect to truncated wing trailing edges that they studied leading up to their 1920 patented wing.
Here is the equation for calculating the drag increase of any degree of wing truncation.5% is considered the maximum that can be removed before a significant drag increase is seen

[freebeard]

*On a streamline body,the velocity and pressure measured transversley,anywhere along the body is the same.

The sum or product of velocity and pressure, surely? I'm thinking of crosswinds.

*On a streamline half-body,in ground proximity,this condition can also be the same if particular attention is paid to the underbody.

This is why I think a flat floor with fences, or better, grooves to absorb the turbulence and channelize it into a central difusser could be beneficial.

[aerohead]

Originally Posted by freebeard

The sum or product of velocity and pressure, surely? I'm thinking of crosswinds.



This is why I think a flat floor with fences, or better, grooves to absorb the turbulence and channelize it into a central difusser could be beneficial.

*Hoerner uses the term 'equipotential'.It's the kinetic energy within each imaginary streamline filament surrounding the body.
*In a streamline body,the filaments surrounding the body are of adequate similarity in both static pressure and velocity,that the pressure gradients all along the path maintain attached flow.No 'kinks' as Hucho calls it.
*As to crosswind,it is measured for,dragwise,but I've never seen a report for where it was designed for.
*The body is designed for zero yaw and let the crosswind devil take the hindmost.
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*As to the belly,all we have is a few investigations.
*The consensus is: do a flat belly pan up the the rear axle area,then a gentle diffuser.
*there isn't any turbulence under the belly pan,just the turbulent boundary layer,which is a GOOD thing for us.
*If the diffuser is 2.8-degrees upsweep,it will give the lowest Cd.
*Since the VW Bugatti Veyron eliminates all it's diffuser vanes/fences/chines/strakes/fins (whatever else people call them) for low drag,it confirms the notion that nothing other than the gentle upsweep is needed.
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*Something of particular interest though,is in Morelli's year 2000 paper, "A New Aerodynamic Approach to Advanced Automobile Basic Body Shapes",which is available online as a printable PDF.
*In this paper they investigate a centrifugal fan,Coanda Effect rear wheel and wheelhouse duct system, linked with blown extractor jets just inside the ring of the rear bulkhead,above the longitudinal centerline of the rear tires.
*These Coanda slots help eliminate the wheel interference drag enough to facilitate the 'Fluid Tail' phenomena.
*At the time of publication,they were still investigating.No numbers were published.

[freebeard]

Found it. It's got me thinking about the 'wheelhouse duct system' reduced down to a basalt-fiber cycle fender.

[aerohead]

Originally Posted by freebeard

Found it. It's got me thinking about the 'wheelhouse duct system' reduced down to a basalt-fiber cycle fender.

It's pretty impressive: a 5kW power reduction for only a 240-Watt penalty.Nice 'voodoo' economics!