Virtual boat-tailing

[aerohead]

I found an image online of the picture which I believe was in 'Race Car Engineering' by Valkenburg.
It's not a great photo but neither was the one in the book.
You can definitely see how the flow is displacing over the very steep aft-body.
Ludwig Prandtl and Oskar Tietgens were able to keep flow attached over a wing flap at 40-degrees of inclination by use of suction.Prandtl had demonstrated the concept with cylinders as early as 1904.

[freebeard]

Rotating cylinders? Could you use a rotating cylinder at the stagnation point to effectively move it to ground level?

[Madact]

In the style of a Magnus effect airfoil huh? I like it. Make the cylinder big enough and it might make the car look like it's recently been hit by a steam roller (roller still spinning cartoon-style)

[ChazInMT]

Something to keep in mind, aerodynamics are not about what is happening within the 1 foot of the skin of the vehicle, rather out to 10 and 15 feet in front of and around the vehicle and trailing behind for hundreds of feet. Everyone's literal myopic obsession with the air flow close in has you thinking about only a small part of a very big equation. And by very big equation, I mean engineers with massive computers have yet to be able to accurately predict what is going to happen without testing it in the real world.

[freebeard]

I don't disagree, maybe excepting the word 'literal'. I'm just asking questions.

In this case, it's a relatively narrow gap with one stationary wall (the ground). A-B-A testing could be done with an on-off switch.

[aerohead]

Originally Posted by freebeard

Rotating cylinders? Could you use a rotating cylinder at the stagnation point to effectively move it to ground level?

*Prandtl worked with a stationary,'sealed' cylinder,in a water tunnel.The slot was formed at one of the downstream sides,with vacuum applied to the slotted 'can.'
*Later,the 'can' would be replaced by a wing section,then flaps.
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*A moving (rotating) cylinder was investigated by Magnus after he noticed the drift of spinning KRUPP-type artillery shells (think of a flying soup can).
*The rotation (from the rifled cannon barrel) created lift,throwing the shell off trajectory.
*Magnus reversed the phenomenon to create the rotating cylinder ship sail.
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*'moving wall/moving skin' boundary layer control has been investigated and is reported about in 'Boundary Layer Theory', Hermann Schlicting's self-published book.
*It hasn't come up yet,so I've not prepared any material on it.
*If the surface moves at local velocity the boundary layer never forms,like at the tire tread/road interface.
*Theoretically,if you had a lubricated,elastic outer skin surrounding a low friction-surfaced body,and you could continuously orbit the skin,matching the local air velocity,you could eliminate the boundary layer.
*A 'backwards-rotating' cylinder at the forward stagnation point would:
- create a pressure spike behind itself as the air crashed into the high Reynolds number 'moving' body surface boundary downstream,triggering an instantaneous transition to full TBL.Probably shockwaves too,which would be audible.
- The boundary surface on the underside of the cylinder would already be moving at 2X the local airspeed plus the Prandtl line of discontinuity acceleration velocity component,and with a TBL already established, trigger separation at what would normally be the stagnation point.
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*The inviscid flow which is typically just outside the boundary layer would still be governed by viscosity effects of displacement dynamics/thermodynamics and remain affected by the isoenergetic Bounoulli rules regarding velocity and pressure within streamline filaments,and we'd still have to 'streamline' the shape,or we'd have separation-induced pressure drag.
*It doesn't bode well.

[Hypermiler1995]

(I wish I could post pics correctly) I drew a car that used ducts through the side to guide air from the nose, around the side, and out the back into the wake without boattailing. But it takes away from the space in the car, would cause some drag itself due to curves the air would go through, and its totally a nightmare for the back glass (requires multiple sheets of glass to fan the air out
and fill the wake and see at the same time)

[Hypermiler1995]

The best option I see (if it works) is to make a mini boattail, make the entire boattail the sharpest angle of the aero template, without the slow angle change, then use VGs to keep the air attached in the transition from the roof to the boattail.

[aerohead]

Originally Posted by Hypermiler1995

The best option I see (if it works) is to make a mini boattail, make the entire boattail the sharpest angle of the aero template, without the slow angle change, then use VGs to keep the air attached in the transition from the roof to the boattail.

Bear in mind that vorticity cannot be converted to pressure recovery.Without the gentle transition into the down/in-slope,you'd always have a drag penalty from the VGs.If the flow didn't reattach,the entire wake would take on the pressure of the point of first separation.
And they'd have to be tailored precisely to match the boundary layer thickness where they were attached.
If only the trailing edges of your mini tail touched the 'Template,'you'd have reattachment without VGs,along with captured-vortices,of which the inviscid flow would skim across.
The low pressure of the vorticity would be sequestered away from the wake,allowing for pressure recovery and a higher base pressure,lower pressure drag.
Without blowing,or suction.there's only so much VGs can do.
I think your plan WOULD reduce drag,compared to doing nothing.It would make for a fine project,and so far as I know,your combination has never been attempted.
Look at the back of the Mitsubishi Lancer,and the trailing edge of the trunk,compared to the roof height.This should guide your mini tail architecture.

[Hypermiler1995]

Originally Posted by aerohead

Bear in mind that vorticity cannot be converted to pressure recovery.Without the gentle transition into the down/in-slope,you'd always have a drag penalty from the VGs.If the flow didn't reattach,the entire wake would take on the pressure of the point of first separation.
And they'd have to be tailored precisely to match the boundary layer thickness where they were attached.
If only the trailing edges of your mini tail touched the 'Template,'you'd have reattachment without VGs,along with captured-vortices,of which the inviscid flow would skim across.
The low pressure of the vorticity would be sequestered away from the wake,allowing for pressure recovery and a higher base pressure,lower pressure drag.
Without blowing,or suction.there's only so much VGs can do.
I think your plan WOULD reduce drag,compared to doing nothing.It would make for a fine project,and so far as I know,your combination has never been attempted.
Look at the back of the Mitsubishi Lancer,and the trailing edge of the trunk,compared to the roof height.This should guide your mini tail architecture.

I am using the link in post #3 of this thread as evidence for my theory, but I am not sure about the amount of drag created By the VGs.