Vortex Generators - hopefully a rational discussion.

[MrInstrument]

The Holy Grail, part two. You are correct that VG's require controlled and intelligent placing. However, with a few tuff test and understanding of flow velocities and separation, you can, as a layman (not having a wind tunnel), get descent results with VG's. VG's are not designed for placement into relative wind as you see so many people doing on you tube and on their VG sales page. VG's are designed to take the energy from the free flow stream and impart the velocities into the boundary layer, thereby increasing the energy of the boundary layer for it to continue to do its job. This same energy that is imparted from the created vortex is the energy you wanted proof of above. The very design of a vortex generator is this proof of energy you desired, that is why they are used, they impart energy. As I stated, you can control this energy by the placement of VG's at the rear of a car to extend the "perceived" rear-of-car, thereby decreasing the biggest form of drag on most vehicles, the deep low created at the rear. Placement doesn't mean, stand the VG's in soldier like form all across the rear, not at all. The relative wind is already flowing that direction so keeping it flowing that direction does nothing. What is desired is the vortex energy that is created by off-setting the VG by approximately 30 degrees. When off-set like this, the VG now gives off a controlled vortex which extends into and over the deep low at the rear of the car. When the upper or above the car flow is controlled, the divergent flow now lessens because for a car, the road is the termination point of the free airflow, in essence, you close the gap of your profile. Just a basic set of VG's on my truck produced 2 mpg difference, that isn't using but on area of the drag profile. I sold the truck before I could finish the top row and do the sides, but I that was an immediate result do to tuff tests and my sons GoPro.

[MrInstrument]

I don't sell VG's. Like everyone here, I find it interesting to bring my 30 years of aviation, aircraft modification, and maintenance and a minor in rotorcraft aerodynamics to the table to see how it can be applies to the everyday driver. I am sure I could find some VG's to suit your need but you have the same Google and search engines I have. I just joined to enjoy the craftiness of the ecomodders that I have been reading about.

[MetroMPG]

MrInstrument - Aside from your results, do you happen to have any sources of study about using VG's on passenger vehicles the way you describe? You're describing a sort of "virtual tail" that I think would be very popular if it were doable.

My own A-B-A testing of air tab style VG's (which supposedly take into account the required angle to the local airflow to generate vortices) showed no effect when placed at the rear of a minivan on the sides & top.

Yet a simple Kammback style tapered extension did measurably reduce drag on the same minivan.

I'm open to the suggestion that I did it wrong, or that the VG's weren't optimal for the conditions.

[aerohead]

It seems that the VG is intended for the notchback in which separated flow fails to reattach onto the boot,yielding a squareback wake.
The Mitsubishi Lancer suffered from this as well as a Honda Accord.
Successful VGs were sized and spaced with critical dimensions specific to these vehicles.
The Beetle has no rear structure for which to enable reattachment.
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In free flight,there exists an axi-symmetrical flow field which provides a jet pumping action behind bluff bodies,which attempts to pump air into the turbulent wake.
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In ground proximity there exists no axi-symmetric flow field,and no jet pumping action.
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While VGs might reduce the effective wake of a cargo plane,they have been shown during wind tunnel investigations to offer no gain within ground proximity,as well as many other aeronautical technologies.
Hucho says they're a dead end for ground vehicles.(unless there's a structure behind the separation point)
------------------------------------------------------------------------- Ernie Roger's Beetle wing provides a structure in which the longitudinal vortice pair can be burst,and a locked-vortex can be captured,allowing the outer flow a 'surface' to ricochet off.

[botsapper]

VGs, what critical stall speeds in...land vehicles?
Aero-TV: Making Good Planes Better -- Micro VGs Offer Aerodynamic Enhancements | Aero-News Network

[MetroMPG]

Originally Posted by aerohead

It seems that the VG is intended for the notchback in which separated flow fails to reattach onto the boot,yielding a squareback wake.
The Mitsubishi Lancer suffered from this as well as a Honda Accord.
Successful VGs were sized and spaced with critical dimensions specific to
these vehicles.

This is the one style of car I'd actually like to try them on - small cars with very short rear decks. Tuft testing the deck would reveal whether it was a good candidate for a VG test or not.

[aerohead]

Originally Posted by MetroMPG

This is the one style of car I'd actually like to try them on - small cars with very short rear decks. Tuft testing the deck would reveal whether it was a good candidate for a VG test or not.

I'm seeing some entry level Kia and Hyundai cars which look like they could definitely benefit.You'd still have the nice utility and outward vision of the notchback design along with more friendly Cd.
We might presume a similar boundary layer thickness and VG height for these other cars as with the Lancer.Also,if the VGs were just like Mitsubishi's,then their location and spacing could also be used,as the delta pattern of propagating downstream vortices would be similar.
These cars would also be good candidates for rear spoilers to help capture a horizontal vortex within the C-pillar side flow.
I've also noticed that Mitsubishi went to an aeroback with the later Lancer,ditching the traditional notchback roofline.An aeroback mockup using 'cardboard and duct tape' design (I love that!) would also be fun to test in an A-B-A comparison.
(no shortage of potential projects!)

[MrInstrument]

I see VG's ruffle feathers around here. For me, it is too late to quote Hucho and any other because I have been driving the proof of concept all winter. My truck never saw 18mpg, never, not even on one hour drives on the interstate, but now I have seen as high as 19.8. No boat tail installed, no engine change, no drafting, no structure behind, no axisymmetric flow, no speed change, only VG's placed at appropriate spacing at 30 degree angles and 2 extra days of driving before refueling. But thanks for the input. Just trying to pull out the extra mileage on each tank like everyone else here.

[MetroMPG]

Most around here (without access to wind tunnels) put stock in controlled-as-possible "A-B-A" testing. Any chance you'd be willing to try that and do a write-up?

EG: http://ecomodder.com/forum/showthrea...ery-11445.html

[aerohead]

*A Clark Y airfoil will stall above 22-degrees AOA
*With VGs,stall is postponed to around 26-degrees AOA
*The VG requires the lower surface boundary in which to broadcast the vortex-induced kinetic energy to prevent stall/burble/recirculation/eddies/turbulence
*The base pressure of the wake is determined by the static pressure of the inviscid flow just outside the Turbulent boundary layer.
*The separation line determines the pressure of the wake.
*On a squareback vehicle,the VG does not alter the separation line,as the body falls away so rapidly that we lose the lower surface boundary;and with it,the turbulent boundary layer which might have received kinetic energy from the VGs.
*Since the separation line,with or without the VGs is essentially identical,there's no facility for pressure recovery,the only thing which could raise the base pressure,reducing pressure drag of the wake.
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*VGs in front of a 'fast' box-cavity or 'fast' boat tail WOULD provide for beneficial locked-vortices all around the perimeter.And this WOULD raise the base pressure,lowering the overall drag.
*Scientifically,I just don't see how VGs can benefit a squareback type aft-body.
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*In Hucho's chapter on commercial vehicles he comments that VGs,turning vanes,etc.,all fail to reduce drag on road vehicles with square backs.
*The box-cavity,next to the boat tail, is the only add-on device to register a drag reduction when subjected to rigorous wind tunnel testing.
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*Suction and blown slots DO show a drag reduction,however,the energy required to operate the pump exceeds the energy saved.
*This is why Hucho emphasizes the importance of 1922 technology body elongation through boat-tailing for meaningful drag reduction.