Using these two "research" writeups as a guide...this is an attempt to try to determine the best vg to use...and how to use one in the most effective way.
Since I don't have a windtunnel...and all vehicles tend to have different shapes...this will involve a lot of guessing and extrapolating.
The surfaces ahead of the vg and those after it will determine how well a vg will work......as least as much as the form of the vg itself?
There are usually 2 most effective areas to use vgs...above a downsloping rear window and at the back of a blunt shaped vehicle like a van....station wagon...or truck.
Using the NASA research...I concluded that a vg approx 1" high and 4" long...set at an angle to air flow of about 16* to 20* would be most effective and practical at the back of a blunt vehicle.
These would be sloped downwards when used on the sides. These are much like short "strakes".
On the top they could be angled inwards in this manner (air flow is from top)..... \-\-\-\-/-/-/-/....
They would be spaced around 4" apart.
I would make them from alum flashing material and mount them to the vehicles surface using regular clear silicone caulk.
* Unlike seen in some of the pics...any sharp corners should be rounded....edges sanded.*
Any type of "angle iron shape" (steel or alum) or clear plastic corner molding could also be used.
One vg might look something like this...where the airflow would be coming from the bottom right corner of this pic...the darker gray area is glued to the surface.
When used above a rear window or hatch window...they might be 1" high X 2" long and angled at 15* to the tested airflow**. They would be mounted maybe 4" forward of where the "break" downward occurs...and spaced 4" apart.
** airflow testing above a rear window can be done by placing a series of oil drops (motor oil) aprox 4" apart in a line around 4" in front of where the surace drops off at the back...drivng your car at usual road speed for a mile or so...stopping and taking note of the angle that the oil flows. Usually the oil will flow straight back except towards the outer edges...where it will tend to flow in towards the center of the car. Right at the edges there is usually turbulent air...so you don't want a vg there.
There are also other DIY vgs that can be used:
* the airtab clone (not my design or pics)...is best used at the back of a blunt ended vehicle?
This pic show the application of these to a pickup truck...
These would be placed as near to the back edge as possible.
This is the pattern for one vg...
This is how I visualize them working...showing the counter-rotating vortexes formed...
* then there is a double vg design as copied from one used on an ultralight...best used above a rear window? Notice that the vgs on the end are angled inward vs the cars centerline so each single vane lines up at around 15* vs the actual airflow.
here is a pattern...
It's possible that the airtab clones and the double vgs create vortexes that interfere with one another LESS than the single vgs....since these vortexes counter-rotate....so that the edges of adjacent vortexes are moving in the same direction.
The single vgs could also be used like the ultralight vgs...using full 1" high vanes around 4" long and angled from 16* to 20* in opposite directions...they could be made individually and put in place one by one....maybe 3" to 4" apart...placed like this...(air flow is from below)...
Here is a pic of a hypothetical vg that might form a boat tail at the back of a blunt ended vehicle...since it is tapered down to the back...and then again...it might not. These would be placed as near to the back edge as possible.
Here is another that might work better...
...these would be used as ~1" high vanes 4" long and angled from 16* to 20* in opposite
directions.....put in place one by one....maybe 3" to 4" apart? The blue arrow shows air flow...the height tapers down at the back to try to force the vortex in towards the centerline of the car...placed like this...air flow from below...
These might not work so well if a vortex is typically formed by air spilling over the top of the vane. Or they might form both a defined air stream AND a vortex at the same time. Anyone have a wind tunnel?
Possibly the best performing vgs would consist of complex shapes such as with the air tabs?
Each airtab would create double counter rotating vortexes and the vortexes from adjacent airtabs would not interfere with one another because their outer "edges" would be going the same direction?
Decided that I couldn't be left behind in the vg race...so I made some more out of ~.011" alum flashing material. A better material might be alum siding which tends to be ~ .022" thick. This stuff can be cut with regular scissors...but don't use your mommy's sewing scissors.
My analysis of the airtab:
The airtabs is around 5" long, but the "effective" angled area is only 2.5" and is angled at 30*. There is a straight "intro" area and an "exit" area that are likely shaped to "condition" the air flow to avoid any extra disruption of the air flow over the tab. The "exit" area probably helps to cause the vortexes to pull towards the centerline of the vehicle slightly? At any rate the complex shape probably makes them more efficient (cleaner vortexes with less drag) than a simple vane type vg.
I made two versions of vg...a smaller one that is just two straight vanes both angled at 20*...and a larger more complex one with the same 20* angle.
With the smaller vg the surface area of each vane is maybe 30 % greater than the airtab, but they are angled at 20* rather than 30*. The NASA research shows that too sharp an angle can cause the vortexes to break up with a straight vane type vg. With this vg it is easier to find a relatively flat place to install them.
The larger vg has a different design and is made to try to use streamlined shapes to cause less disruption of the air flow. The cross section of each vane is maybe 30% more than the smaller vg, but are also angled and not vertical.
The larger vg could be used on a van or on the hood of most vehicles. (pay no attention to those people in the other lane that are laughing about something)
* right click and click "view pic"...or paste url into address bar and hit ENTER??
the patterns (large squares are centimeters):
This material...espec the .011" stuff is pretty flimsy...I sanded the edges, painted them, and used about 3 dabs of silicone caulk to stick them on...along with a level to get them straight. They should be placed less than 10" from the back edge (if you have a squared off back end...no not yours...your car's) and spaced maybe 4" to 5" apart on center.
They probably shouldn't project out much farther than the greatest width of your vehicle.
Depending on whether you have the materials laying around...$0 to maybe $10.
Various vg related LINKS:
VG SEARCH TERMS:
boundary layer separation
turbulator OR "vortex generator" OR tripper OR invigorator