Help choosing vortex generators

[AeroModder]

When I was using the Vortekz brand delta VGs as instructed on the package, they multiplied the intensity of the counter-rotating vorticies that swirl over the trunk of my sedan, most likely increasing drag. I'm thinking about trying them again in the opposite direction to cancel out the vorticies instead of intensifying them, and seeing how that changes things.

[aerohead]

Originally Posted by AeroModder

When I was using the Vortekz brand delta VGs as instructed on the package, they multiplied the intensity of the counter-rotating vorticies that swirl over the trunk of my sedan, most likely increasing drag. I'm thinking about trying them again in the opposite direction to cancel out the vorticies instead of intensifying them, and seeing how that changes things.

The vorticity is a function of the pressure differential between the top and sides of the car,which is a function of the velocity gradient at the two locations.
I don't see how VGs could affect this.

[AeroModder]

The induced vorticies likely alter the pressures in different areas.

Package instructions say to position them like this when standing at the rear looking forward:

/ / / / / \ \ \ \ \

That lead to the intensified counter-rotating vorticies and a concentrated area of high pressure at the center of the base of the rear window. The vorticies were strong enough to draw water droplets in the shape of the vorticies.

I tried running with them reversed:

\ \ \ \ \ / / / / /

This lead to two high-pressure areas on the base of the glass:

| x x |

(the vertical lines being the edges of the window and the x's being the high pressure)

That's telling me there's two systems of counter-rotating vorticies, one pair on the outside and one pair in the middle, with the downward directions meeting at the x's. Flow seemed straight on the glass, but I'll need to tuft test to verify. I'm guessing the vorticies weren't strong enough on the surface to affect water droplet flow.

Right now I have them set up like this:

\ \/ \/ \/ \/ /

I'll see what effect this configuration has.

[aerohead]

Originally Posted by AeroModder

The induced vorticies likely alter the pressures in different areas.

Package instructions say to position them like this when standing at the rear looking forward:

/ / / / / \ \ \ \ \

That lead to the intensified counter-rotating vorticies and a concentrated area of high pressure at the center of the base of the rear window. The vorticies were strong enough to draw water droplets in the shape of the vorticies.

I tried running with them reversed:

\ \ \ \ \ / / / / /

This lead to two high-pressure areas on the base of the glass:

| x x |

(the vertical lines being the edges of the window and the x's being the high pressure)

That's telling me there's two systems of counter-rotating vorticies, one pair on the outside and one pair in the middle, with the downward directions meeting at the x's. Flow seemed straight on the glass, but I'll need to tuft test to verify. I'm guessing the vorticies weren't strong enough on the surface to affect water droplet flow.

Right now I have them set up like this:

\ \/ \/ \/ \/ /

I'll see what effect this configuration has.

When Ecommodder gets it's big wind tunnel,we'll be able to do pressure surveys and blow lots of smoke over things and get a good 'look' at what's going on.Wake flow is especially complex.

[slowmover]

Came across this story recently:

An Expediter and his Air tabs


" . . I had the rare opportunity to get my Sprinter into the wind tunnel at a local college to observe what happens before and after the Airtabs, as well as with and without the bug shield and rain guards. What we observed was rather dramatic.

Turns out, the bug shield and rain guard added a combined 7% increase in wind resistance, which translates to about a 3 ½% to 4% hit on fuel economy. At 22 MPG that’s about .8 MPG, which is a lot.

After a day of the students gathering the “before” data, Airtabs were mounted as close to the trailing edge of the Sprinter as you can get and still have them on a flat surface. With the Airtabs installed, the drag was dramatically reduced and showed a wind resistance reduction of more than 10%, which translates into a fuel economy savings of about 5% or 6%, which coincides with my own actual calculations.

Without the bug shield I could expect at least a 3% to 4% gain in fuel economy, so the wind tunnel math and the 2% to 4% claims made by Airtab are the same. With the Airtabs and no bug shield, I'd be getting 22-23 MPG instead of the 21-22 that I currently get. (I need to dump a couple hundred pounds of junk that have accumulated inside the van, too). . . . "

.

[ChazInMT]

One of the main sponsers of this "ExpiditeNOW" website is Airtabs....FWIW.

IMO Airtabs are worthless under most circumstances, and you're a sucker if you buy these gimmicks.

[MetroMPG]

Good eye, Chaz.

FWIW, the maker of the Vortekz VG's once offered to advertise on EcoModder. (EM must rate high for VG's on Google or something.)

I said (in a friendly way), "of course, but be prepared for the members here to openly challenge your company's claims in the absense of acceptable evidence about the effectiveness of the product."

[AeroModder]

I recently tried only 4 VGs in the center, and tuft testing showed a recirculation bubble over most of the glass. Removing the VGs made the tufts stand still.

Just out of curiosity, I ordered some Airtabs, and I'll see if I can finally get the airflow straightened over the rear glass.

[ConnClark]

Note: A sprinter van is big enough to benefit from air tabs at the back just like a semi

[ChazInMT]

Show me a real scientific study that shows airtabs have helped with fuel efficiency on anything. The studies done by air tabs themselves hold as much credibility as the studies on websites done by powerbands, LED light therapy, or water ionizers for health and HHO generators for fuel milage.

Here's some.

What is Water Ionization

- Light Relief

Power Balance - Performance Technology

Running Your Car On HHO Gas - Scam or Solution?

These sites use pseudoscientific babble to make you think that they are legit, when in fact they simply dazzle with bull s#!t in order to sell their junk. Kind of ironic that a device purported to help lower the suction on the back of a vehicle really is made for suckers.

Here's a website the eviscerates water ionizers.

"Ionized" and alkaline water: snake oil on tap

I acknowledge that the vortex generators may do some good in situations where the attached airflow is right at its limit on the back of the roof on a sedan, but these applications a very specific and the only credible paper I've seen was done by Mitsubishi.