ahagge said:
I have them on my ICE (Mitsubishi Evolution IX MR) - they're stock on Evo MRs. Mitsu calls them "vortex generators". They're claimed to help reduce turbulence in the airflow coming off of the passenger compartment. Do they do anything useful? I doubt it. Certainly doesn't seem to help my mileage at all.
To be fair, Mitsubishi's goal was to decrease lift on the rear of the car by directing more air to the rear spoiler.
The VG's upset the boundary layer causing airflow to stick a bit closer to the rear window so more wind hits the rear spoiler thus reducing lift.
A side benefit is that drag was also reduced slightly.
Mitsubishi was nice enough to publish a paper showing their testing methods and results to come up with the optimal design. Going to be tough for someone without at least some good CFD software to duplicate.
http://www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2004/16E_03.pdf" onclick="window.open(this.href);return false;
Mitsubishi's study found a maximum of a 0.006 reduction in Cd with well placed VGs. That's about a 2% reduction in drag. Note that you can't necessarily apply these results to the LEAF since the LEAF is a hatchback and the Evo is a sedan.
As mentioned previously, airplanes use VGs on wings to decrease stall speed - a plane stalls when airflow separates from the top of the wing. But on planes under normal conditions, it increases drag so manufacturers work on wing designs that eliminate the need for VGs while maintaining minimum stall speeds.
I'm not aware of any real wind tunnel studies on VGs on vehicles with square backs so I am skeptical of any drag reduction claims.
On the LEAF, you can see the little bumps on the trim below the windshield, but they are positioned in the direction of airflow - these are not VGs. Larger fins can be seen on the undertray under the bumper. The fins are used to help clean up turbulent air - in the rear this can help reduce drag - before the windshield I'm sure they are used to help reduce wind noise primarily.
My educated guess at the most effective location for VGs on the LEAF to reduce drag? The spoiler and sides of the car, particularly around the rear "haunches". Nissan sacrificed drag for style here as around the mid point I'm sure that airflow separates over the curve increasing drag. If they had tapered the lower part a bit more and created cleaner line from the rear bumper to the top half of the tail-lights I'd bet they could have dropped a tenth off the cD figure.
Compare the rear of a LEAF to a Prius. Pretty easy to see how much bigger of a "hole" the LEAF leaves in comparison.