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nmgolfer · 10-09-2010, 11:36 PM

Seems some people don't understand what vortex generators can and cannot do.

First... lets get one thing straight... Of course they work!

But what does "work" mean?

Vortex generators are a band-aide for one type of problem not a cure-all. Vortex generators "energize" the boundary layer to help prevent/delay the onset of separation in the presence of an adverse pressure gradient. But they cannot perform miracles and no one should expect them to.

Putting vortex generators on the roof upstream of a typical hatch is a complete waste of time and money. Why? Because even vortex generators cannot prevent separation in the presence of extreme adverse pressure gradients.

So when can vortex generators be advantageous? The Mitusbishi ap. is a good example. There was undoubted a separation bubble at the base of the rear window on that sedan prior to their putting vortex generators upstream. With the generators in place that separation is undoubtedly gone consequently the pressure is higher (Cd lower) with them installed.

The key is that the rear window slope was gentle enough so that there existed the POSSIBLITY that the flow could be coaxed to remain attached. In this case a "band-aide" (vortex generators) stopped the "bleeding" (flow separation). That will NOT the case in all (even most) applications. Its certainly not the case when it comes the the rear window of a fit. (too steep!)

And its not the case in a (I believe it was Sentra) that some one else here on ecomodder tried. Sorry dude... THEY DON'T WORK MIRACLES! The Sentra rear wind0w is WAY TOO STEEP to have any chance of keeping the flow attached. As you learned ... that was a waste of time and money.

Anyway... Frank's right ... spend no money and make them easily removable.

In my own research I learned that the Toyota Yaris Sedan (which I own) had the same Cd as the Yaris hatch (.29)... that made no sense since the Sedan is "clearly" more aerodynamic... now I know why... Tuft testing showed that the sedan has a separation bubble at highway speeds at the base of the rear window. This is not good and something had to be done about it.

A local plastics shop gave me a small scrap sheet of ABS from which I fashioned some delta wing vortex generators (based on a template I found on that other web site clean mpg) I placed these on the glass at the top of my rear window using glue (goop) I know will not come off until I want it to. I then put the tufts back on... lo and behold this time they lay down BEAUTIFULLY. (I don't hear any noise from them either which was one of my concerns). Anyway I know the pressure is now higher on the rear window and trunk (and my Cd is lower) with the vortex generators installed. No I did not do A-B-A. To me the proof of the pudding is in the eating... I only care about my mpg at the pump which is now darn close to 50 depending on how much city driving I have to do.

10-09-2010, 11:36 PM	#30 (permalink)
nmgolfer EcoModding Lurker Join Date: Aug 2010 Location: New Mexico Posts: 41 Thanks: 3 Thanked 3 Times in 3 Posts	Seems some people don't understand what vortex generators can and cannot do. First... lets get one thing straight... Of course they work! But what does "work" mean? Vortex generators are a band-aide for one type of problem not a cure-all. Vortex generators "energize" the boundary layer to help prevent/delay the onset of separation in the presence of an adverse pressure gradient. But they cannot perform miracles and no one should expect them to. Putting vortex generators on the roof upstream of a typical hatch is a complete waste of time and money. Why? Because even vortex generators cannot prevent separation in the presence of extreme adverse pressure gradients. So when can vortex generators be advantageous? The Mitusbishi ap. is a good example. There was undoubted a separation bubble at the base of the rear window on that sedan prior to their putting vortex generators upstream. With the generators in place that separation is undoubtedly gone consequently the pressure is higher (Cd lower) with them installed. The key is that the rear window slope was gentle enough so that there existed the POSSIBLITY that the flow could be coaxed to remain attached. In this case a "band-aide" (vortex generators) stopped the "bleeding" (flow separation). That will NOT the case in all (even most) applications. Its certainly not the case when it comes the the rear window of a fit. (too steep!) And its not the case in a (I believe it was Sentra) that some one else here on ecomodder tried. Sorry dude... THEY DON'T WORK MIRACLES! The Sentra rear wind0w is WAY TOO STEEP to have any chance of keeping the flow attached. As you learned ... that was a waste of time and money. Anyway... Frank's right ... spend no money and make them easily removable. In my own research I learned that the Toyota Yaris Sedan (which I own) had the same Cd as the Yaris hatch (.29)... that made no sense since the Sedan is "clearly" more aerodynamic... now I know why... Tuft testing showed that the sedan has a separation bubble at highway speeds at the base of the rear window. This is not good and something had to be done about it. A local plastics shop gave me a small scrap sheet of ABS from which I fashioned some delta wing vortex generators (based on a template I found on that other web site clean mpg) I placed these on the glass at the top of my rear window using glue (goop) I know will not come off until I want it to. I then put the tufts back on... lo and behold this time they lay down BEAUTIFULLY. (I don't hear any noise from them either which was one of my concerns). Anyway I know the pressure is now higher on the rear window and trunk (and my Cd is lower) with the vortex generators installed. No I did not do A-B-A. To me the proof of the pudding is in the eating... I only care about my mpg at the pump which is now darn close to 50 depending on how much city driving I have to do.