Vortex Generators - hopefully a rational discussion.

[fjasper]

Originally Posted by kstruve

I have a WRX and the back window never has air flowing over it and it's annoying. I don't have any "wind" to keep it clear when it gets wet or snowy.

I did this to keep the back window of my Audi clear, works great. Just make sure you put the VGs far enough forward that the vortices have enough space to attach to the surface so they'll make the bend from the roof to the window. About a foot forward of the rear window worked good for me.

[Pendragon]

You could also try some airflow modifiers/deflectors on the sides to try to channel some air over the rear window & trunk to see if it helps. You would need to be careful not to make them so large that they added a lot of drag.

Maybe something that looks a little like the vertical slats at the front of the side pods of some F1 cars. The loads on it would be fairly substantial. Perhaps it would be quicker to add a few VGs to the rear pillar (on the sides) to see if they modified the airflow in any useful way.

[kstruve]

Thanks fjasper and Pendragon for your input! I'm sure it will take some R&D to get them working just right. The VGs from vortekz.com are only $20, so that's not a huge outlay of money.

[dcb]

yah magnets are not very expensive either

[kstruve]

Originally Posted by dcb

yah magnets are not very expensive either

I'm not expecting any kind of reduction in drag or corresponding increase in MPG due to VGs. But from what I understand, I can very well expect better airflow over my rear window, which is what I'm looking for.

[MrInstrument]

Good example of VG use. However, for the ecomodder, one thing is missed. VG's impart energy, useable energy, that we can use to our advantage. In consideration of the relatively slow speeds we drive compared to the speeds of aircraft, VG placement and design will need to be somewhat different. I.e., a glider can effectively use the Z strips because the wings are designed around laminar flow, a boundary layer so close to the wing that those 1/16 or less bumps are beneficial. On a car however, Z strips are all but useless because cars are not designed with laminar flow in mind but flow more so related to the old cumbersome wings you see on 30 plus year old airplanes. Back to imparting energy. Knowing most all cars and trucks have flow separation at the rear, beyond that of the boot, tailgate, trunk, or hatch, we can now take this energy imparting device and strategically place it to "create" the boat tail the adds to aerodynamic efficiency. Look at the post below with the car and arrows representing drag, by placing the VG's at the very rear, tuff test to find where flow starts loosing energy, of vehicle. By placing the VG's here, we lengthen the effective (aerodynamically perceived) rear of the car thereby decreasing considerably the deep low pressure area created. So, when you think of VG's, don't think of the minuscule effects of air over roof and trying to straighten them but rather think of VG's as you would a permanent mod on the rear or anywhere you have the low pressure areas created. In contrast, VG's are not hard mods but merely imparting controlled energy into the air. Therefore if we are generating energy from the air, some of that generation is taking from the force to move the vehicle and therefore hard mods will always out perform VG's, but for quick, inexpensive flow control, with $$$ benefits, VG's are the way to go.

[euromodder]

Originally Posted by MrInstrument

On a car however, Z strips are all but useless because cars are not designed with laminar flow in mind but flow more so related to the old cumbersome wings you see on 30 plus year old airplanes.

Even 1mm or 1/24" high dimple tape works on the front of a car.
(mirrors, top of the windscreen, A-pillar)

Empirical testing by myself showed they reduced wind noise and altered the wind / rain pattern on the side windows.
It's likely not enough to improve fuel consumption beyond the daily variation, but they DO have an effect.

No effect observed @ rear of vehicle.

we can now take this energy imparting device and strategically place it to "create" the boat tail the adds to aerodynamic efficiency.

That's the holy grail of VG placement.
Can you prove it ?

In contrast, VG's are not hard mods but merely imparting controlled energy into the air.

They're easy mods by themselves, but hard to make efficient ... hard to control.

Therefore if we are generating energy from the air,

You're not creating any energy with VGs.
They are COSTING energy.

The only way to make 'm work is applying them in such a way that the vortices result in an overall improvement, despite the drag they or the induced vortices cause themselves.

That'd require a lot of trial and error experimentation for most, or lots of wind tunnel research by the happy few with access to one.

As a result, adding VGs won't be a

quick, inexpensive flow control, with $$$ benefits, VG's are the way to go.

Why do I expect you to pop up and start advertising some miraculous VGs

[ecomodded]

If the Vg's are improving attached flow they would have better effect being used near the front of the vehicle where they have some distance and time to be attaching the airstream in.

example: above the windshield not above the rear glass.

or On the side of the car ahead of the front doors.

[freebeard]

Wow, zombie thread shambles forth.

The rear aspect of the front and rear fenders of a VW Beetle curve inward too fast. Maybe a vertical row of VG abaft the wheelwell would work there.

[MrInstrument]

Your comments of dimple tape would be expected for sorts when applied to the front of a vehicle where you have higher velocities and forced flow characteristics. But as I stated, they have very minimal effects of "controlling" airflows to provide an aerodynamic advantage. You cited an airflow advantage but I can say without hesitation that without plastering the whole front of the vehicle with them your aerodynamic gain will be next to nothing because they where designed for laminar flow and the flow outside the boundary layer, a glider wing is perfect example. Glider wings depend on the boundary layer to achieve the tremendous lift that their aspect ratio provides and the Z tape provides the insurance for the critical lift areas they need.