Vortex Generators - hopefully a rational discussion.
 

[If this devolves into crazy-talk, kick it to the corral, but we can at least start out trying to improve understanding of this topic.]

VGs can be discussed rationally, but claiming they'll do anything other than energize the boundary layer, and that the energized boundary layer will do what energized boundary layers do, gets off into "guess & hope" territory pretty quick.

VGs are thoroughly studied and well-understood devices, but that understanding is, while fairly solid, not particularly widespread. Their direct aerodynamic function is usually at least one order removed from the reason for their employment.

For instance: a row of vortex generators on the vertical stabilizer of an airliner reduces drag overall. However, the way in which it reduces drag is not direct. The VGs energize the boundary layer behind them on the vertical stabilizer. This allows the airflow to remain attached across the junction between the vertical stabilizer and the rudder (the movable part of the vertical fin) making the entire airfoil more effective (capable of producing more [horizontal] lift) at a given airspeed. Since the vertical fin assembly is more effective, it can be smaller, reducing friction drag and weight. It also means that it takes less deflection (angle of attack of the entire vertical fin assembly) to produce the same amount of horizontal lift, which reduces induced drag (by reducing the rearward lift vector). (The size/weight/friction reduction and reduction of rearward lift vector are factors that have to be balanced against each other and optimized.) Clear as mud?

Now, the reason all this matters to you as a frequent flyer is that rudder effectiveness is most important in a situation where one engine (on a typical two-engine airliner) is not producing thrust (broken, on fire, or has fallen off, for instance).

A more effective rudder allows more differential power to be applied without making the airplane uncontrollable (It's a factor in Vmc, for you pilot types). Since one engine is making no power, an increase in differential means an increase in total power. More available power lets you carry more weight in this situation, so the VGs on the tail allow the airplane to carry more weight in a given situation, which allows the operator to sell more tickets & make more money. (Operators are limited to carrying the amount of payload that would still allow the flight to be completed even if the most important engine fails at the worst possible time.)

So in this case the VGs reduce drag and allow the airplane to carry more weight. It's an indirect connection, though, and if the direct increase in parasitic drag of the VGs themselves isn't overcome by the other factors, they won't make sense, and won't be used.

Briefly mentioned here.

Applying this to vehicles, we have some different goals. Essentially, we're usually looking for direct drag reduction, not increased lift (or downforce). So the VGs will tend to (through energizing the boundary layer) help the airflow remain attached. If attached airflow will allow a reduction in drag, greater than the increase in drag of the VGs themselves, there will be a net decrease in drag. However, it is not at all clear that the reduction in form/pressure drag is likely to be greater than the increase in parasitic drag.

In situations where the airflow separates (relatively far forward), AND doesn't reattach, AND this is a major factor in increasing the (form/pressure) drag of the object, a row of VGs place forward of the separation plane MAY energize the boundary layer enough to allow the airflow to remain attached and reduce overall drag.

For experimenting, a rectangle of aluminum, bent in half crosswise and taped to the surface at an angle to the airflow will suffice to play with VG effects. There's no need to buy some specially-designed VG to see what effect energizing the boundary layer will have.

Nomex on, awaiting correction, happy to learn.

Sweet! I was wondering how to get that smiley on this site!

It may take a while, but I am curious to see if I can make a rear spoiler similar to Ernie Rogers' Beetle more effective with those VG devices. I have to build the spoiler first, but if I can keep the airflow more attached down the back glass, I think it may reduce the drag of my UFO.

Blowing the Vortex, Part 1
Vortex generators: just a fad or are they effective on road cars?
Blowing-the-Vortex-Part-4


http://us1.webpublications.com.au/st...0/3058_4lo.jpg


Reference: Masaru Koike,Tsunehisa Nagayoshi & Naoki Hamamoto; Research On Aerodynamic Drag Reduction By Vortex Generators, Mitsubishi Motors Technical Review, 2004, No 16.

I'll be doing some ABA testing with delta VGs on my Aspire when the weather clears up. Tuft testing shows good straight flow on the rear window, but I haven't noticed a reduction in drag.

The simple version of the VG discussion is that they're not very effective for reducing drag. They do lower the Cd by a small ammount (6 points on an Evo), but other mods such as a Kammback will get you far greater results. VGs do have their applications, but there are simply better choices in terms of drag reduction.

Some here may prefer the ultimate drag reduction of a boat/kamm, but for those of us who do not wish that labor (or appearance), more subtle techniques have appeal. It would be nice to validate more than one type of drag reduction technique.

That's essentially what a VG does - whatever it's being put on : bending the airflow and keeping airflow attached, where it would otherwise separate.

Looking at what gliders use - zig-zag tape and turbulator tape - all VGs I've seen here are way too big, too tall, and as a result, too draggy despite there being too few of them.

Though he didn't see a reduction in fuel consumption, when MetroMPG tested a few of the big AirTab VGs, they did smooth the airflow over the rear window.

You must understand that any chance of improvement will have to come with very vehicle specific VG tabs and installation instructions (and data to document the testing of the tabs on a previous particular vehicle configuration). And even then it sounds like the gains of VG on a car are on the slim side (i.e. .006 CD with a whole lot of tuning I'm sure). I do not find such a large effort for a small (if any) gain very appealing myself :) Maybe I'm wrong.

lots of raindrops make a lake. that's how i look at it, if there is a positive effect i'll likely try it (depending on price obviously)