Swirling vortices from vortex generators?

[Hermie]

I ordered a set of the delta-shaped "Vortekz" generators thinking that it would smooth airflow on my back end, but after some tuft testing, it showed something I wasn't expecting at all.

I don't have a camera for pictures, but airflow was attached down the middle of the window all the way to the base, where the airflow apparently strikes the trunk surface fairly hard and causes it to move towards the sides of the trunk. Behind that, airflow looks smooth rearward.

On the sides of the window, I noticed rotational movement of the tufts giving the impression of airplane-style swirls off of the tail end. From standing behind the car and looking at the passenger's side of the rear window, the air rotates in a counter-clockwise half-circle on the glass, and it looks like it'll make a full swirl off of the car.

So, has anyone else observed this, or know if that's a detriment to drag?

[MetroMPG]

A pair of counter-rotating vortices are present at the rear of many vehicles. They were probably there before you added the vortex generators.

And yes, they're a "detriment to drag".

The way to avoid them is through shape optimization. Ideally you'd have a Kamm shape to your car (ie. extend & taper the greenhouse to the end of the vehicle) to get rid of them.

[Hermie]

Actually, they weren't there before I added the vortex generators. Before I added them, tufts were striaght along the sides and "bubbled" in the middle of the glass to the middle of the trunklid where the flow detached.

I'm wondering if I may have put the VG's too far forward.

[MetroMPG]

The VG's may have strengthened the vortices, but I'm betting they were more than likely there before.

[vtec-e]

Did they help your mpg?

ollie

[Hermie]

Ollie: Not sure yet. I haven't had them on for long. I also got an alignment and new tie rod last week, so I won't be able to tell the difference in the MPG increase.

I also have a higher-flow muffler that I'll be installing this weekend.

[Hermie]

According to a French study (Peugeot/Citroen), the large counter-rotating vortices I'm getting are really reducing rear lift while also slightly reducing drag.

Their tests show that vortex generators are MUCH more effective at reducing lift than drag, and the large counter-rotating vortices I'm seeing on my rear window means that the VG's are doing their job quite well:

"It appears that the largest drag and lift reduction is clearly associated to a strong increase of the size of the recirculation bubble over the rear slant."

Here's the paper:
SpringerLink Home - Main

This is exactly what I want- Reducing lift, with drag reduction as an added bonus.