Forced flow separation on tail light. Why?

[CfdAero]

Hello everyone, in the last years I have seen more and more cars, with the tail lights with the final edge that sticks out a little bit (you call it a 'nolder' if you want). If it's not on the tail light itself is integrated on the body of the car. Anyway I think the trend is to create a precise line of separation at the rear end. But I was wondering, why does having a flow separator help the aerodynamics (or simply drag reduction)? Why doesn't allow the flow to maybe create a less wide wake by separating further down?
thanks

[McMrocks]

Just leaving this: http://en.wikipedia.org/wiki/Kammback

basically saying the same as Shooty81

somewhere in the deeps of the world wide web is a video of a Mercedes in a windtunnel demonstrating that effect. You can see the airflow keep its direction "without any" turbulences behind that edge

[CBeck113]

Avoiding the 30° angle is very important (it actually starts at about 25°, but that's not very relevant for the explanation) - the edge at the 30° angle creates a strong vortex, which is very bad for aerodynamics on street cars. The goal is to avoid furhter reduction of the pressure behind the car.
Here is an image (if this works) of a vortex generator, check the angle:



In case it doesn't, here's the link:
http://www.autospeed.com/cms/article.html?&A=3059

[Just_a_fan]

I have a feeling that these devices are used to reduce buffeting caused by Karman vortex shedding typical at the rear of bluff bodies.

[DaveKillens]

Dirt.

Any dirt particles carried by the air are subjected to this disturbed air and instead of flowing around the side and coming into contact with the rear tail lights, and possibly cover the rear tail lights, will most likely never have a chance of attaching.

Safety, it is desirable to keep the rear tail lights, and especially the brake lights clean and visible.

[shelly]

I am not sure about the effect of dirt.

The main aim is to achieve lower drag.

If you leave the flow go around the radius/ corner, it will accelerate lowering its pressure and then detach.

The base pressure on the back face of the car depends on the pressure at the separation line. So if the separation line has lower pressure, all the surface facing the separated wake flow will see a lower pressure, and since this is the backward facing surface.
If you make the flow separate more upstream, and you create a local higher pressure with a convex shape, you get an higher pressure on the backward facing surface of the car and so lower drag.

Reanult recently launche an interesting prototype called Eolab which has two edges actively coming out on the sides of the rear bumper to achieve lower drag.

Kamm and so on is a similar but slighltly different topic

[DaveKillens]

But shelly, can you not agree that for a road car the requirement to keep the rear tail lights and brake lights free and clear of dirt is of prime importance from a safety perspective?

[shelly]

Yes I agree that it is important and I know that some aerodynamic studies involve the effect on aero on mud and dirt on the rear winow for exampl (I remember an articel by Volvo some years ago).
It could be that there is an effect on dirt also - that's why I said I am not sure.
I remember well about the effect on drag

[DaveKillens]

Thank you very much, it may supplement aero benefits but IMO keeping the rear lights clear is much more important than eking out a miniscule gain in fuel mileage.

[CfdAero]

Thanks everyone for the replies. They are all very interesting. I personally think that the main use of such devices is to lower drag, i.e. aerodynamics, the rest is secondary. I also personally think that the answer given by shelly is the most correct one, it's not the first time I hear that the base pressure depends on the one at the separation point, but could you please provide me with some scientific articles explaining the concept? thanks

[MadMatt]

I am not sure about the effect of dirt.

The main aim is to achieve lower drag.

If you leave the flow go around the radius/ corner, it will accelerate lowering its pressure and then detach.

The base pressure on the back face of the car depends on the pressure at the separation line. So if the separation line has lower pressure, all the surface facing the separated wake flow will see a lower pressure, and since this is the backward facing surface.
If you make the flow separate more upstream, and you create a local higher pressure with a convex shape, you get an higher pressure on the backward facing surface of the car and so lower drag.

Reanult recently launche an interesting prototype called Eolab which has two edges actively coming out on the sides of the rear bumper to achieve lower drag.

Kamm and so on is a similar but slighltly different topic

Thanks, I never heard of that prototype, looks good, active rims as well, that's a good idea:

[Just_a_fan]

Dirt.

Any dirt particles carried by the air are subjected to this disturbed air and instead of flowing around the side and coming into contact with the rear tail lights, and possibly cover the rear tail lights, will most likely never have a chance of attaching.

Safety, it is desirable to keep the rear tail lights, and especially the brake lights clean and visible.

This is an interesting idea however on my vehicle, which has these spoiler features moulded in to the lights, the lights get as dirty as the rest of the back. I.e. they don't appear to reduce dirt on the lights.

[g-force_addict]

AFAIK the only function of these protuding lights is to make them visible from wider angles.
And yes, they seem to get as dirty as regular taillights.