Active Flow Control

[trebuchet03]

So in the past I've discounted active flow control as something just too complicated to implement into an already existing car....

But with my kamm project, I've got some renewed interests....

Things like VG's, zigzag shapes (also a VG), lips, etc. are passive methods of controlling flow.

Active flow control is basically perturbing flow with your own controlled flow... You can do this with vacuum (sucking), pressure (blowing) or mechanically with things like fluttering spoilers and such.

On aircraft wings, that looks something like this




But on a car.... It's a little different



So the C-Class merc takes in fast moving air under the car, and ejects it out of slits in the tail light lenses. The C-Class has a cD of .27 - probably not all tail lights... but they did spend the R&D time and money and then more time and money to make it... One would be led to believe that they decided gains were to be had....



The idea is redirect flow behind and away from the car, rather than wrapping around and doing nasty things against the car.

Here's a better photo showing the little slits on the outside edges



Just thought I'd share.... As far as how I personally would implement active flow control... Not sure yet, but I'm experimenting

[Tango Charlie]

Do you suppose they did this purely for drag reduction, or are they trying to eliminate build up of snow and grime on the tail light lenses? Or both?

[trebuchet03]

I imagine moreso for aero rather than built up gunk - that surface is rather vertical as is.... If the car is parked overnight, it won't prevent any snow build up, which I imagine is the more common snow build up scenario...

If it does prevent snow and grime, I imagine that's a bonus side effect.

[dichotomous]

Originally Posted by trebuchet03

I imagine moreso for aero rather than built up gunk - that surface is rather vertical as is.... If the car is parked overnight, it won't prevent any snow build up, which I imagine is the more common snow build up scenario...

If it does prevent snow and grime, I imagine that's a bonus side effect.

I'd actually think this would keep a ton of grime off the tail lights, mine get nasty from plenty of negative pressure pulling road grime up, this would keep all of that away

[DieselHybrid]

"the C-Class merc takes in fast moving air under the car, and ejects it out of slits in the tail light lenses."

Can you provide a link to this information, because I find it highly unlikely.

First, this, in and of itself, would not reduce the drag coefficient to 0.27. There are plenty of vehicles with Cds in this range and lower that do not employ ducting. Second, stop and consider the losses incurred (not to mention added weight, complexity) of ducting high speed air from the bottom of the car to exhaust vents in the taillights. The added ducting (if it exists) would actually increase vehicle drag.

Perhaps Daimler employed this to reduce the pressure in the rear wheel wells. It seems more plausible- as there is a build-up of pressure in the wheel wells and the shorter, linear distance between the two.

However, a simpler, much more effective (and elegant) solution would be to partially cover the rear wheels.

[dichotomous]

anyways not on the mercedes but active flow control, this has been something of a thought for me as well since I have a pretty aero car with an adjustable rear wing that could either make the car very stable with a lot of drag and downforce, or very slippery aerodynamically because it hangs off the top back of my hatchback. it curves down then back up. but what if the trailing edge height was controlled not by bolts (as is current and limited) but by an electric servo. when I feel the need for speed I could just crank it up via a button and lock the rear down (its very effective).

another thought is to have the front edge variable and on a spring, with a cover between the leading edge and the start of the wing mount (the front dips down to get more downforce) and installing a sliding sheet between these. so that before a certain speed the whole wing would be even with the roof and a good aero extension, but when going faster the pressure there would push (or pull from the vaccume behind) the leading edge down for more downforce (and drag, and stability). no electronics, no motors, very little weight, have my cake (good downforce giving wing that looks cool) and eat it too (have an very aero car). cause, if I am going to be driving over 70mph, I will NOT care about fuel economy (or I wouldnt drive that fast) I WILL care below that, but I will care about a functional wing.

I like this idea, and I think I will be trying it out

[trebuchet03]

Originally Posted by DieselHybrid

"the C-Class merc takes in fast moving air under the car, and ejects it out of slits in the tail light lenses."

Can you provide a link to this information, because I find it highly unlikely.

Sure
New C-Class: Ventilated tail lights - worldcarfans

Here's another source
eMercedesBenz - The Unofficial Mercedes-Benz Weblog

OFFICIAL PRESS RELEASE


New C-Class: Ventilated tail lights for better aerodynamics

Stuttgart, Feb 07, 2007
Two Mercedes patents that bring significant aerodynamic advantages are being used in the new C-Class for the very first time. Mercedes engineers have invented innovative "ventilated tail lights" which replace conventional spoiler lips....


The first technology, ventilated tail lights, have been implemented to replace conventional spoiler lips and help direct air flow along the side of the vehicle. The lights, which are hermetically sealed against the vehicle, function by taking air sucked from the underbody and forcing it through ventilating slits. Without them, the slipstream would be conducted behind the rear of the vehicle at the tail lights, resulting in unfavorable turbulences which negatively impact air resistance, rear axle lift and yaw characteristics.

I can't post where I originally found it because of licensing... But I did find it originally in a tech journal

Originally Posted by DieselHybrid

First, this, in and of itself, would not reduce the drag coefficient to 0.27.

Seriously, did you not read the original post?

The C-Class has a cD of .27 - probably not all tail lights... but they did spend the R&D time and money and then more time and money to make it... One would be led to believe that they decided gains were to be had....

Originally Posted by DieselHybrid

The added ducting (if it exists) would actually increase vehicle drag.

Thus far, my cfd results show otherwise (depending on location, of course).... weather or not they are right is another story Can you provide some links/evidence for that claim?

[dichotomous]

I wonder if bigger tail light vents could give better results. I hate my tail lights so cutting some holes in them wouldnt bother me a bit, and running some dryer hose would be pretty easy. I wonder if would help more on a hatchback with a typical vaccume back there... high pressure under car air would naturally move to low pressure rear

man, now I wanna do this, and the wing idea, and rear wheel arch covers, and an aero front bumper. darn it trebby, you've planted another modding bug in my ear!

[trebuchet03]

^^ I'd try and find more details on how theirs works before doing any cutting It's all about the details at this point....

[dichotomous]

Originally Posted by trebuchet03

^^ I'd try and find more details on how theirs works before doing any cutting It's all about the details at this point....

heh heh, you havent seen how ugly my tail lights are...


now you have