[Snowman811]

To add an annecdote, the leading edge of an Owl's wing has little bone barbs that protrude ever so slightly. THis has the same affect as we've learned above. The result is that small mice and rodents, who's hearing is phenominal, cannot detect an Owl swooping in for their prey dinner. The resulting turbulent air flow removes the singing resonant 'hissss' as the Owl flys down over its prey and grabs they cute little furry, soon to be bloody dinner, in its talons.

You know, in case your daughter wants to have an exmaple of this affect in nature.

[e90fanatic]

Originally Posted by ksfrogman

When I look at the satellite antenna, the angel eyes, and the toothed wind deflector, they remind me of shark parts.

what shark did u see that had angel eyes????

[pander5]

e90wraith and others who would have corrected my inaccurate description - Thanks! I asked the same question of a pilot friend when looking at my wife's car parked in the driveway and looking at the little "toothed thingy" on her sunroof while open. He went on to describe to me what I then posted above. Sorry for the misinformation gang and again, thanks for the clarification. (Oh, and by the way, I like this guy so I won't tell you what airline he flies for!!!)

[human]

Respect to you e90wraith. Smart dude.

[poldim]

Originally Posted by e90wraith

Perhaps I should clear up some misconceptions. First, the black toothed thingies are not creating (or more accurately, maintaining) "laminar airflow" (smooth airflow) over the car. In fact, they do the opposite - they mix the air by introducing more turbulence (opposite of laminar flow - chaotic flow). The main sound you hear is not the "air flowing over the sunroof" but the acoustic excitation of the air in the cabin, i.e., the Helmholtz effect. The same effect you hear when you blow over a Coke bottle. The goal of the black toothed thingies is to mix the air as quickly as possible to minimize the excitation by the shear layer (the interface between the slow air in the cabin and the fast air flowing over the car). The quick mixing is accomplished by making the air more "turbulent," not laminar. This also has an added benefit that less air enters the cabin to mess up your hair. Finally, the black toothed thingies create more drag, not less.

There was a discussion of this in the context of "helicopter" noise when a window is open:

http://www.e90post.com/forums/showth...t=15922&page=1

Second, the winglets have nothing to do with "laminar flow." Their purpose is to reduce induced drag by: 1) increasing the aspect ratio of the wing. 2) modify the shape of the trailing edge vortex in the near field (less than 10 wingspans behind the wing). Perhaps you are confusing them with "riblets," which are designed to maintain laminar flow over wings and aircraft bodies: They are basically slits made on wing or body surfaces which are parallel to the direction of flow. These help maintain laminar flow over wings and airplane bodies. The riblets were inspired by shark skins which have microscopic grooves - they help sharks maintain laminar flow over their bodies - hence reducing drag.

You're making it seem as though the Fluid and System Dynamics classes I've taken weren't a complete waste of my life...

[e90wraith]

Originally Posted by Josh49

Haha, I was about to get on this as well and explain how the purpose is to create turbulence, and break up laminar flow.. Beat me to it as well.

Another effect of the teeth that I could think of is by introducing turbulence into the laminar flow above the roof, a result is the alternating Von Karman vortices behind the e90 are weaker. Since these low pressure zones are not as "low", the difference between the high pressure in the front of the car and rear of the car has now decreased, aka the drag force has decreased. This is the same reason/effect that a golf ball with dimples will fly further than a golf ball without dimples. The turbulence is much better, as it doesn't adhere to the smooth surface like laminar flow..

Whether or not this is enough to lower the drag force of the full e90, not sure, it could also add drag as it increases the frontal area of the car slightly.

If people are really interested in this effect I could dig up some wind tunnel photos from school sometime.. its much easier to see in the wind tunnel with smoke lines..

They might move the "separation line" a little farther back - although I think the sunroof deflectors are too far forward to serve this purpose. I suppose you can easily conduct an experiment by putting some water droplets on the rear window. Since I see very little movement of water droplets at speed during rain, I suspect the separation line of a "clean" E90 is either right at the top edge of the rear window, i.e., E90 has huge base drag. If you see the water droplets move down the rear window with the sunroof deflectors up, then they are able to move the separation line farther back - and should reduce drag somewhat.

P.S. There is recent research suggesting that the old belief of how golf ball dimples reduce drag might be incorrect. It is widely believed that they delay separation by making the laminar boundary layerf transition into turbulent flow as you mentioned. However, new evidence suggest a slightly different mechanism: inside the dimples, little unsteady eddies which squirt high momentum flow (like little jets) re-energize the boundary layer. Even today, there's still a debate on how golf ball dimples really work.

P.P.S. I am an escapee of Caltech and JPL...

[pander5]

There was an interesting post either here or somewhere else that suggested the ribbed rear tail lights on slightly older Mercedes were functional for helping to reduce dirt buildup on the rear lenses because they produce little "eddies" of turbulence when the car is in motion that helped to discourage the dirt in the air from settling on the lenses. Somewhat like the golf ball dimple affect?

[poldim]

Originally Posted by pander5

There was an interesting post either here or somewhere else that suggested the ribbed rear tail lights on slightly older Mercedes were functional for helping to reduce dirt buildup on the rear lenses because they produce little "eddies" of turbulence when the car is in motion that helped to discourage the dirt in the air from settling on the lenses. Somewhat like the golf ball dimple affect?

I wish there was something like that for the rear bumper lip, right below the trunk lid. That ledge just loves to collect dirt and dust.

[Broken1]

Thanks to this thread, I decided to try it out yesterday. I was driving on the highway at 120km/h, about 65mph and my sunroof was open. I put my hand on the black toothedged thing and pulled it down, and there was a huge helicopter effect buffeting noise. I let it back up again, and it dissapeared.

Obviously, with it up - there was still wind noise...but I just turned the music up :-)

[e90rocks]

Thanks everyone for posting. I did the same thing (pulling down the toothed sunroof leader) demonstrating by its absence the increase in cabin noise to my daughter. I will tell her about the owl once I get a picture.

[stressdoc]

My brothers and I tried using polyOx in various patterns on our old sailboat bottom in a failed attempt to optimize friction layers. But like sunroofs, we could get different sounds while on a plane from the different combinations we used.

[Josh49]

Originally Posted by e90rocks

Thanks everyone for posting. I did the same thing (pulling down the toothed sunroof leader) demonstrating by its absence the increase in cabin noise to my daughter. I will tell her about the owl once I get a picture.

I tried this as well. Its much quieter without it, however much more uncomfortable on your ears from this effect. With it up, there is a lot more "white noise" but its comfortable..

Have been thinking about this topic some (what I talked about above). It would be interesting to throw a model of the e90 into the wind tunnel and install a strip (or strips) across the roof (and vary the location) and see if it is possible to significantly reduce the drag force by boundary layer separation across the top of the e90 (explanation of how this works earlier in thread). Maybe I could buy a model sometime, or obtain the 3d model that one e90post.com member designed and make SLA models (rapid prototype) with strips designed into it.. I've been really curious about this, especially with the price of gas lately.

[visor]

Originally Posted by Josh49

Have been thinking about this topic some (what I talked about above). It would be interesting to throw a model of the e90 into the wind tunnel and install a strip (or strips) across the roof (and vary the location) and see if it is possible to significantly reduce the drag force by boundary layer separation across the top of the e90 (explanation of how this works earlier in thread).

From what I understand from reading forums and car mags, the modern car is almost fully optimized (without looking like a tear drop) in terms of aerodynamics of the top, front, sides and back.

The only improvement left is the bottom of the car, which BMW and other car manufacturers have addressed over the years.

[Josh49]

Originally Posted by visor

From what I understand from reading forums and car mags, the modern car is almost fully optimized (without looking like a tear drop) in terms of aerodynamics of the top, front, sides and back.

The only improvement left is the bottom of the car, which BMW and other car manufacturers have addressed over the years.

I highly doubt this, except for high performance cars. Most modern car companies places a very small emphasis on aerodynamics. I'd say in the United States, there are zero aerodynamic/fluid engineers. If there were, you wouldn't see the latest trend in cars like the chrysler 300sc, chevy hhr, every suv, jeep, hummer.. American cars are getting bigger and boxier every day. They are going backwards from what aerodynamically makes sense. And I disagree even for some german/japenese cars as well. The new Audi has a huge front vertical surface on the front of the car. There is an enormous high pressure zone in front of this, its running a vertical flat wall into the wind. It seems aerodynamics in a lot of cars comes second to aesthetics (and the sad part is the front of the new A4 is fugly). The element, hj cruiser, scion xB, etc, are definitely not optimized at all for aerodynamics!!

I'm going to pull up some research from school one of these days that I did, and try applying it some models of the e90 in the wind tunnel.

Again, it'd come down to aesthetics. I'm sure even if this worked, its application wouldn't look too good on the cars (having strips across the roof), and it wouldn't be designed into the cars. I really do think aesthetics are a huge compared to aerodynamics in modern cars (until you hit high performance).

If when you said modern cars you meant cars starting at the e90, g35, etc level and up towards ferrari and lamborghini in terms of performance, I apoligize for the rant..

[poldim]

Did this yesterday. The rear of the car sounds like its got an Apache in it.

[e90wraith]

Originally Posted by visor

From what I understand from reading forums and car mags, the modern car is almost fully optimized (without looking like a tear drop) in terms of aerodynamics of the top, front, sides and back.

The only improvement left is the bottom of the car, which BMW and other car manufacturers have addressed over the years.

I would have to agree with Josh49 as well. As my wife went to Art Center (the "premier" automotive design school in the US - Chris Bangle is an alumus - hmm, maybe it's no longer a premier school... ), I know that car designers (more accurately, car stylists) have only rudimentary knowledge about aerodynamics. For cars, style is everything. For the external body, the fluid/aero engineers are left to only optimize aerodynamic performance by making minor tweaks such as side mirrors, door handles, wipers. Since they can't touch the external body much, they can make much more difference in aero performance by playing with the airflow through the underbody and the engine. These are the areas where most of the improvements have been in recent years. As I mentioned in another thread - this is why the old Lexus LS400 was such a standout in aerodynamic performance even though it was shaped like a brick.


For the external body, the largest drag component is the base drag - when the air separates at the rear of the car because the air cannot follow the contours of the car and creates negative pressure (or vacuum). The solution would be boat tailing (teardrop shape), but no one wants a Citroen (at least in the US). Josh49's quest to increase the turbulence in the boundary layer to delay separation is another method. I would say it will work - although it might not be pretty. Given how much dust settles on the rear window, trunk, and bumper of the e90, it probably was not optimized for aero performance very much in the rear.

[ward]

wraith, thanks for the info in here

really nice to see some info around that brings back memories from school!

keep up the good work!

[Fletch]

Here's a dumb question: My sunroof makes a lot of noise on the highway when open and the "toothed thing" (OK, so I'm no physicist...) doesn't seem to help. Has anyone had better luck with the sunroof visors you can buy from the BMW dealer or online, and any recommendations on brand?

[Ghunger]

Nice to know I'm not the only "enginerd" that drives a bimmer.

Originally Posted by Josh49

I tried this as well. Its much quieter without it, however much more uncomfortable on your ears from this effect. With it up, there is a lot more "white noise" but its comfortable..

Have been thinking about this topic some (what I talked about above). It would be interesting to throw a model of the e90 into the wind tunnel and install a strip (or strips) across the roof (and vary the location) and see if it is possible to significantly reduce the drag force by boundary layer separation across the top of the e90 (explanation of how this works earlier in thread). Maybe I could buy a model sometime, or obtain the 3d model that one e90post.com member designed and make SLA models (rapid prototype) with strips designed into it.. I've been really curious about this, especially with the price of gas lately.

Were you thinking of something like on the new Evo's?

[Josh49]

Well this isn't what I am thinking at all, but to sum up this design:


From their website:

Originally Posted by Evo website

The ultimate Lancer Evolution, the MR is redefining sport compact performance expectations around the world. It comes standard with the kind of technology that pushes the absolute limits of street and track performance. Take everything available on the Lancer Evolution IX and add one-piece forged aluminum BBS® wheels to save a little more rotational weight. Add a close-ratio six-speed gearbox to keep you in the fattest part of the powerband. Let the revolutionary Vortex Generator funnel air to the rear wing and then give the wing a wickerbill extension to increase the level of downforce it generates. Throw on some inverted Bilstein® shocks to push lateral-acceleration (g-force) numbers to the absolute limits ever seen in a street car. Then add an aluminum roof to reduce center of gravity for even better cornering performance, and you've got an MR that can go head to head with just about anything in production. And win."

Its all about being in the phattest part of the powerband with the vortex generator funnel, and wickerbill extension, yo... :rocks:

So not exactly (or anywhere close to) what I had in mind.. What I am thinking about (boundary layer separation) is installed perpendicular to the direction the car is traveling in (across the roof from driver side to passenger side)..

Who you calling a nerd?

[mmeads]

Originally Posted by poldim

You're making it seem as though the Fluid and System Dynamics classes I've taken weren't a complete waste of my life...

Don't kid yourself they were. :P

In college I actually had a class titled "Gas Dynamics." I loved telling people about my Gas Dynamics exams, of course.... i am an engineer, and find nerdy things hilarious.

[Ghunger]

Originally Posted by Josh49

Well this isn't what I am thinking at all, but to sum up this design:


From their website:

Its all about being in the phattest part of the powerband with the vortex generator funnel, and wickerbill extension, yo... :rocks:

So not exactly (or anywhere close to) what I had in mind.. What I am thinking about (boundary layer separation) is installed perpendicular to the direction the car is traveling in (across the roof from driver side to passenger side)..

Who you calling a nerd?

Well, ya, I realize it's ugly, that wasn't the point. So you're thinking about something more like a roof spoiler? Like the ACS one for example? Or were you thinking of something farther forward?

Oh, and ya, I was callin' you a nerd!!