sawtooth trailing edge
 

I'm inclined to think a spoiler with a sawtooth trailing edge might have advantages over a straight one.

here's how i arrived at the idea.

first of all vortex generators have the ability to reduce drag, while their effectiveness is debatable in some applications their fuctioning si quite well understood.
now when i compared vortex a vortex generator with a naca intake, the first thing you notice is they're very similar in shape, in fact a naca intake also uses vortexes to draw air into it's orifice, so basically a naca intake is an embedded vortex generator that draws air from outside in.

so what if we where to use a series of naca ducts in a lip spoiler. likely the naca intakes would draw air from the top of the lip to the bottom of it!
all of this would be a low drag affair, and no additional frontal area is generated.

one could simplify this setup by cutting of the rear of the naca duct leaving a trailing edges with a curvy sawtooth pattern, wich might even be further simplified to just real sawtooth, as are used on turbulator tape for example.

the whole idea starts to make even more sense, when you realize humpback whales have fins which rounded sawtooth like protrusions at the end, which in turn have been copied to design low drag blades for huge wind turbines.

so what do people with a better understanding of aerodynamics think? am i drawing the wrong conclusions or is there something there?

Not to push you in the wrong direction, because I think you are onto something here, but these Austrian people design computer case fans with notches in them to keep them quiet and push more air:

Noctua Fans

Also this is (was a while ago anyway) a topic of discussion in this thread:

Whale Tubercles
This may assist in your venture. Good luck and keep us informed. :thumbup:

I think you may be onto something. Fab one up in fiberglass, tuft test it, and post the video here so we can all see! We're easily entertained! ;)

lunarhighway -

This sounds really neat. If you already had a spoiler on your car, then maybe you could extend some serrated coroplast to create the sawtooth shape.

CarloSW2

Trailing?
 

I'm looking at this stuff and thinking those teeth would work on the leading edge.
The way I understand the effects, is you can get a much higher
AOA before stalling the wing (whale fin).

http://www.biomechanics.bio.uci.edu/...04biomech2.jpg

I've been thinking about adding a sawtooth leading edge to one of my
Coroplas model airplanes to test out the AOA idea..

http://i46.photobucket.com/albums/f1...SPA3D/654d.jpg

This type of pizza box wing design really doesn't have real lift, but is fun to
fly around real slow angled up about 45 degrees. (A good engine is a must).
I suspect that adding Whale Tech to the LE will improve the high AOA slow speed hovering ability a lot..

Now, I just have to figure out how to implement the idea.. :confused:

naca
 

I thought that the submerged NACA inlet provided the perfect teardrop taper curvature,both in elevation and plan, to allow the air to decelerate without vorticity,and provide the highest static pressure for ram effect to the heat-exchanger.Turbulence I can see,but I thought vorticity was always a "no-no".----------------------------- With regards to the "tubercules",would the upswept "kick" as GM has just modified into it's VOLT rear spoiler,provide the same effect as the "sawtooth"? If the wake is with turbulence but without vorticity,as a normal spoiler would create,would the sawtooth create a local interference drag adding to overall drag?

Vortex generators increase induced drag, but have the potential to reduce from drag. The net effect can be positive, but lower overall drag would be very hard to achieve by trivial placement as even on airfoils, which are much easier to work with than the back of a car, the effectiveness of vortex generators is highly dependent on chord placement, vg angle of attack and vg density, and you would not position them at the same place wheter your goal is drag reduction or, as in the case of an airfoil, lift.

I don't see what good they could do right before the wake break point.

The image I've attached is that of the engine of a 787 Dreamliner. The guy who worked on the sawtooth design wrote that it reduces noise. This means less energy wasted energizing air to produce that noise. I'd guess that there would be small vortices forming at each point on the trailing edge, but that all the vortices combined would take less energy to maintain than a possible single large one trailing from a smooth trailing edge.

NASA has been studying serrated trailing edges, and serrated leading edges are of interest, too. Google for it. Also, check into Barred Owls, which have serrated feathers for ultra-quiet flight. Works.

Sayyad -

Wow, that 787 engine also looks totally Batman!

CarloSW2

this is what wikipedia has on te naca intake

looking at this example it's hard to overlook the similarities between two vg's and a naca intake.

http://i77.photobucket.com/albums/j5...y/vgvsmaca.jpg

so it's my understanding that they work on the same principle where the vg's reënergise the boundy layer by drawing in air from outside the boundry layer and naca's draw do the same but draw in air below the boundty layer, and the skin.

this suddenly made me think that an upswept lip spoiler ofte found on sporty hatchbacks, but also on car with very low drag quoefficients could be an every simplified vg, epsecially since the once that can be suspected to be there for true aero reasons are often rounded and heigher in the middle

here a picture on my string of thoughts that might make more sence...

http://i77.photobucket.com/albums/j5...lipspoiler.png

but then that last configuration looks like this test autospeed did
on their hoda insight and they recorded no benefit. but of course the insight is already highly optimised and might not need any band aid somutions as it's basic form is already so good... and all the separation angles are very optimised

quiet
 

NASA's Paul Soderman sent me a paper he did from his research at NASA Ames.He had studied owl flight and I believe the premise of the work was acoustic Stealth,and not necessarily drag reduction.---------------------- At full take-off power,does the sawtooth blend the interface between the turbo-fan's jet stream and the slower surrounding nacelle air?

For CarloSW2:

Humm, Leading Edge VGs, sorta like Whale Fin Tech..
http://us1.webpublications.com.au/st...0/3061_7lo.jpg
AutoSpeed link

VGs up on the nose didn't hurt the MPG, but helped the handling..
I looks like they were installed at an angle (hard to tell)..
Does anyone here know what that Insight angle might be?

I have an angled area on my skidplate that might be a good place to try some VGs!
http://i46.photobucket.com/albums/f1.../Skidplate.jpg
--------


I've been thinking about making a Coroplas rear spoiler.. Maybe a ZigZag TE
would be better than trying to make it rounded like a baseball cap bill..?.

Sayyad -

This is soooo cool! science fact meets science fiction! Batman's cape will never look the same :

Attachment 1749

I can't wait to explain this to friends and watch as their eyes glaze over and they get the brain-dead look on their faces, :D !

CarloSW2

lunarhighway -

Wow (again). And it's just been staring us in the face for the last few years.

EDIT: I have always wondered what the "negative volume" between two Airtabs could mean, so this is kind of neat. On a huge semi, the fact that the airtab "comes out" of the trailer may be relatively unimportant in relation to creating the quasi-naca duct.

CarloSW2

Yes. the sawtooth edge improves on the ducted fan, where the fastest inner core is buffered from the ambient air, reducing the shearing effect between layers, which causes the noise. The sawtooth makes for micro-vortices that to some extent cancel each other out, as the upside flow from one micro vortex impinges on the downside flow of the next.

Where could I get a look at the Soderman paper?

The owl, btw, uses his serrated edge feathers more for noise reduction than for most efficient lift. That's why raptors fly faster, though more noisily--they get breakfast by dint of speed and power, vs. being sneaky.

Otto,

Suggest you go to NASA - Advanced Search and and do an Soderman, etc. search.

That reminds me of a rear spoiler on a car I once raced against:


The car was very fast (low drag) and hard to draft. If I remember correctly, the driver/owner claimed that the saw-toothed spoiler made more downforce and less drag than a spoiler that was the height of the bottom of the saw teeth. His business is data acquisition systems, and his car is fully instrumented to measure such things, so I tended to believe him.

Also, when I saw the car, I believe the spoiler was about the same height but I remember the teeth as being a lot more prominent (taller), fwiw.

Paul's paper
 

Otto,if you can't find the paper through GOOGLE,as CobraBall recommended,I'll dig it out and mail you a copy.Might take a little time to find it but I'm pretty sure I can put my hands on it.

Naca/vg
 

Thanks lunarhighway for the data on the NACA duct.I have only seen military versions of them,and the sidewalls diverge at such a small and gentle angle,it's hard to imagine how there would be any separation,however,without any radii at the skin interface,I can see how air could spin into the void.---------------- And yes the similarity between the two devices almost makes one believe they were conceived from the same stock,forming male,and female counterparts simultaneously.------------------------ I still hit a mental glitch,with them used at the very back of a form,without another trailing surface for the flow to re-attach to.----------------- If the wake is turbulent and without major vorticity,my understanding is,that this is about as good as we can hope for,and any additional drag reduction would necessarily come from lengthening the vehicle while reducing the cross-sectional area,to reduce the wake itself.------------------------------------- If the saw-toothing can actually reduce drag,then its a minor coup! Lower drag from shorter vehicles is a very desirable target.Stranger things have probably happened.I'd like very much for it to work!

ok i did a little more reading on wikipedia, as their info is usually fairly easy to digest and not overly complicated.

one interesting thing is this:


this propperty makes them usefull in aviation appli
" "cations, where one wants to delay wing stall by keeping the airflow attachted with vortices where it would otherwise separate.

however www.airtab.com - The smart way to cut the cost of your highway fuel costs manifacturer of the airtabs state in their instalation guidelines:
this indicates that their purpous is not to reattach air to the surface but that a different purous is intended.

if we look back at the naca intake :
than perhaps the VG's like airtabs seen on cars work the same as a naca duct by drawing air into the wake of the car, and thereby increasing the pressure slightly. since this negative pressure behind the car actually pulls it back an increase in pressure would be a very good thing.

further the presence of of the vortices might cause the wake to form in a different way wich is more beneficial.

tuft testing is obviously fairly useless in this case as all the action goes on behind the car... (unless one uses very long strings, but that could lead to hairy situations). pressure measurements behind the vehicle might reveal a difference in pressure tough.

sometimes i really wich i had a windtunnel

It seems that the only explanation for drag reduction would be a higher base pressure.Perhaps the VGs create a "more" homogeneous wake,of smaller vortices,or "micro" vortices,which viscous-attrition can erode quicker,converting kinetic energy into static pressure.Maybe Santa Claus will bring ecomodder.com a BIG windtunnel where we can do easy flow imagery and force measurements!

I think my spoiler/wake boards might be making too much wind noise..
Not sure yet, I need to drive on a highway w/o a million trucks buzzing by me.

http://i46.photobucket.com/albums/f1.../BBlftside.jpg

So, this mod may not last too long.. However, I wonder what (if any) effect a Sawtooth on the TE would have? (Besides making it look Batmobiley)..
I'm thinking about little 1.5" deep 'V's...

If it was making a noticeable noise, and the Sawtooth got rid of it, that may be interesting..

I've also just added the mini side-skirts, so those might actually be the source of the noise.. I can quickly find out, since the removing the skirts takes 4 bolts.. :cool:

Hi Rich,

I think that the Kammback panels need to be flush with the surfaces of the vehicle in order to work the way they are designed? Obviously, the location of the taillights on the CRV make this a little tough to do.

Xringer

The "spoiler/wake boards" are designed to do what?

Those side boards are supposed to be like the wake-boards as shown here:
Solus - Cross-Flow Vortex Traps
Or maybe kinda like the Frame Extension indea Solus - Frame Extension

But I think the sides came out too streamlined and the VG effect isn't there.
I'm betting the flow around the tail lights is remaining somewhat attached..
(Or may be better attached, if I added some ZZ tape forward of the tail lights)!

The top section bends down just a tad, maybe giving it a little boat-tail effect.
You're right about the tail-lights. Plus, I didn't want anything that was Too big.

Here's all my pics http://ecomodder.com/forum/showthrea...ards-5152.html

Xringer

It would help decrease your drag by getting rid of the tie-down "stuff" on the roof.

I would think vortex generators on the top and sides would be more effective.

Have you tuff-tested the "spoiler/wake boards"

In high speed racing Kammback spoilers/bodies are used to increase downforce and drag. At bonneville the really high speed cars 300+mph often use a kammback design to move the center of pressure aft of the center of gravity, thus providing more stability. NHRA Funny cars use the kammback spoiler for downforce mainly but it also adds to stability.

That 10 foot long gutter that I installed the other day,
http://ecomodder.com/forum/showthrea...suvs-5140.html
came home tied on that roof rack..

Back in January of 2000, I ordered and installed those myself.. They are there to stay.. (Loctite) ;)

From what I have learned so far about VTs (like AirTabs) is they can add drag.
Don't need any more high profile stuff sticking out into the air flow.. :)

I might want to add some ZZ tape just ahead of the tail lights, to keep the flow attached until it gets to the back edge of the Wake board..
The top piece is pretty streamlined, so it will likely stay attached up there..

No real testing yet. Did 25 miles last night, but traffic conditions were bad and the test wasn't what I wanted. I did get a pretty good 31.2 MPG anyways.. :)

And, no.. I don't know what I'm doing.. :o

sawtooth
 

PBS Television re-broadcast "Raptor Force",a program which undertook to decribe how aeronautical engineers are borrowing technology from raptor birds.------------------------------ Part of the program dealt with the Grey Owl,which uses stealth and extremely sensitive hearing to locate Voles beneath snow.---------------------- The back of the owl wing is sawtoothed,and they made particular mention to it as a noise abatement aerodynamic adaptation.As air sheds off the top and bottom of the back of the wing,the sawtooth feathers break up shedding vortices with an accoustic signiture which would otherwise interfere with the owl's ability to "hear" the voles beneath the snow.Feather- down also absorbed "noise" in the boundary layer and further aided the birds ability to fly undetected.------------------- No mention is made to drag reduction,however in a different segment,regarding the F-117 Stealth Fighter,they did mention that aerodynamacists had definitely made the connection between "noise" and power loss,and while stealth was the overriding consideration for the aircraft,the notion of stretching the JP-4 a little,was,no doubt,a fringe benefit they wouldn't argue about.--------------------------------- So the sawtooth appears to definitely affect the separating flow,perhaps establishing a micro,rather than macro turbulence environment.---------------- Perhaps Santa will bring ecomodder that big windtunnel for Christmas and we can find out.

interesting find
 

Serrated trailing edges for improving lift and drag characteristics of lifting surfaces - Patent 5088665

i'll have yet to read this trough very carefully but this looks very much like what i mean!

For cd, I can guess how that might work on the trailing edge of a swept-back wing (jet fighter). Instead of generating one massive vortex behind each wing tip, a bunch of 60 deg saw teeth cause multiple small vortexi all along the TE..?.

When this topic first came up, it really got me thinking that my CRV rear spoiler+wake boards should have some 60d saw teeth added.. :D

And this morning while planning the micro-front wheel skirts, I made this
edit:
http://i46.photobucket.com/albums/f1...frontskirt.jpg
(Tape holding a crescent of coroplas).

Not smooth at all.. But, would cutting a sawtooth pattern in the TE & LE of the opening be beneficial to cd??
(With the teeth points orientated in the direction of travel of course).

Hehehe! Something like that is going to be considered a real eye-sore by my wife!! :p

it's worth a try, but remember aero is a complex matter, what works in one area may do the opposite in another area.

A common front wheel well wayout on low drag cars seems to be a mild traditional flare out in front of the wheel, and an indent behind the wheel. it would seen that you don't want to obstruct air from getting out of the wheelwell, but some obstruction in front of the wheel is good. so perhaps you may want to experiment with leavint the rear leg of the upide down U-shape off so that the air could exit there.

just an idea

You are right about the wheel openings. It is the same principle as sunroof spoilers. The goal is to get a spoiler just high enough so the flow reattaches asap past the opening. They added leading front wheel openings extensions when they did the aero development of the z06 corvette. I wonder why they just didn't design the bumper and fender accordingly in the first place though.

I see what you guys mean.. Like the Jetta taped front wheel wells, just posted:
http://ecomodder.com/forum/showthrea...mods-4869.html

When I drew the trailing tail of the crescent, I was thinking about the benefits of the pizza pan wheel cover. How do they help??

Maybe, fast air flow from the front end would bridge the forward gap and flow mostly unimpeded over the tire and smooth wheel cover, then bridge the gap on the trailing end of the tire and onto the door.

If a large gap exists, fast air over the tire will pull air out of the wheel well and increase the turbulence across the door and down the whole side of the car. That gap could be the point where a big separation of flow occurs..
http://us1.webpublications.com.au/st...0/1023_9lo.jpg
http://photos.revolution-press.com/J.../veyron_12.jpg

If the upside down crescent was really close to the tire, with a really small gap, with a moon disc installed, how much different would that be from a full skirt??

sawtooth
 

It appears that he uses them ahead of the wing,as with a Fieseler Storch,after the wing,but ahead of the flap,and also at the end of the flap on wings chopped at up to 20% of the cross-section for the flap.No performance claims are listed.

I didn't see this in the thread.....

At least one computer fan mfr makes "sawtooth" blades - really, staggered notches..... But the claim is for noise reduction....

http://ecomodder.com/forum/attachmen...1&d=1227151269

Noctua.at - sound-optimised premium components "Designed in Austria"!

They do also claim higher efficiency..... But, it's not a uniform notch - each successive blade has a slightly different offset from the last. So cross application is questionable.


Of course, we could very well be dealing with different Reynolds numbers etc. etc.


With respect to the Dreamliner nacelles.... So the claim too is to reduce noise.... It takes energy to make noise - energy that doesn't make you go forward... But the sawtooth allegedly breaks up larger vorticies... So to do that, I suspect it takes energy. If you use energy to cancel out other acoustical energy - I imagine there's a penalty. Think of luxury car acoustical noise cancellation or noise canceling headphones - it costs power to do it, but you'll experience less noise. This is just intuition speaking here :) The accuracy of that is all up for discussion.

I guess it's worth mentioning the very strict noise regulations for aircraft taking off :p

------
Personally, rather than playing with funky edges/passive control - I'd like to see more research and testing on active flow control. Hyundai apparently has a cool feature in one of their cars that scoops air from under the car and blows it out small vents (at higher velocity) in the tail lights to combat the intersecting surface flow shenanigans back there... Aptera dumps their exhaust out the tail in a similar fashion too for related reasons....

Has anyone seen the Pagani Zonda S? I doubt it's purely aesthetic that they have 4 tail pipes arranged in a square dead center of the height of the vehicle... not the vehicle's body height, but from the ground to the top of the roof-line, I believe.

Something about that says "high velocity flow to combat vacuum at the rear of a vortex" to me. in fact, I believe it absolutely screams it.

Having a side-exit, or even rear-side exit exhaust is a blatant waste of an aero-tool that should be utilized. Someone write congress.

Heavens! It'd be easy to bolt on a tailpipe trim piece to a stock exhaust to redirect flow upwards, or centrally. On cold days you'd see the effect.

I'd think there's a little more involved in it than that, exactly... like legality issues with having exposed sections of exhaust beyond the bumper... illegal in PA.

However, for testing purposes, it might work out... so if it's legal for anyone, maybe a test is in order of different placements of the exhaust flow compared to the dynamic airflow of their particular chassis?

I'm thinking that even light-throttle exhaust flow will counteract a certain degree of wake caused by not having a sharp point to detach flow... plus, on a sedan (full trunk) configuration, having exhaust gasses contaminating that airspace between the upper deck and bumper cover might actually cause a flow extension, allowing the flow to stay attached to an artificial surface far beyond that of the actual sheet metal of the car's chassis.

Obviously, the exhaust gasses are under pressure, so would that not have the added benefit of canceling the vacuum space that traditionally tails an auto? Adding any kind of pressure to that area has to be aerodynamically positive, yes?