sawtooth trailing edge

[aerohead]

Originally Posted by lunarhighway

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.



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...



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

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!

[lunarhighway]

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:

Vortices readily deflect and attach themselves to a solid surface...

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:

Location On Vehicles
In general terms, and for maximum effect, AirtabsTM should be installed as close to the back (trailing) edge of the vehicle
as possible while ensuring proper adhesion surfaces. They are effective in shielding gaps or reducing turbulent air from
entering undesirable areas. Any part of a vehicle that presents an edge that is at right angles to the airflow is a possible
Airtab® location. This includes forward of wheel wells etc. In order to accommodate rivets etc during installation,
AirtabsTM can be mounted so that the leading edge of the Airtab® is 14 or less inches from the trailing edge of the vehicle.
Mounting further forward will reduce performance.

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 :

This design is believed to work because the combination of the gentle ramp angle and the curvature profile of the walls creates counter-rotating vortices which deflect the boundary layer away from the intake and draws in the faster moving air,

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

[aerohead]

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!

[Xringer]

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.



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..

[NeilBlanchard]

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.

[CobraBall]

Xringer

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

[Xringer]

Originally Posted by NeilBlanchard

Hi,

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.

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

[CobraBall]

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.

[Xringer]

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..

[aerohead]

Originally Posted by lunarhighway

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

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.