sawtooth trailing edge

[donee]

Hi Christ,

I cannot find the thread. Its on this board.

Here is a link to the paper though:

Information Bridge: DOE Scientific and Technical Information - Sponsored by OSTI

Yes, a Turbocharger, used to pump air out the back of the vehicle would be an example of an air-amplifier. But there are others with no moving parts. Do a google of Exair and look through their catalog.

[trebuchet03]

Originally Posted by donee

Tebuchet, its not the volumes that needs to be compared, but the masses of gases. :-). The gas in the exhaust is under pressure, so its density is more.

I don't see where I said anything about volumes

[donee]

Originally Posted by trebuchet03

I don't see where I said anything about volumes

Oops Sorry ,

Should have been addressed to Aerohead.

[binarycortex]

Originally Posted by Christ

Still, that Zonda has a 7.3 liter V12... you know from science that if at any point, it is working with 100% VE, it is consuming and exhausting 7.3 liters per revolution..

Speculation from here on:
IF that engine works with 100% VE at (speculatively) 2000 RPM, it is then using/exhausting a total of 7.3L*2000=14,600L of air per minute. (Damn, that's alot of air!) For a total of 515.6 CFM exiting the tailpipe of that car.

An engine actually does half its volume per revolution so you need to divide your findings by 2.

Think about a single cylinder 4 stroke engine. 1/2 rotation for intake, 1/2 rotation for compression (nothing leaves), 1/2 rotation for combustion, 1/2 rotation for exhaust. If you add those up its 2 full rotations.

[max_frontal_area]

Originally Posted by Sayyad

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.

love it! horseshoe crabs have a trailing edge that looks quite similar but it has likely been put there for survival purposes instead of streamlining.
GO blue blood ;~>

[Christ]

What about turtles? Some of those have rather large jagged trailing edges... I always figured it was just b/c of the way the shell is constructed, from several large "scales".

[aerohead]

Originally Posted by Christ

You could, but I wonder if it would be as conclusive as real world testing? and You'd have to perform several sets of calculations.

1. Engine speed increases as vehicle speed does, until a gear change
2. Exhaust CFM/airflow changes with pedal modulation... (think: turbocharger)
3. Air mass over/around the vehicle obviously changes with vehicle speed, not to mention every time the wind blows...

There are too many variables, IMO, to answer a question like this with mathematics alone. At least not for someone like me, who isn't interested in sitting around crunching numbers to attempt to cover every possible variable/scenario. Someone else can take that liberty. LOL.

Real world testing may be in order, but most probably on a track. (Unless someone plans some serious vehicle modifications). But even modifying the vehicle to put the exhaust in the rear end's low pressure area would breed questions such as "would it help more at the top of the area or the bottom? either side?" etc and so on.

Speculatively, I can say that adding ANY amount of pressure to a low-pressure or vacuum area would reduce the amount of vacuum (increase overall pressure) and SHOULD help, even if marginally.

Less speculatively, I would wonder if the necessary modification would prove it's worth for the return on investment over a viable period of time.

Also, a question... if the exhaust were in the wake area, would that not create a vacuum and aid the scavenging cycle in the exhaust, since the area the exhaust is exiting into would obviously be a vacuum... thus "Drawing" the gasses out of the pipe.

It might be beneficial to performance in more than one way. Speculatively.

ConnClark was noodling around with this one.Here's my take on it.For a wagon,SUV,or van type vehicle,it probably makes no difference where the exhaust is.The reason being that essentially,the constant-section body prevents any static-regain of the surrounding flow,and when the flow finally separates at the rear,the base pressure behind these types of vehicles is at the same pressure as anywhere along the side of the body,up to the windshield.So moving the exhaust from the side to rear of one of these types of vehicles would have no effect with respect to scavenging.-------------------- With respect to a fastback-style car,with attached flow all the way to the rear,it would make a difference.On a fast-back,from anywhere on the body,say from the headrest back,pressure would be steadily increasing as the flow moved closer to the tail.So actually,for a fast- back,a side exhaust would potentially provide the highest scavenging,as the pressure behind the tail would be at a higher base pressure,respecting the Bernoulli Theorem of delta-V vs delta-P.

[Cd]

So has anyone here tested a sawtooth trailing edge on a Ksmmback ?
I did a rough test in a CFD program of a half teardrop ( cut halfway like a Kammback )
One version with a straight edge, and the other with a sawtooth edge.
Each time I tested it, the sawtooth version gave a lower Cd by one count.
The CFD program isn't accurate though on many models, and I'd like to know what the actual results would be in real life.

[freebeard]

My impression has been that a serrated edge affect mostly the acoustics.

Did you test varying scales of serration?

[Cd]

Originally Posted by freebeard

My impression has been that a serrated edge affect mostly the acoustics.

Did you test varying scales of serration?

Did I test at varying scales ? No. The CFD software is very unreliable.
But it was repeatable, which i found intetesting.

What got me curious, is that birds have semi serrated edges to the shape of their wings. ( Not the end feathers, which, as you know, actually do reduce drag by acting like winglets on aircraft )
Since this is common on birds such as hawks, I thought that this might be a feature to aid in stealth.
However, you also see it on vultures as well, which would have no need for stealth.
I'd show an example image here, but I'm posting using my Android phone.
Links don't seem to work.

I see that some wings use serrations for 'vortex shedding'.
Wind turbines use them for noise, but I haven't seen anything on making them more efficient because of it.
I would imagine a serrated edge on a Kammback would be useless, or perhaps worse.
A bit like putting vortex generators on the very back edge of a semi trailer.