sawtooth trailing edge

[aerohead]

Originally Posted by Christ

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?

You could calculate the volume of exhaust gas exiting the tailpipe in a given unit of time and compare that to the volume of air passing over and around the wake of the vehicle in the same time element.

[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.

[trebuchet03]

Originally Posted by Christ

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.

That's what CFD is for... Setup mass flow outlet surfaces and such... Trying to solve a problem like that analytically is really too much to do by hand

Originally Posted by Christ

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.

It's even more simple than that.... You want your pressure differential (front versus back) to be as close to zero as possible

[Christ]

Originally Posted by trebuchet03

It's even more simple than that.... You want your pressure differential (front versus back) to be as close to zero as possible

I seem to have over-evaluated the problem, which, if you talk to my friends, is quite common for me. I analyze things entirely too far... back to the point.

If you were to use a simple pressure analysis tool to check the vacuum at the tail of a vehicle in real time (read: water in a tube.), one could effectively determine the effect of adding pressure to that area.

I would guess that the highest rate of airflow would be at the absolute top, bottom, and sides of the wake, while the lowest pressure overall would be the area absolutely furthest from the movement of air (most central area, closest to the object).

Keep in mind, that is only a guess, and could very well be incorrect.

I'd be willing to bet that introducing some flow into the lowest pressure area would definitely interfere with the wake, and might smooth airflow so as to gain a positive result.

Once again, speculative, but I believe this may have been a consideration in designing the Pagani Zonda S.

Pics:
Zonda C12, introduced in 1999:


Zonda F, introduced 2005:


It appears quite aero to begin with, doesn't it?

For kicks, one of the first concept drawings of the Zonda:



This is taken from a post on SuperCars.net:
Zonda F: 1kg of downforce at the front / 25kg of downforce at the rear
Drag Coefficient: .36

No idea how accurate it is, I can't find the information right off.

[Christ]

The design just seems intentional, like the engineers knew that they could alter the wake's profile by putting the exhaust there... maybe I'm reading too far into a purely aesthetic feature?

EDIT:

The best thing I can find is that placing the exhaust in the wake of the vehicle actually increases torque across the RPM range, due to added scavenging... that, in combination with exhaust length/size tuning, could yield a very admirable result, but short of having a turbine engine, or a very small wake area in the rear of the vehicle, after some good 'ole google research, I don't think the average car could really benefit in aero from placing the exhaust in the wake.

Turbocharged cars could benefit from quicker spools, due to the lower pressure differential, N/A cars and supercharged cars could benefit from increased cylinder exhaust pulse manipulation (scavenging), but no aero benefit has been evidenced that I could find.

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.

Compared to the obvious wake area of the car, it seems like this might make a bit of difference in the wake's profile at speed, and possibly during acceleration.

That's the furthest I can get, I don't know how to calculate airflow at speed, and I dont know the wake area of the car, nor what speed it would be going during this exhaust profile.

Hopefully someone else can take over from here?

EDIT again:
Is it possible that manipulating the shape of exhaust flow could further alter the wake's profile?

[trebuchet03]

Originally Posted by Christ

If you were to use a simple pressure analysis tool to check the vacuum at the tail of a vehicle in real time (read: water in a tube.), one could effectively determine the effect of adding pressure to that area.

And there's the rub.....

"Low Pressure"

It's not intuitive... but the pressure back there isn't exactly "low" relative to atmospheric... you should totally measure it for yourself... Vacuum gauges are cheap (or do the tube and water thing)

It is "low" relative to the front of the car, however

[Christ]

See, that's why I stopped calling it "vacuum" and started calling it a low-pressure area.

The pressure there is lower than that anywhere else around the vehicle... that said, if the water gauge was calibrated to a high pressure area, such as the windshield cowl, it would immediately show the difference between each area.

I'm not sure what, exactly, the results of the water test would prove, other than (for my car, specifically) how much of a difference there between each end of the vehicle.

And, I don't have a way to video-record the results. I'll probably do it anyway, just for kicks, since that's pretty much why I do everything to my car.

[trebuchet03]

Originally Posted by Christ

The pressure there [back of car] is lower than that anywhere else around the vehicle...

This isn't quite accurate

Originally Posted by Christ

I'm not sure what, exactly, the results of the water test would prove, other than (for my car, specifically) how much of a difference there between each end of the vehicle.

Just about everything is car specific... What applies to your car probably won't apply to another car Aero "rules" generally are ratios depending on geometry (such as width versus curvature). When doing that test, don't compare to the front of the car, compare to inside your car (which should be "0" - atmospheric as long as you have the a/c blower off). You don't care so much about the difference between each end as much as the difference from atmospheric (plus it's easier to setup that test).

That said, it's not so much what the results would prove - it's to free your mind from your own intuition. Intuition doesn't really work here unless you've actually observed what's going on (which technically is more observation less intuition).

Here's a pressure plot


It's all just colors - blue is lower, red is higher... Green is about 14.7 lb/in^2 (aka, atmospheric). Low pressure happens (drum roll), on top of the car! Where, as you may have guessed, flow velocity as at it's maximum. The pressure differential between max (front of car) and minimum (top of car) is ~14.9-14.3 ln/in^2 - .6psi (the validity of those numbers should be tested).


Now, I'm not saying direct exhaust on top of your car,=... That'd be bad... The idea behind putting exhaust behind your car is to keep the flow moving back, away from the car rather than swirling about. The exhaust has the capability to push some of that out of the way.

[donee]

Hi Christ,

Some of this was discussed in another thread. There as a link to an article in that thread. The researchers found that unless the thrust was applied to all 4 edges at the rear of the vehicle at balanced levels, drag was increased. I think some kinda air-amplifier technique might do something. An air amplifier uses the pressure in a gas to pump massive volumes (in comparison to the air volume under pressure) of ambient air. Industrial Air Amplfiiers are designed for typical shop-air pressures - 80 to 125 PSI, however. One designed for the common back pressures in an a car exhaust would require the engineering background of the original designs.

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.

[Christ]

Could you find the thread and post it? Was it on this forum?

Without reading it, I can't understand how it would add drag to place the exhaust in the wake of the vehicle... doesn't make sense to me without numbers to back it up.

I'm not doubting it, just questioning it, since I haven't seen their testing or results personally.

If I correctly understand the air amplifier thing, you're basically stating a form of Turbocharger, no? Uses small amount of airflow to create a much larger flow of air through induction?