rear spoiler

[Anonymous]

Marketing ?   s:wink: :wink: s:wink:

i think if SP had not got involved with TTE then id be the only one in the UK driving one!

Marketing? thats a good joke

[philster_d]

The Germans have an eye for beauty it seems.

Uhuh

[markiii]

[quote="simontan]That's why there are vents at the decklid so that hot air will rise up from the engine bay to fill the vacuum spot, while cooler air rushes in from the bottom of the 2. :[/quote]

see I'm not convinced of that, airflow is actually down and through thast why teh TTET fan fore downards, because if it went up it woudl fight natural airflow.

alsobare in mind teh drip tray stops hot air coming out

[rtbiscuit]

would agree with mark on this.

the direction of air flow is pulled over the car. as the air flow passes under the car it sucks the air down as well pulling it over the engine. and out behind the exhaust.

i can't remember the name for the process i believe it is similar to a chimney. as air passes over a chimney it then sucks the smoke up.

[loadswine]

The venturi effect!

[rtbiscuit]

bingo! thats the one!   s:D s:D

[aaronjb]

i can't remember the name for the process i believe it is similar to a chimney. as air passes over a chimney it then sucks the smoke up.

Venturi effect.

However there's more in play than that - a car is very roughly wing shaped (which is why most road cars, certainly until recently, had negative downforce at speed).

The air passing over the top of the car has to travel considerably further than that passing under the bottom of the car (because over the top has to follow a very curved path). But, as with a wing, it still has to arrive at the back at the same time - so it goes faster over the top, which creates lower pressure over the roof of the car...


First person to stick some bits of wool to their engine lid vent and drive at speed should find out exactly which direction the air flow goes in  s;) s;)   If it gets sucked in - downward, if they all stand up on end, upward.


Go on. You know you want to.

[evileye_wrx]

The venturi effect!

Come on Nigel, everyone knows that's from the episode of Star Trek where Kirk makes out with the green Orion woman!

Phil   s:wink: :wink: s:wink:

[Anonymous]

see I'm not convinced of that, airflow is actually down and through thast why the TTET fan fore downards, because if it went up it would fight natural airflow.

alsobare in mind the drip tray stops hot air coming out

Mark, I'm not quite sure why you are ignoring the obvious physics behind this. I've been through the exact same discussion in SC 2 years ago. I can't remember where the thread is, but I do know the theory that I just talked about had been agreeable. It's not my theory if you know what I mean.

I take the liberty to cut&paste the related aerodynamic involved just so you understand why I say what I said.

Before I start, let me briefly describe what's Bernoulli Principle for the sake of the general board members.

Bernoulli's Principle states that in an ideal fluid (low speed air is a good approximation), with no work being performed on the fluid, an increase in velocity occurs simultaneously with decrease in pressure or gravitational energy.

Wikipedia definition of Bernoulli's Principle

In car-related definition, it means :- the faster a gas moves the lower its pressure becomes or vise versa.

Now here is the physics...

It is a well known fact that if you drop a feather and a steel ball in a vacuum then they will both fall at the same rate.  However outside a vacuum this is not the case and the ball falls much faster.  This is the due to the air resistance being greater for the feather than the ball.  This principle applies in aerodynamics and is known as drag.  The air molecules have to find the easiest path around the car so the better shaped the car is, the less drag will affect and therefore the more energy can be saved.

Drag at the front and the rear of the car is very different so these will be considered separately.

The frontal pressure relates to how the air molecules move around the outside of the car.  As they hit the front end they compress and the pressure increases.  The air moving next to the car is at a much lower pressure so the high pressure air moves towards the lower pressure air and thus moves around the car.



This is the reason for sleek edges on performance cars and racing cars as their shapes decrease the pressure at the main pressure points which are the front bumper and the windscreen.  On family cars with a primary target of low costs, these expensive aerodynamic designs are not heavily used whereas with a car such as the McLaren Mercedes SLR, every single part of the body is designed specifically designed for optimum performance.

The other main area that is affected by drag is the rear of the car.  As a vehicle moves through the air, the air molecules are moving around the body but then they need to fill the space left by the vehicle.  This is sometimes known as the rear vacuum.  Consider a truck moving along the motorway at 70mph.  The large area of the rear end of the truck will need a large number of air molecules to move into it very quickly.  However the truck is moving too quickly for the air molecules to continually move into the area so a partial vacuum, technically known as the flow detachment is created.  This can happen at any part of the car where a gap can occur behind.  The main affected areas are the rear bumper and the rear windscreen.  However it can also occur behind wing mirrors and tyres as well.




The full article can be found here and here.

So now are we agreeable that our 2 has a rear vacuum similar to the one shown above?

Now let's continue...



Theoretically or pratically speaking regardless, both the air (See Figure 2) travelling from Point A (marked in red and green to illustrate air moving over and under the car) must reach Point B at the same time. Thus, the air moving over the car MUST travel faster in order to reach Point B at the same time as the air underneath the car, since the distance from Point A over the car to Point B is longer than the undercarriage distance of the car Point A to Point B. Please don't tell me our 2's undercarriage is not perfectly flat, we all know that. But the distance is shorter regardless perfectly flat or lesser.

Therefore by applying Bernoulli's Principle, we can conclude the area above the car has lower pressure compares to the undercarriage area. That's why there are more lifts above the car, and less drag underneath. (See Figure 1)

We can thus relate such effect of Bernoulli's with general fluid's convection theory, which in the context of our discussion, states that hot air rises due to greater bouyancy force, and cold air settles. A state of indefinite continuous cycle of such state is called a convection current. As the air in the engine bay gets hotter, it rises naturally. The void resulting from the rising hot air is then replaced continuously by the moving cold air that's feeding from the bottom of the car. That's why there are vents on the rear decklid, so that the hot air can escape from the engine bay.

So you said the diptray will hinder such process. But please realise that the diptray is not SEALED. It has openings on the longitudinal sides, which will permit the exit of hot air. Yes, perhaps the convection cycle is not as ideal or as efficient as without the diptray. Perhaps the presence of the diptray will promote turbulence to a certain extend. But let there be no doubt that it is not enough to counter the coupled effect of the convection cycle and the rear vacuum which is practically sucking air from any open vents. The diptray is here for a different reason and that's to stop the rusting of bolts and nuts of various motor components (think rainy days). It will hinder the rising of hot air, but it cannot conquer the laws of nature and physics. Please don't associate such hinderance with Brownian motion, because Brownian motion exists only in a fluid of static state. If the diptray is stopping air from rushing out of the vents, why wouldn't it stop air COMING INTO the engine bay then?  s:roll: :roll: s:roll:  

Now why does the TTE fan blows and not suck? I have no idea. I'm a student of nature. Nature never fails, human does. That explains why I never understand the many things the Germans do. The Japanese on the other hand, are the selected specimens of the rare species.

I can understand that the aerodynamics of the spyder may be different from the explanations given for general vehicles posted above. Thus if my understanding does fail me, please correct me accordingly and pardon my folly. I rather be a foolish wit than a witty fool.

I'm also a keen learner of knowledge, and a willing repenter.

[Anonymous]

Dont worry Simon, im the witty fool! .. just wait im not even witty  s:( s:(

[Anonymous]

Dont worry Simon, I'm the witty fool! .. just wait I'm not even witty  s:( s:(

Still... you are a nice and friendly fool.  s:wink: :wink: s:wink:

That's if you are a FOOL!!  s:twisted: :twisted: s:twisted:    s:twisted: :twisted: s:twisted:

[Anonymous]

Dont worry Simon, I'm the witty fool! .. just wait I'm not even witty  s:( s:(

Still... you are a nice and friendly fool.  s:wink: :wink: s:wink:

That's if you are a FOOL!!  s:twisted: :twisted: s:twisted:    s:twisted: :twisted: s:twisted:

Oh Simon youre so nice   s:mrgreen: :mrgreen: s:mrgreen:  

but i am a fool, i think you know that   s:P s:P  

Hey and things finally got moving today  s:bounce: :bounce: s:bounce:  you know what i mean   s:wink: :wink: s:wink:

[northernalex]

Physics looks spot on to me!  

However the TTE spoiler may be a good idea with the roof up as it may act to 'smooth out" the rear of the soft top. Reducing the drag shown behind the roof on the above diagram.  Maybe its the same idea as when someone sticks a small spoiler on the top of a hatchback, it extends the length of the car and reduces drag slightly.

[Anonymous]

A physicist to the rescue!   s:bowdown: :bowdown: s:bowdown:

[Anonymous]

Hey and things finally got moving today  s:bounce: :bounce: s:bounce:  you know what i mean   s:wink: :wink: s:wink:

WOW! That's GREAT NEWS!   s:D s:D    s:D s:D

[rtbiscuit]

i concur!

although never read up on theory!

did a bit of fluid dynamics at uni! not enough to be of any help here though!

[Anonymous]

did a bit of fluid dynamics at uni! not enough to be of any help here though![/quote]

that will be drinking beer then?  s:D s:D

[rtbiscuit]

lol both,

spent more time doing the beers than the science  s:oops: :oops: s:oops:

[northernalex]

"We can thus relate such effect of Bernoulli's with general fluid's convection theory, which in the context of our discussion, states that hot air rises due to greater bouyancy force, and cold air settles. A state of indefinite continuous cycle of such state is called a convection current. As the air in the engine bay gets hotter, it rises naturally. The void resulting from the rising hot air is then replaced continuously by the moving cold air that's feeding from the bottom of the car. That's why there are vents on the rear decklid, so that the hot air can escape from the engine bay"

However I'm not too sure about this bit, your physics is comepltly correct regarding the convection currents but I think there will be a crossover point at a particular speed where the partial vacuum behind the car is such that it overwhelms the convection current causing air to flow into the engine bay from the top and out of the rear of the car".  All sounds a bit too complicated for what we phsycists call "Hand waving physics"  but I think we got the gist of it.

[evileye_wrx]

Dr Alex to the rescue, he's been waiting for a thread like this to come up since 1996. You tell 'em Alex, your years of study have been building to this moment!   s8) s8)  

On the subject of wings, if you used the grill on the engine lid to attach your wing, the C/F type which bolts on, would it stay in place or be to unstable? Just thinking whether a person could avoid drilling the lid.

Phil

[rtbiscuit]

my honest advice is not to,

yes it is all attatched to the car, but it could flex or move. especially once your hitting higher speeds. any flexibility will be sent straight to its weakest point. and you wouldn't want it ripping of your rear. would dread to think of what damage a flying spoiler at speed would do.

i may be wrong, see what the others say.  i cant remember if the grill is flexible or not!

[markiii]

I don't pretend to fully understand teh physics but until someone practically proves it one way or another my money is on the TTE actual testing result which is that the air goes in and down

[alancdavis]

Down force,,,,,down  draft,,,,,all i wanted was a spoiler thwt looks good   s:roll: :roll: s:roll:

[Anonymous]

but I think there will be a crossover point at a particular speed where the partial vacuum behind the car is such that it overwhelms the convection current causing air to flow into the engine bay from the top and out of the rear of the car".  All sounds a bit too complicated for what we phsycists call "Hand waving physics"  but I think we got the gist of it.

What you said is very possible. What I'm interested about is what's the particular speed for such crossover point? 20km/h? 80km/h or..? This could mean a duo flow scenarios for the general public if the crossover speed is generally attainable during normal driving.

If such overwhelming vacuum is present, perhaps TTE wouldn't have stuck a fan down the rearlid in the first place because nature would have done its job. Either the crossover point is at too high a speed, or the crossover effect is not significant enough, thus rendering a need fo an additional fan during general driving speed.

You make a very valid point Alex. I'm very interested to find out.

[Anonymous]

you lot crack me up   s:lol: :lol: s:lol:    s:lol: :lol: s:lol: