SW20 aero theory. (56k = if u patient)

[RomanV]

Another interesting use for a Manometer, would be measuring the effectivness of the stock SW20 spoilers. (which is another unresolved topic of debate!) If you placed a pressure sensor at the rear of the car, it would show a pressure drop (or not!) depending on whether or not the spoiler was attached. (or effective for that matter.)

If the spoiler TRUELY was functional, as opposed to aesthetic, surely there would be a pressure difference.

If the spoiler was functional, the air flow would be rejoining further down the cars wake. So surely this would have an effect on the air pressure at the rear of the car? Im not sure if it would be higher or lower however.

Aaah! Thats right.
All of this testing could be used in conjunction with the 'wool tufts taped to the car' idea.
Which would be another good way of establishing positive/negative aspects of various mods.

[fivebob]

More importantly thou, what is yor intent with all that gear 5bob?

Ahh, I having a cunning plan to get all the gear I need to run my own Automotive research facility, then I can accumulate all the info that no one else has, and sit back an laugh and tease all the plebs when threads like this one come up

Next on the list is a PicoScope, then a Dynapack and I'll just wait a few years until desktop CFD and Engine analysis packages become affordable and practical.

It's all true I tell you, except for the part about laughing at and teasing all the plebs, I'll probably just give the info away

[fivebob]

All of this testing could be used in conjunction with the 'wool tufts taped to the car' idea.
Which would be another good way of establishing positive/negative aspects of various mods.

As far as the spoiler goes I think the wool tuffs would be all you need to show there's no flow back there, no flow = no downforce. Been there, done that and I was not impressed with the results

An idea I've had is to get someone in the UK with a SW20 and work out a way to test as many configurations as possible in one day, then hire the MIRA wind tunnel and sell the data.

[vvega]

is there any reason we cant do this with a model ??
we may have a couple of devices at work that may assist on this little venture ill see tomorrow how "obtainable" they are for private use

i also remeber last yeah there was a few experments done on aerodynamics as part of a phd at waikato uni ..ill make a few inquireys about how the results were obtained and what may have happened to the software... perhaps a "backup" copy was made as so often happens with uni software

v

[fivebob]

The problem with using a model is that the Reynolds number is radically different unless it's close to the actual size, and building a big scale model is a time consuming and expensive task.

Might give some indication but IMO it's not worth putting a lot of effort into as the results could be radically different on a real car.

[vvega]

so we now need a big fan and a long garage

[fivebob]

Yes a really long garage, last home made wind tunnel I had anything to do with was 7m long for a 0.25m test section, with a giant extraction fan on one end. We used it to test propellors for model aircraft speed engines using a 2hp Router motor to spin them to 30,00rpm and a series of water manometers to measure the pressure drop in front of the prop.

[vvega]

hehe we have a test room for the lycomings in the ct4e has different fixtures for most other engines be turbo prop or piston power
massive difuser and it exits out the roof
there tested prop on for the correct loading and ther eare shutters to "starve " the prop to simulate higher altatude :S

its used for testing as well as runup's

so realistically fivebob how much airflow is gunna need to be pushed to get a a viable result ??
are we gunna be better to set up the car for the tests and drive it on a "controlled" road ??



though in a tunnel cornerwieghts could be used to guage any increase or decreace in downforce

v

[fivebob]

It depends on the scale of the model e.g. A 1/4 scale needs 4 times the wind speed to keep the Reynolds number constant. So it's not really practical on small models that need to simulate high speed testing. Even if you could fit a 1/4 scale model in, you would need an airstream at 400km/h to simulate a 100km/h full sized car. The "Scale effects" section of http://www.aerodyn.org/WindTunnel/wind_tunnel.html should give you some ideas of the problems of scale model testing.

The reality is that you need to test on the car with the best quality instruments you can afford to get repeatable useful info. A properly instrumented car with pressure sensors, load sensors on the suspension on a smooth track could provide good data. Doing it in a comparative form using water manometers would tell you if there is a difference in pressure, but quantifying the amount of downforce generated would be difficult, still better than having no info at all though.

Best of all would be full size wind tunnel testing, but AFAIK the only suitable tunnel that's available for public use is the MIRA one in the UK and IIRC it cost about $5000 per day 5 years ago.

[RomanV]

What sort of info could you gather from wind tunnel testing?

I mean I have seen the smoke trails, but what else? I suppose you could have scales under the car on at each corner, and measure the downforce/lift generated.
I suppose you would still need a lot of the sensors on the car to gather any useful information.

Also, on a semi-related note.

How is the Coefficient of drag calculated?
Is it a mathematical calculation, or can it only be established using wind tunnel testing?
I would imagine that it is a calculation based on the frontal area, in relation to the drag created.
How exactly do they determine how much load the drag is creating in a wind tunnel? Obviously it would be a force pushing the car backwards, but how do they test this?


P.S. That is very interesting about the scale models needing HIGHER wind speeds! I would have thought it would be the other way around.
But at the same time, I always wondered that about scale models. I mean the properties of air dont scale down aswell, to suit your model.
So it makes sense I suppose.
Its like the scale model would be cutting through thinner air, correct?

I remember reading that a lot of insects etc. wouldnt be able to survive/fly if they were 20 times bigger. Because they rely on phenomena that only occurs on a smaller scale.

eg. Those little creatures that scoot across the top surface of ponds, using water tension. Try making that 20 times bigger!

Woops! Heh, so easy to wander off topic.

[RomanV]

Ive just been reading a few pages on aerodyn.org. Which has cleared up a few things.

Quite a good read so far.

[RomanV]

Another aspect of SW20 aerodynamics that I find interesting is the rear 'drop off' behind the windshield.

Because obivously the airflow will be different at different speeds.
Fivebob, you say that you have tested the rear wing of an SW20, and it is in a 'deadzone'. However, at what speed? I mean the low pressure area behind the car will be more easily 'filled' at lower speeds, correct?
As the vehicle speed increases, the air detaching from the rear of the roof will rejoin further down the vehicle.

Hmm. Time again to explain with pics.




At lower speeds, it is easier for the air flowing over the top to fill the low pressure zone created behind the rear window. Also there is less vaccuum created. Basically, the hole that the car is punching through the air is filled more easily.
So more of the cars body is 'touched' by the airflow. So at some speed, perhaps the spoiler is actually in a position to be functional?
When the flow is rejoining the vehicle at a certain point ahead of the spoiler, at a certain speed. Perhaps this is the speed where the SW20 needs the spoiler?
However, the at this speed, the airflow is probably so slow that it would make a negligable difference.
When the speed increases, the air flowing around the car rejoins further behind the vehicle. (to the extent that it is well beyond the spoiler) So in this scenario, the spoiler would be out of the flow of air over the car, and therefore in a useless 'deadzone'.

I dont have any hard facts to prove any of this, it is just a theory.

[Dell'Orto]

Does that take into account the underside of the car's aerodynamics? I thought the point of those undertrays was to help bring air up through the engine bay to aid cooling?

[RomanV]

What part of this thread are you talking about? Im confused.

[RomanV]

(Huge bump! )

If you look at the sard mc8r race car, you can see that it runs the vented hood. So it cant be a terribly bad idea!

[CozmoNz]

(Huge bump! )

If you look at the sard mc8r race car, you can see that it runs the vented hood. So it cant be a terribly bad idea!

[img]http://www.mr2.com/TEXT/GRAPHICS/sard/sw20-sard-mc8-lemans.jpg[img]

go look @ most Mr cars.

Ferrari, Lotus, well, most things out there Mr

how many have bonnit vents... HEAPS.

cant be THAT bad

[RomanV]

Erm... Yes, I already know that. Infact I stated that in my first post.

What was being debated was whether or not it was a good idea for the SW20.

[Dell'Orto]

I think one main reason they wouldnt have done it on the MR2, is practicality. Look at the likes of Elises, F40's etc that are MR and have bonnet vents - not really your daily driver are they?.

[RomanV]

Yeah thats for sure!

And also the aggressive look of the bonnet vents would perhaps put off some buyers.

[DJ]

Hmmm I need a new bonnet....