more aero...Trapped Vortex

[meb]

Milliken discuss a trapped vortex in the aero chapter. I'm intrigued and have wondered about an application to a relative small hatchback design. Since the car in question produces modest power, 210bhp, I would prefer to concentrate on priciples that reduce drag rather than create downforce.

How sound is this principle in practice? I also imagine a rear diffuser might work in concert...the basic idea is to reduce the wind wake.

[AU N EGL]

Originally Posted by meb

Milliken discuss a trapped vortex in the aero chapter. I'm intrigued and have wondered about an application to a relative small hatchback design. Since the car in question produces modest power, 210bhp, I would prefer to concentrate on priciples that reduce drag rather than create downforce.

How sound is this principle in practice? I also imagine a rear diffuser might work in concert...the basic idea is to reduce the wind wake.

First look at the Tech rules of the SCCA or NASA as to what you can and can not do for aerodymanics.

Racing rules dictate almost everthing.

rear diffusers only work with the whole underbody is closed off, the car has side skirts too.

[tristancliffe]

Originally Posted by AU N EGL

rear diffusers only work with the whole underbody is closed off, the car has side skirts too.

This is NOT true. Why do a lot of cars, from road cars to F1 have diffusers? Because it smooths the airflow at the back of the car, reducing drag whilst improving the [limited] ground effects possible. To say they only work on skirted cars is just silly.

[AU N EGL]

Originally Posted by tristancliffe

This is NOT true. Why do a lot of cars, from road cars to F1 have diffusers? Because it smooths the airflow at the back of the car, reducing drag whilst improving the [limited] ground effects possible. To say they only work on skirted cars is just silly.

have you spent any time in a wind tunnel?? I have.

The side skirts are for highspeed cornering.

[tristancliffe]

Yes, as a matter of fact I have.

Now you say skirts are only for highspeed cornering - correct in a sense that a skirted ground effect car will have lots of aero-generated grip. But you're original statement was that diffusers only work with skirts, and this CANNOT be the case.

Show me an F1 car with skirts from the last 10 years? Or show me one WITHOUT skirts that DOESN'T have a diffuser. I'm sure you can't. Does a Ferrari Enzo (no skirts, and lots of ground clearence [compared to single seaters]) have a diffuser? Yup. Elise? Yup (sort of) Even some hatchbacks have slight diffuser mouldings to clean up the air flow and reduce the turbulence off the back of the car = less drag.

[meb]

So, is my goal possible? My 2005 Mini is basically a brick at high speeds. I've seen lots of folks try to employ aero down force aids with this car. But to my way of thinking, this robs an already inefficient aero design of high speed acceleration. In most situations, this car will never reach speeds over 130 mph. So reducing drag by organizing air as it exits the rear of the car seems to be a good idea to me...but perhaps a little far fetched for my limited resources - no wind tunnel.

I also imagine that a splitter factors into this equation at some point since it in theory may help a trapped vortex work more efficiently...or simply keep some air from being caught up by the underside of the car.

The outside rear view mirrors and the non-functional wing are items I can easily play with; the wing can be removed and the mirrors exchanged for a type that fit into the A-pillar.

Thanks for your help!!!

[trikesrule]

I reckon au meant they work (diffusers) better if sideskirts are present. I've built quite a few Supercarts - with and without has been tried and the vote was with side skirts (panels) is better.........trikes

[trikesrule]

In a cross wind (yaw) the unskirted Supercarts were easier drive at high speed. Cleaning up the air at the back of your Mini can only reap rewards. Everything I've been able to reduce drag (aero) on was always quicker. Keeping the sides 'clean' is important I've noticed. The underneath -uumm well just clean it up as best you can. The V8 Supercars fill in the gaps and fit smooth panels under the cars. I've seen this as occasionally they fall over - DOH. I would fit dimpled panels under the car if it's that important. ....trikes

[Locost47]

I've got a little bit of inside knowledge of the R56 Mini and what they did to improve the aero over the R50.

Both are stubby little beasts, which doesn't give you a lot to work with, but the simplest things you can do to reduce drag are improve the shielding of the front wheels and better fixing of the separation points on the C-pillars.

The extremely short front overhand means that the air has relatively little 'warning' of the obstructions ahead and so doesn't start turning out of the way very far in front of the car. This means the front wheels are at a very high angle of yaw relative to the airstream that hits them and so you get a lot of drag generated. Fitting deep deflectors in front and slightly inboard of the front wheels helps. Look at those on a 1-series BMW for an example of what i mean. This isn't the best photo but it was the best that Google could find:
http://www.fotos-de-coches.info/foto.../foto-3541.php

At the rear the highly-curved C-pillars cause problems. The newer Mini has a sharper edge moulded into the plastic to effetcively fix the separation point and reduce drag. The one that ended up on the car was much more subtle than the one the aero people wanted, but it looked too ugly to the stylists. By your Milliken reference i'm guessing you've also considered the old Ford Fiesta-style fences/spoiler combination that kind of offsets the perimeter of the rear hatch from the back face of the car by 30mm or so, and therefore both fixes separation and ensures that the low pressure from the trapped vortex thus generated does not act on the back face of the car. If you can modify it that far, then great!

Then, as Trikes said, try covering up as much of the lumpy stuff on the underside as possible or, possibly, deepen the front air dam by 20mm or so (which would also help in the job of better shielding the front wheels, but could also move the lift distribution too much to the front).

[Chris Y]

Congratulations, Locost

[Locost47]

Thank you HypnoToad, err, i mean cheers Chris.

[zac510]

For my own interest I looked up the R56 mini and found this photo so decided to share it:



So far searches for 'Fiesta spoiler' are not really producing the results I was expecting

[rcarr]

What is the difference?

[Locost47]

Flipping heck! No-one's supposed to have a photo like that! That's a pilot build R56 kitted up for a hot or cold environment test. Someone must have been pretty opportunistic with their camera.

The difference between the C-pillars on the R50 and R56 Minis was that the newer one replaced the longer glass side window with a short one plus plastic body panel. This panel had a sharp edge moulded into it to fix the separation point on the corner and help improve drag (and cross-wind stability, to a lesser degree).

[rcarr]

Originally Posted by Locost47

The difference between the C-pillars on the R50 and R56 Minis was that the newer one replaced the longer glass side window with a short one plus plastic body panel. This panel had a sharp edge moulded into it to fix the separation point on the corner and help improve drag (and cross-wind stability, to a lesser degree).

Doesn't the sharp edge also help with soiling on the rear window?

[Locost47]

I don't think that's the final production c-pillar panel. It looks too much like what it should be like and not what a stylist has 'prettied-up'.

I don't know much about soiling at all, although that didn't stop them from putting me as chairman of a soiling session at a big international aero conference and making a muppet out of myself by trying and failing to ask intelligent questions about stuff. D'oh!

My guess would be that the c-pillar trip line would worsen soiling on the rear screen by reducing the local flow velocity there. Most estate cars and steep hatchbacks suffer from soiling issues so I would figure that these devices making the car more like an estate, from the air's point of view, would have the same effect. It may well be more complicated than that though, i don't know.

[zac510]

Is that the final production C-pillar panel though?

[meb]

What's interesting, is I have given a lot of thought to that area and wondered how it might work with a diffuser and wing designed for down force or for vortex generation...but in addition, I also gave some thought to creating an opening from the inside of the car along the vertical length of the C pillar where it mates to the rear hatch...trying to eliminate the parachute effect created with side windows open. Another interesting complication.

In essence, the ridge in the pillar help to create a vortex??? How the different are masses merge from the roof, C pillars and from under the car is a curiousity of mine...can you spare a wind tunnel for a few days???