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).

[meb]

Wow! Lots of great info here. I'll digest the information for a day or two and come back with a couple of questions I'm sure.

I also tracked a ford fiesta in the early 80s...I must like bricks Once, at Lime Rock, I forgot to roll down my windows...near the end of the stright, the very thin door frames bent out and blew the glass all over...I was very thankfull for generous run-off area. The car was quicker with the windows closed, a possible no brainer here. But, with regard to the Mini, I've also been thinking about ways to open up the rear hatch from the inside; with the windows open, this car is a giant parachute. The difference here is that at speed, another 20hp or so is eaten up by this parachute effect...windows closed and another 6-9mph more top speed on average is possible. That is significant on Lime Rock's short straight.

I'm not worried about rules, yet...

[meb]

Locust47,

In addition to my above reply, I viewed the photo. My 2005 mini have a this flap (for the lack of a better word), but it runs the width of the car. Now, I've given a lot of thought to cutting the center section of this away which should allow more air to pass under the car. I keep looking at the Ferrari 460's front end and this shape appears on many cars...not that it is applicable to the mini...

...there is yet another rigid flap just aft the front sub-frame. This one sticks down 2" into the space below the mini...this will go too.

I fear if I do not move air under the car as well, it might just take off.

[Locost47]

Woah there! It's highly likely that they put that secondary flap behind the subframe for good reason. It is quite common for normal production cars to have various little flips scattered around underneath in order to get the airflow to 'jump' over obstacles or features which would otherwise cause greater disruption. They are especially common around the exhaust and fuel tank areas, particularly if there is a void or sudden change in floor height.

A few years ago I had to reduce the drag of a hatchback for a far-eastern company but without making any visible changes to the car, both for styling and cost reasons (visible parts normally have to be painted, at extra cost).

In the end I got 10% just from better shielding of the front wheels and other problematic areas under the car.

The full-width front air dam can be sometimes just there as a kind of blanket measure for stopping air going under the car and smacking into nasty things down there. Often though the effect of the air dam on lift can be a problem so the centre section, which usually offers the least drag benefit, is cut out.

On the F430 this is for a different purpose. It has a flat floor so doesn't have to worry about the air hitting things and the raised nose section increases the flow to the diffuser. Look at the Challenge Stradale or some sportier BMWs and see how they still achieve shielding of the front wheels by deepening the main part of the front bumper only in front of the wheels.

[rcarr]

Reduce the drag, put a large peice of aluminium or GRP under the car, this may reduce drag, with a Mini you won't get more than 130mph out of it anyway! Block up a few ducts to get higher speed, it will overheat/blow your engine. Take off the wing mirrors, don't think downforce is what you need, think about mechanical dynamics rather than aerodynamics.

[meb]

Locust47,

Removal of the piece behind the sub-frame is experimental - it can be reinstalled easily. I do understand the relative differences between my Mini and the 460, and, the danger present in cross-applying techniques. I'm poking in the dark here for sure...and that extra piece hanging down IS just ahead of a major depression in the underside which houses the exhaust and CAT. It's easy to simply cover these up, but then, heat becomes trapped and another bag of worms opens up...

..a splitter's job is to move air over the top of a car in an effort to make a rear wing more effective? By moving more air over the top of a car, are we creating more lift? Airplane wing...

rcarr,

I have the 05 JCW - better gearing, more power, same great taste. It would probably see the up side of 140-145 mph, but not on a track; there isn't one with a straight long enough . If by mechanical dynamics you mean the removal of stuff like a faux wing and mirrors, I'm all for it.

Above all, thank you for your help. I wanted to design cars for a living...mom and dad said no...too bad, Mark Sterhenberger sent many of my drawings back to me with lots of encouraging notes.

[Locost47]

"It's easy to simply cover these up, but then, heat becomes trapped and another bag of worms opens up..."

Tell me about it! I've spent the last 8 years always trying to cover these types of things up and fighting against the Thermal Management people who want everything as exposed as possible. I see their point though, there's no point having a quick car which melts after 5 minutes!

With regard to the front slpitter thing, they do force more air up and over the car (as well as starve off the region underneath them, leading to very low pressures there.

Just from a simple Newtonian action/reaction thing you can see that forcing the air up must force the car down. The wing/lift aspect comes in if that large mass of air needs to be brought back down again at the rear of the car - if you have a fastback or saloon type car then it can be a bit of a problem. If you have a steep hatchback or estate/wagon type car then it isn't a really a problem and you get a big downforce benefit. It's actually one of the reasons TWR ran a Volvo estate in the BTCC a decade or more ago.

[AU N EGL]

Originally Posted by meb

I have the 05 JCW - better gearing, more power, same great taste. It would probably see the up side of 140-145 mph, but not on a track; there isn't one with a straight long enough . If by mechanical dynamics you mean the removal of stuff like a faux wing and mirrors, I'm all for it.

You dont hit 145 at Watkins Glen? Then you need to do Pocono Long course or come down to my neck of the Woods and Run VIR.

[rcarr]

meb, what I mean is, if you want to increase top speed you will have to lower the frontal area and all the large openings, but as locost has just said you cannot jeopardise the cooling for straight line speed.

As for mechanical dynamics, if you want it to go quicker, concentrate on cornering speed, ie mechanical grip. A fast car is useless if it can't corner well.

I am a car designer, if you want to become a race car designer, don't go through an artistic routeway, do an engineering degree. If you want to become a stylist, do a car design degree but then you can only dabble in aerodynamics. I have tried to get into race car design but without an engineering background, I have not been taken seriously by people in the industry!

[AU N EGL]

Originally Posted by rcarr

meb, what I mean is, if you want to increase top speed you will have to lower the frontal area and all the large openings, but as locost has just said you cannot jeopardise the cooling for straight line speed.

As for mechanical dynamics, if you want it to go quicker, concentrate on cornering speed, ie mechanical grip. A fast car is useless if it can't corner well.

I agree

I am a car designer, if you want to become a race car designer, don't go through an artistic routeway, do an engineering degree. If you want to become a stylist, do a car design degree but then you can only dabble in aerodynamics. I have tried to get into race car design but without an engineering background, I have not been taken seriously by people in the industry!

Dont most aerodynamists come from the aviation industry first? i.e. airplane design engineers

[rcarr]

Yes! I didn't specify aerodynamicist or anything, I said race car designer. And I said engineering degree, I didn't specify which discipline, be it aeronautical or mechanical.

Saying that, John Illey, Chief Aerodynamicist of Scuderia Ferrari, is from the same background as me, same degree course and uni.

[Locost47]

"Dont most aerodynamists come from the aviation industry first? i.e. airplane design engineers"

I think in F1, yes, but in the rest of the auto industry it seems to be the case that mechanical engineers often get moved or choose to move into aero roles and then pick a lot of it up as they go along.

I did Physics originally at Uni then realised my fate would probably in either computer programming or accountacy so did a Masters' in aero before trying unsuccessfully to get into F1. I then spent 8 years doing aero consultancy on road & race cars of all kinds plus trucks, buildings and all sorts of random things (like bird tables!).

As of next week though, i will finally have achieved my goal since i start as an Aerodynamicist at Williams F1. It's like the American Dream, only British.

[Chris Y]

Congratulations, Locost

[Locost47]

Thank you HypnoToad, err, i mean cheers Chris.

[phoenix]

Yes, well done Locost! But I expect you will have to be a bit more tight lipped about aero suggestions outside the office from now on......? Which is a shame, because I had another question for you!

[meb]

No, but I also don't know this course as well. My only opportunity is thru the esses and into the bus stop - wicked fun that area!!! I ran out of fuel thru the esses my last time there...thought I blew the motor, given the change in elevation and Gs. I was obviously relieved!

Originally Posted by AU N EGL

You dont hit 145 at Watkins Glen? Then you need to do Pocono Long course or come down to my neck of the Woods and Run VIR.

rcarr...unfortunate...because the first man didn't have a degree either. I've been in my business for 33 years...the individual with the most spirit (excitment) is the the one I go after. You obviously love what you do and that should, at some level, matter...

[meb]

Locust47,

Can I send you a PM?