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 Wind Tunnel Project (currently 6,177 views) |
| Graham Bichard |
| Posted on: December 17th, 2014, 10:08:47 |
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Maximum Member2
Posts: 751
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Got a couple of questions I'm hoping people on here can help me with.
I've constructed a mini wind tunnel for a college project with the aim of investigating the aerodynamics (lift) of a mini marcos:
From the photo you'll be able to see the general set-up of the tunnel. From the frame on the left (filled with straws to create a laminar air flow) you'll see I've had to cut down the scale of the tunnel - my vacuum box (from my flow bench) wasn't able to produce the air speed I was after.
You'll also I'm sure, be familiar with the car model I'm attempting to use:
The car I'm actually using I've filled with blue-tac in order to give the car more weight (records better on the strain gauge) and to fill in the door window area. You can also get an idea of the aerofoil shape of the side profile of our little plastic cars.
However, the first readings I've recorded with this set up has given results exactly the opposite of those expected!
So to the questions:
Does anyone have records of the corner weights for their cars? I'd like to build up a table of evidence showing the rear of the cars are lighter than the front, and also by how much.
Does anyone have any stories/anecdotes/experience regarding the rear of the cars becoming 'light' at speed?
Thanks. |
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| Steve_Schmidt |
| Posted on: December 17th, 2014, 10:41:49 |
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Medium Member
Location: Gippsland, Victoria, Australia
Posts: 167
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Interesting wind tunnel project.
Corner weights for my Marcos are:
Front Left 191 kg
Front Right 182 kg
Rear Left 107 kg
Rear Right 118 kg
Please keep us updated on your results. |
Steve (MM 7056) Downunder
http://www.mm7056.wordpress.com |
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| jimnaylor |
| Posted on: December 17th, 2014, 13:52:40 |
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Big Member
Location: Bedfordshire
Posts: 222
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About 120 is the fastest I've been in mine and no noticable lift at that speed.
You might try filling in the underside to get it smoother and more closely resemble the MM. Flow under the car is very significant for lift. But it wouldn't surprise me if there was no lift despite the apparent aerofoil shape. The winscreen produces significant +ve load on almost all cars and the roof despilte appearing areodynamic is almost certainly too steep for the flow to remain attached, so the air will stall, killing any lift. You should be able to test that last point in the wind tunnel using smoke.
I'll be very interested in your results. |
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| Graham Bichard |
| Posted on: December 17th, 2014, 17:31:27 |
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Maximum Member2
Posts: 751
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Thanks Steve, Jim.
It's the underside of the model which I think may be causing the discrepancy from what I expected.
Rather than trying to smooth the model, I think it may be a case of an actual car being worse in this respect.
By this I mean - the flat-ish floor produces a venturi effect, but where the rear subframe attaches the air can expand to fit into the available space, and then hits the front face of the fuel tank (causing drag and possibly lift). The model doesn't suffer this.
I had thought of creating a flat piece to attach (via dzus fasteners perhaps) to the bottom of the subframe as a way of retaining a flatter floor on the car. Also of creating a 'sump guard' type arrangement for under the engine (probably with a cut out for the gearbox/sump). This would've probably been the conclusion to counteract the lift I was expecting to see  .
I guess the model just isn't accurate enough!
As for the air travelling over the top of a car, my reading suggests the base of the windscreen produces a high pressure area, and where the air travels over the top of the windscreen/over the lip of the roof produces a low pressure area (lift).
All those mini owners with raised rear bonnet edges (presumably to aid cooling) - it isn't actually letting air out, but the high pressure area pushes air in, which will travel down the bulkhead and exit by the steering rack under the car. Introducing the air in this area should also increase lift.
I still welcome all corner weights and experiences, and I will keep all updated with any findings (perhaps even a magazine article, similar to my flow bench experiments  ).
ETA Vizard states in his engine book that air can cope with a sharp change of direction ~15 degrees without adversely affecting attachment too much (from memory). Given the curve of the roof and the size of an air molecule I would expect the air to remain attached over the length of the roof (but you're right - smoke should show this. Need to get a smoke machine!)
Another thought - why would you need this if there is no real amount of lift:
http://www.rory.uk.com/MK%20VI%20Extras.html (I'm talking about the rear spoiler).
This will presumably produce more downforce the faster you go, but also more drag. Given that I don't think any MM will be going through Eau Rouge at 130mph+, I'm not sure the benefits outweigh the disadvantages? |
Last modified December 17th, 2014, 17:38:38 by Graham Bichard |
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| jimnaylor |
| Posted on: December 17th, 2014, 22:54:49 |
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Big Member
Location: Bedfordshire
Posts: 222
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Regarding stalling airflow see http://en.wikipedia.org/wiki/Stall_(fluid_mechanics) or search for a more reputable source. But it looks accurate. I think the back of a MM is well steep enough to generate stalled airflow, but only testing will actually tell. Let us know when you have any results.
Frank Costin in all his designs (not that the MM is one of his) was fanatical about getting a flat floor to aid aerodynamics, and I know got annoyed with Vauxhall when doing work for them because they refused to smooth out the underfloors... now of course the eco versions of Vauxhalls have flat floors, only about 40 years after he suggested it!
As you say there is room at the rear of the MM to fit a diffuser, and I did think about it a while ago but it ended up on the 'not enough time' pile. But it should work. A couple of people have added diffusers to the original Lotus Europa in that way with apparently good results.
You might want to look at this for model sensitivity http://www.lotus-europa.com/manuals/misc/aero/Europa%20Aerodynamics.htm |
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| jimnaylor |
| Posted on: December 17th, 2014, 23:06:58 |
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Big Member
Location: Bedfordshire
Posts: 222
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| Just remembered the main reason I didn't pursue the diffuser. The big problem with a rear diffuser on a MM, is it's FWD. A down force generating rear wing or a rear diffuser, if it generates downforce, it might help high speed lift but it will almost certainly make cornering worse. The main reason FWD cars understear is because there is more available grip at the rear wheels than the front, the most common way to dial out understear is to reduce rear wheel grip (hence the +ve camber at the rear of a standard mini). Adding downforce at the rear increases rear grip........ so only if you have a super sticky front end will adding downforce at the rear help |
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| Graham Bichard |
| Posted on: December 18th, 2014, 16:55:16 |
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Maximum Member2
Posts: 751
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Jim, I think the only way to create a diffuser on the MM would be to extend a structure out the rear of the car. The diffuser rising allows the air that is travelling under the car to expand into a larger space thereby lowering the pressure (I know you know this, but in case anyone doesn't). This isn't exactly a vacuum, but as long as the pressure is lower than the air pressure above the car you'll get downforce.
With the fuel tank being at the very rear of the car I think any upward sweep of a diffuser structure would have to extend out the rear.
My idea was/is to put a 'board' under the gap in the middle of the rear subframe, the idea being to control the flow of the underbody airflow. As you correctly say, this is done in modern cars to improve economy and to an extent, noise.
I've also had another look at my car in the garage - there does appear to be a noticeable increase in the curvature of the roof once you get back to the hatch area.
As an aside, do you suffer from understeer much in your car? Do you have a +ve camber rear set up? And have you ever checked the corner weights on your rally car?
Glad I started this thread |
Last modified December 18th, 2014, 17:51:14 by Graham Bichard |
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| mike brown |
| Posted on: December 18th, 2014, 17:14:20 |
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Big Member
Location: Southampton
Posts: 419
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Myself and my brother spent a little time on this with his race car we moved the tank into the spare wheel well this allows fitment of a rear diffuser. We also spent some time on the front of the car with a big blower, video camera, some wool and quite a lot of talc powder. With this we discovered as you have the issues around lifting the rear of the bonnet. The main reason we did all this though was to perfect the shape of a bonnet air exit for the front rad.
I'll prod Allan to see if he can upload the video if he still has it, it makes interesting viewing.
Mike |
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| Graham Bichard |
| Posted on: December 18th, 2014, 17:43:24 |
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Maximum Member2
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That'd be great Mike. Is your car hillclimb, sprints, or circuit? (It's not the green one is it? Seems to ring a bell).
Looking over the Lotus Europa link, it demonstrates that it's more important to control the flow under the car than try and eliminate it.
This is where the protruding spitter comes in - as a generalisation, the longer the better. This prevents the air that hits the front of the car from spilling under the car, causing turbulence. The splitter allows laminar flow to continue under the car while directing the air above the splitter to flow down the sides of the car.
All interesting stuff.
Should add - if you look at the front of a 997 GT3, you'll see a black strip running across the top of the nose. This is an air extraction vent for the front. What solution did you come up with for cooling the MM? In this area I think you could direct the air quite sharply upwards towards the front of the bonnet area and use the high speed/lower pressure airflow over the bonnet to aid extraction of the hot air (i.e. the air exits before the high pressure area at the base of the windscreen). Easy to say in theory, not sure how easy it would be to make in practise!
I think a similar extraction system was used in the LM600 Marcoses and GT40's. |
Last modified December 18th, 2014, 17:53:39 by Graham Bichard |
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| mike brown |
| Posted on: December 18th, 2014, 17:59:35 |
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Big Member
Location: Southampton
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| Allan Brown |
| Posted on: December 18th, 2014, 18:49:36 |
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 Administrator
Location: Billingshurst, UK
Posts: 452
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I will try and find the video, but as we have just moved it is boxed up somewhere and it is on video so I might have to record it off the tv to get it in digital format.
I will try and find some photos of my old bonnet. and my rear diffuser.
Allan |
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| jimnaylor |
| Posted on: December 18th, 2014, 18:54:49 |
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Big Member
Location: Bedfordshire
Posts: 222
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I'll try and dig out my corner weights. I tend to run about -0.25 degrees negative at the rear but it does depend on the tyres. My car does not particularly understeer but I do have fully adjustable suspension front and rear so it (hopefully) is set up fairly neutral. On most rallies the surface is usually poor anyway so the finesse of setup required by circuit racers is not really required.
All FWD cars naturally understeer, the same as all RWD cars naturally oversteer. It's a function of the driven wheels having to share the available grip for both drive and cornering. It can be dialed out, ideally by increasing grip on the driven axle, but that tends to be difficult, so in most cars it's dialed out by reducing the grip on the non driven axle, which is comparatively easy. Adding rear grip almost always helps a RWD car, but can be a mixed blessing on a FWD car. But as with everything you only find out the effect on a particular car by doing it.
It will look better with a diffuser, and might have less drag. |
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| Graham Bichard |
| Posted on: December 19th, 2014, 12:01:21 |
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Maximum Member2
Posts: 751
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Thanks Allan.
With your car being for sprints and hillclimbs, do you think the speeds you achieve, you would 'feel' the benefit of a diffuser? (Forgive my ignorance - Prescott is the only hillclimb I've walked (Marcos 50th the other year)) I imagine there will be relatively few high speed corners, and if you're braking hard for tighter corners, the effects would be reduced with the lower speeds.
However, if speeds are generally that bit lower, I suppose you could run a high drag/ high downforce type set up. Ever experienced the rear trying to come round, or feeling floaty?
If the results of my experiments did end up showing what I expected (bearing in mind I've only done one 'run' so far) I was looking to play around with a Guerney strip to see if I could cancel out the expected lift as an alternative (more elegant?) solution to the Heritage spoiler, as well smoothing the underfloor/rear subframe area. Something like this:
http://www.tvrblog.com/wp-content/uploads/2010/10/tvr_t350t_tvr_unofficial_blog_3.jpg (Right colour car too)
Talking about understeer, I can't say I've noticed this as a problem in the many years of driving my mini (albeit running a reasonable amount of -ve camber at the front, and I don't tend to drive it as spiritedly as I used to). I appreciate manufacturers tend to design in understeer into mainstream car (my daily driver now the mini is in semi-retirement is a Merc diesel (RWD, heavy-ish engine in the nose and an auto to boot!)).
I think this could be the topic of a separate thread - once I've got a car on the road to experiment with!  - I read Alan Staniforth's book on suspension design a few years ago and found it interesting! |
Last modified December 19th, 2014, 12:04:06 by Graham Bichard |
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| Allan Brown |
Posted on: December 20th, 2014, 21:25:47
 Attachment: pict0063a.jpg - 134.46 KB (5163 views) |
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Administrator
Location: Billingshurst, UK
Posts: 452
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Unfortunately I can't find the video, but here are some photos of the bonnet and diffuser.
I remember when we did the bonnet we originally dipped the the whole front of the bonnet but found the air flowed better with about 5" of the front of the bonnet left in place.
As for the rear diffuser it was more about creating a smooth air flow under the car and therefore less drag than improving the handling.
As for speed on a hillclimb or sprint, I used to get 80mph at Gurston down and 115mph at Goodwood. |
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| Allan Brown |
Posted on: December 20th, 2014, 21:26:53
Attachment: dscf7404a.jpg - 146.80 KB (5139 views) |
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Location: Billingshurst, UK
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| front of rear diffuser |
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Mini Marcos Forum > General Boards > Chit - Chat > Wind Tunnel Project
