Over and Under or around the sides

Post here information about your own engineering projects, including but not limited to building your own car or designing a virtual car through CAD.

Post Wed Apr 22, 2009 7:33 pm

I've been working over the Off season on my latest Electric Car and underneath the chassis is by far the most efficient I've made.

Due to time restraints though we've been unable to mould the bodywork as planned and have been forced to make a temporary ply body in Brawn colours. To do this we used some templates made for the mould and fibreglass taped them to some 4mm ply.

ANYWAY!

My question is;

Is it better to direct air over and under the car in a cambered aerofoil (60mm ground clearance) or is it better to direct it round the sides. I've always been lead to belive the sides but recent figures have made me think over and under could be better.

What do you all think?? i'll (tryyy) to post some pictures later, hopefully you BrawnGP fans will approve :P
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greenpower dude reloaded
 
Joined: 29 Dec 2005
Location: Portsmouth, UK

Post Wed Apr 22, 2009 7:57 pm

Hmmm, I guess it depends.

Road cars prefer around the sides, I guess thats because it is better than the underbody and going over a steep windscreen.

For an aerofoil though I believe over and under would be best :)!
PNSD
 
Joined: 3 Apr 2006

Post Wed Apr 22, 2009 8:44 pm

It is about where it would fit right.

We dont know how the car looks so we cant say it actually, if it is an formula style car it is about where the air is coming from and where it is flowing.
"Bite my shiny metal ass" - Bender
wesley123
 
Joined: 23 Feb 2008

Post Wed Apr 22, 2009 9:14 pm

Your quite right wesley here are a couple of pictures

Image

Image

Unfortunately I haven't got any better pictures than that but if you use your imaginations a bit.

Imagine the tail (shown in the top pic) attached to the rear and the top fully attached.

also worth noting is that the body is still awaiting a bit more paint
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greenpower dude reloaded
 
Joined: 29 Dec 2005
Location: Portsmouth, UK

Post Wed Apr 22, 2009 9:26 pm

It depends on what you want to achieve. If it's downforce you want and your underbody is really smooth, you might want to take advantage of the venturi effect by sealing the sides and directing and accelerating the air underneath the body. If your underbody is exposed, then you might actually want to seal off the front of the underbody as well, in order to direct the air around and over the car.

Regarding the question of directing the air around rather than over the car; unless you use dams or winglets, you just might notice that the air will tend to do as it damn well pleases ;).
Gecko
 
Joined: 5 Sep 2006

Post Wed Apr 22, 2009 10:26 pm

Sorry I suppose I ought to tell you a little bit about the series.

Basically we're given 4 12v 75ah(50ah in reality) batteries only being allowed two on the vehicle at any one time.

We're also given a 240watt motor which we run nearer 500w and travel at speeds of up to 45mph. The greatest distance covered over 4hours wins. So its purely an efficiency challenge. So no downforce required. we almost have too much grip as it is anyway!

We're no longer allowed fully enclosed cars so im looking for the lowest drag shape possible with four wheels.
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greenpower dude reloaded
 
Joined: 29 Dec 2005
Location: Portsmouth, UK

Post Thu Apr 23, 2009 12:57 am

How many decks does your diffuser have? :)

It looks to me in your case that there would be more work involved, and more weight, to get the air directed around the sides.

no idea if those factors would cancel out gains from directing air around the sides.
Before I do anything I ask myself “Would an idiot do that?” And if the answer is yes, I do not do that thing. - Dwight Schrute
Giblet
 
Joined: 19 Mar 2007
Location: Downtown Canada

Post Thu Apr 23, 2009 1:06 am

That general body shape looks like it will generate significant lift.

A la VW Beetle.

I wonder if putting vortex generators on the top would help, as given how abruptly the rear tapers down I'd think it would generate a decent wake as it is.

Also, given that you're not looking for DF maybe running a Nascar-style splitter on the front to direct all the air around the car would cut down drag.
Last edited by Jersey Tom on Thu Apr 23, 2009 1:13 am, edited 1 time in total.
Grip is a four letter word.

2 is the new #1.
Jersey Tom
 
Joined: 29 May 2006
Location: Huntersville, NC

Post Thu Apr 23, 2009 1:11 am

That would be good if he is going for efficiency, and considering he has an excess of grip at this time....

Crank the tire pressure and float on contact patches the size of dimes.
Before I do anything I ask myself “Would an idiot do that?” And if the answer is yes, I do not do that thing. - Dwight Schrute
Giblet
 
Joined: 19 Mar 2007
Location: Downtown Canada

Post Thu Apr 23, 2009 10:49 am

Vortex generators could indeed make a difference thats a very good point, it doesnt actually taper away quite as quickly as you think, I think when I worked it out it was within the general 15/16deg(depending on who you ask) rule which states anything greater than that will cause turbulence. It could be foreshortenning that makes it appear steeper and of course the fact the tail isnt really attached it will follow the line of the rest of the body when all together.

I would take more pictures but i'm 150miles away at the moment. I'll get some more on sunday when it hits the track.

I agree it will generate a fair amount of lift but not enough to cause a problem i darent say as its designed to average about 35mph over 4hours at Goodwood.

The reason I ask you all about the shape in the first place is that being so far away from the car and having very limited testing pre bodywork, I'm desperately trying to work out how I can expect it to place on sunday all being well.
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greenpower dude reloaded
 
Joined: 29 Dec 2005
Location: Portsmouth, UK

Post Thu Apr 23, 2009 12:18 pm

There seems to be a large enough radius at the front, which may give a bit of drag, if that was to replaced with a sharper edge, it may lower the drag at the front.

John
johnny99
 
Joined: 9 Apr 2009
Location: Killucan Westmeath Ireland

Post Thu Apr 23, 2009 6:05 pm

Image
the nose is quite rounded, yes, but the thinking behind that (after years of cars with square edges in that place) was that rather than force air a particular way let it choose its own natural path a little easier or does that all just sound a bit "new age"?? The tail does turn upwards the same amount at the rear to equalise pressure.

Here's another slightly more together picture..
Image

... I know still no tail on there
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greenpower dude reloaded
 
Joined: 29 Dec 2005
Location: Portsmouth, UK

Post Thu Apr 23, 2009 6:43 pm

greenpower dude reloaded wrote:I think when I worked it out it was within the general 15/16deg(depending on who you ask) rule which states anything greater than that will cause turbulence.


I thought that was in regard to venturi's and a 15/16 degree included angle, which would be 7-8 degree in your case.

I could be wrong. Plus I'm not sure what the Reynolds # would be in this case.
Grip is a four letter word.

2 is the new #1.
Jersey Tom
 
Joined: 29 May 2006
Location: Huntersville, NC

Post Thu Apr 23, 2009 7:34 pm

johnny99 wrote:There seems to be a large enough radius at the front, which may give a bit of drag, if that was to replaced with a sharper edge, it may lower the drag at the front.


The skin friction drag due to accelerating the air over a more curved surface near the front is really negligible in the current design.

The biggest sort of drag you can expect from the shape as shown is the vortices generated from the air spilling detaching at the sharp angle at the sides near the front, and similarly at the back. It might be worth taping some threads on the car and see where the flow detaches.

For this reason, it might be worth extending the side panels near the front and the rear a bit in order to create fences to better direct the air over the car, but again, just how far to go with that isn't easy to figure out.
Gecko
 
Joined: 5 Sep 2006

Post Thu Apr 23, 2009 9:45 pm

Image
to give you a bit more of an idea, this was last years car. we had a bit more time then to make a fibreglass body but clearly not enough to do it all.

As you can see underneath was really rough and there were bits sticking out of the car everywhere. The ground clearance was also much lower, about 30mm but it would seem a low ground clearance isn't always the best course of action.

anything 15mm and under is brilliant as it stops air going underneath unfortunately rules do not allow this. The minimum is now 40mm so it's better to raise it off the deck a little bit and allow it to flow through. I suspect that there was a huge amount of interference drag under the previous vehicle... What do you think??
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greenpower dude reloaded
 
Joined: 29 Dec 2005
Location: Portsmouth, UK

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