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Old 22nd March 2013, 14:50   #1
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Default Wind Tunnel testing of Cars

I hear that Maruti are in the process of setting up a wind tunnel for cars. No information about the size, etc.

I was at the inauguration of a workshop on Wind Tunnel Testing this morning. This was being organised by our National Facility which has a 3m low speed tunnel (upto 90m/s).

The keynote address was delivered by Prof Olivier Cadot from France, whose lab has both a full size and a 40% size tunnel for automotive testing. They do testing for both PSA and Renault, amongst others.

I am not an aerodynamics / wind tunnel man so this is what I could comprehend. Some interesting information:

1. At 80kph about 50% of the power goes to overcome the air drag. This figure rises to 80% at 130kph.

2. There is a lot of work going on in an attempt to reduce the drag from ~0.33 (best is just under 0.30) at present to 0.20 in the future. gas guzzler tax is a big incentive!

3. Of what does not go into overcoming the gross resistence above, about 10% goes into cooling, 5% to door mirrors, 25% is for underbody friction, and 30% overcoming the drag at the rear. So this explains why there are major efforts to make the underbody smooth and the rear end generate less turbulence.

4. Another interesting finding, there can be instability a the rear under certain conditions. The drag is not symmetrical and constant, as one will expect, but due to some instability, partly oscillates between the left and the right. This can result in the car trying to zigzag and is obviously not good. I wonder whether the latest Ferrari has two 'aerofoils' behind the glass on either side, with the glass / body in between? Is this an effort to reduce / minimise this?
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Old 22nd March 2013, 16:29   #2
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Default Re: Wind Tunnel testing of Cars

Quote:
Originally Posted by sgiitk View Post

I am not an aerodynamics / wind tunnel man so this is what I could comprehend. Some interesting information:

1. At 80kph about 50% of the power goes to overcome the air drag. This figure rises to 80% at 130kph.
Sir, it is known, that drag resistance increases with speed ( drag is a function of v*v), but 50 % at 80 kmph, how is this figure arrived at?

Is this for cars with poor drag coefficient?

Quote:
4. Another interesting finding, there can be instability a the rear under certain conditions. The drag is not symmetrical and constant, as one will expect, but due to some instability, partly oscillates between the left and the right. This can result in the car trying to zigzag and is obviously not good.
Could not understand this part.

Spike
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Old 22nd March 2013, 22:32   #3
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Default Re: Wind Tunnel testing of Cars

OT
Hi Spike,
Given the name, thought it would interest you.
Long time back, Ford (England) had launched a car called the Scorpio. Very slippery for a family saloon of its day. Was unpredictable in crosswinds. Solution was surprisingly cheap, a couple of plastic aerodynamic add ons. The actual price was that it significantly raised the CoD, negating the millions Ford had initially spent to get it slippery!

Regards
Sutripta
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Old 22nd March 2013, 22:45   #4
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Default Re: Wind Tunnel testing of Cars

The drag is proportional to velocity squared. From what I have read, most of the energy is spent clearing the air by the time car has crossed 60 kmph.
The second generation Honda city had Drag Coefficient of 0.29cd.

A few merc and lexus have 0.26cd, which is very low for street legal regular series production cars.

F1 cars have a lot of drag due to open wheels. Hence the reason for semi closed rear wheels in older cars like standard herald.

Please correct me is I am wrong.
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Old 22nd March 2013, 23:17   #5
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Default Re: Wind Tunnel testing of Cars

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Originally Posted by Sutripta View Post
The actual price was that it significantly raised the CoD, negating the millions Ford had initially spent to get it slippery!
Thanks for the trivia. Getting the drag coefficient around 0.2 will be one challenge. Will cars start looking more like aeroplanes? I have no idea, may be you may know, which car till date has the best CoD values?

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Old 23rd March 2013, 02:06   #6
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Default Re: Wind Tunnel testing of Cars

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Originally Posted by aaggoswami View Post
F1 cars have a lot of drag due to open wheels. Hence the reason for semi closed rear wheels in older cars like standard herald.

Please correct me is I am wrong.
Some of the drag is actually intentional. It is utilised to generate downforce- keeping the car planted on the ground, and allowing it to corner at speeds otherwise impossible.

Wikipedia Link

Wind tunnel testing is good news for cars designed here. However, I have doubts how much it will affect design- unless I am wrong, most automobile manufacturers here use designs from other subsidiaries\parent companies in other countries. Prime example- Maruti using Suzuki designs.
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Old 23rd March 2013, 03:05   #7
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Default Re: Wind Tunnel testing of Cars

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Originally Posted by SPIKE ARRESTOR View Post
which car till date has the best CoD values?

Spike
Maybe this will help. http://ecomodder.com/wiki/index.php/...t_of_Drag_List
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Old 23rd March 2013, 11:05   #8
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Default Re: Wind Tunnel testing of Cars

Drag actually goes up as the fourth power of the velocity in streamlined flow. 50% at 80 kph is from their measurements, and should be an average.

@spike; The had monitored the rear end drag/turbulence (multiple sensors) as a function of time. The drag was not constant but one end and then the other was higher (and the opposite lower) at below 1Hz. The total / average was the same.
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Old 25th March 2013, 00:05   #9
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Default Re: Wind Tunnel testing of Cars

Hi All,

I did a coursework on Aerdoynamics in my MS, I have attached a pdf for the interested ones, not a very heavy technical report, but a good read with explanatory graphs.

Also, the drag force experienced by the car is

Aerodynamic loss = 0.5 * rho * A * Cd * (u*u)

rho = Air viscosity (typically 1.225)
A = frontal area of the car in square metres
Cd = drag co-efficient of the car
u = speed of car in m/s
Attached Files
File Type: pdf aero_corsework_report.pdf (137.0 KB, 803 views)
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Old 25th March 2013, 00:31   #10
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Default Re: Wind Tunnel testing of Cars

Hi dgupta, nice to have you here. Cross posting w.r.t another thread on tbhp, your thoughts would be welcome on this thread, specially on air flow.

http://www.team-bhp.com/forum/4x4-te...mperature.html (Hood Vents - Do they work in lowering temperature?)

Spike

PS- If you could come up with some CFD simulations, that would be awesome.
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Old 25th March 2013, 00:56   #11
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Default Re: Wind Tunnel testing of Cars

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Originally Posted by SPIKE ARRESTOR View Post
Spike
PS- If you could come up with some CFD simulations, that would be awesome.
Hi Spike, thanks for the pointer, I will go through the entire thread and see if I can dig some more info. As for CFD, i am far from understanding anything beyond what's in my aerodynamics report, I am an electrical guy, just managed to gather that much and get good grades

Having said that, I will find some time to get more info and see if one of my classmate who did his dissertation in aerodynamics has anything he can share with me and I can post here with his permission. He was the rockstar CFD dude in our class!
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Old 25th March 2013, 04:05   #12
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Default Re: Wind Tunnel testing of Cars

When they were trying to build hypersonic aircraft, they were toying with the idea of polarising the air in front of the aircraft and then pulling it over the aircraft using alternating magnets [much like mag lev technology, but with charged air rather than a magnetic track].

That would have aided the acceleration and reduced the drag, doubt that could be installed in cars though... ionised air on the roads could hamper pedestrians and definitely electronics.

Another thing they did was to have a slightly porous material covering the surface of the wings, where by the relatively slow moving boundary layer would be sucked into the wings. That greatly reduced the drag, but maintenance was prohibitive as the pores would get clogged.

Always wondered if they made a porous underbody for a car with significantly larger holes and had some sort of passive suction device that diverted the air to the rear of the vehicle, then not only would ground force increase, but drag would reduce also...

Vortex generators also reduce drag, having a couple at strategic areas could reduce drag also.

Another thing i was wondering if they have alloy wheels where the spokes act like fans sucking the air out from under the car and pushing it outside could also reduce drag, improve cooling of the disc brakes and improve downforce.
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Old 25th March 2013, 09:52   #13
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Default Re: Wind Tunnel testing of Cars

Unfortunately, the air flow behind the esp. a hatch or worse is totally turbulent, so a lot of stuff we are discussing becomes irrelevant. This is why a boot top spoiler is more cosmetic than real. On the other hand the small lip on top of the rear end of a hatch is very effective.
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Old 4th May 2013, 04:22   #14
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Default Re: Wind Tunnel testing of Cars

The one and only car which I have ever been able to drive safely at speed (over 160 kph) in gusting and strong side winds was a Citroen CX (manufactured from 1974-1990). The French car company Citroen had a very long history of high quality engineering, not least in vehicle aerodynamics with as much attention to aerostability in a range of circumstances as the simple drag coefficient. They were interested in this aspect of a car as far back as the early 1930s.

Radically different steering and suspension (both the medium and layout) also helped contribute to this other-worldly experience which I miss to this day. The stability at high speed was mesmerising in still conditions, too - especially through corners. I have left many a fast BMW behind through fast curves - there aren't many straight roads at 200kph+!
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Old 6th May 2013, 09:23   #15
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Default Re: Wind Tunnel testing of Cars

@Flatout; The DS from the fifties will be considered high tech even today, Hydropneumatic suspension, variable ride height, auto levelling, etc. The Two Horses (2CV) also boasted a linked rear and front suspensions to even out the bumps. I saw on a channel that Bentley used their dampers in their suspensions. Viva La France!!
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