Where to find info/discussion on aerodynamics

[kalniel]

Google has failed me, so I wonder if anyone here knows a good place on the interweb that I can find to read up on/discuss aerodynamics of non-race cars? As far as I can tell, it's practically ignored for all intents and purposes.

Cheers.

[midzt]

Some very basic stuff here:

http://www.autozine.org/technical_sc..._aero.htm#Drag

The only place I think I've really come across aero stuff on road going cars.

[Akwaaba]

I guess these aren't much use?

http://www.mustangsandmore.com/ubb/D...odynamics.html

http://www.edmunds.com/advice/specia...4/article.html

and that you already found these!

[kalniel]

Yup I've already read the autozine and the mustangandmore articles - the edmunds one is new but isn't any good.

The excellent mustangandmore one is very good about most of the theory (including debunking some of the info in the autozine article - tubulence does NOT equal drag!) , but unfortunately doesn't really supply any answers other than 'here's a way you can test changes to your own car' which is neat, but I'm more interested in theoretical musing of advantages before I even went as far as to make a change to a car

Maybe we should start discussion here. Anyone have a pet aerodynamicist they can loan us?

[chriswood_7]

Hi guys,

Im NOT an Aerodynamicist but I studied Aerodynamics of a Race Car for my Final Year Project when at Uni (3 years ago now).

Basically, the biggest force acting on a car up until around 60mph-ish is rolling resistance (i.e. tyres and bearings etc) so generally up until this point Aero is ignored. If your wanting to add bits and bobs to your own road going car, my advice would be not to bother. Adding parts will generally add drag to your car, and add no value whatsoever to non-competition driving (and it will eat up your fuel)

Hope this is of some value,

[kalniel]

Thanks, yup I'm aware rolling resistance is a major factor, though I thought aero played a bigger factor sooner than 60.

I'm certainly not looking at adding any supposedly downforce generating things to my car - no point whatsoever, however I was musing about what's happening to the airflow of my astra, specifically around 60-70mph. Namely, I do get a pretty sharp increase in drag at 70 compared to 60, and while of course drag increases at square of speed or whatever, the difference seems more noticable than in some other cars I've driven.

In terms of MPG it's quite a difference, 3/4s of a tank of mostly motorway driving gets me about 61mpg. 3/4s of a tank of mostly B road driving (which includes lots of corners, braking, accelerating etc., unlike motorways) still gets me about 65mpg.

[Vimeous]

Funnily enough we recently had a similar discussion on Honda Revs.

It's been even longer than Chris since I dabbled in Aerodynamics at Uni but....

It's as Chris says until you get a higher speed when shape becomes everything.

Take the front of a car, cutting all those holes in the front for the rad etc doesn't help fuel economy. Shaping the ducts correctly etc can minimise the drag caused and even be used to suck more air into the car from brakes for example.

Remember the first Audi TT's? They had a tendancy to vanish off the road backwards at 130+.
The primary reason (as I understand it) was rear lift caused by the shape of the rear being too curved. Smooth airflow over the car did not break away until too far back at high speed, causing a pressure imbalance making the rear wheels 'go-light'.
Adding a spoiler makes the airflow breakaway from the car much earlier and creates a high pressure area over the rear wheel keeping them planted.

Reading it back I didn't get it quite right.
On a TT the pressure difference between the smooth flow at the cars rear surface and the air under the rear axle generated lift.

The spoiler they added created a high pressure area over the rear axle and reduced the smooth airflow.

It might be worth noting the current Civic's wing is the result of the trade-off between slippery smooth airflow and the need to

--------------------------------------------------------------------------------
Originally posted by Jonny
seperate trailing vortices inorder to reduce drag
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.
Odds on it having a decent cd factor.

I've a few books at home that might help but maybe the following will be of use?:

http://en.wikipedia.org/wiki/Drag_coefficient
http://www.mayfco.com/tbls.htm

The references from Wikipedia may be most useful

[chriswood_7]

Thanks, yup I'm aware rolling resistance is a major factor, though I thought aero played a bigger factor sooner than 60.

I'm certainly not looking at adding any supposedly downforce generating things to my car - no point whatsoever, however I was musing about what's happening to the airflow of my astra, specifically around 60-70mph. Namely, I do get a pretty sharp increase in drag at 70 compared to 60, and while of course drag increases at square of speed or whatever, the difference seems more noticable than in some other cars I've driven.

In terms of MPG it's quite a difference, 3/4s of a tank of mostly motorway driving gets me about 61mpg. 3/4s of a tank of mostly B road driving (which includes lots of corners, braking, accelerating etc., unlike motorways) still gets me about 65mpg.

Hi again,

The more parts you add to the car, the more you generate downforce and drag (all downforce generates drag). How can you tell that there is a significant increase in drag at 70 to 60? Could there be a flatspot in your engine torque curve, where you notice it more at this speed?

I know a lot of big car companies now consider whats called "Under-hood Analysis" when designing a car, i.e. the flow around the engine bay and underneath as they are now the main sources of drag. If you notice now a lot of the newer high performance cars are coming with smoother sump guards and diffusers? Also a lot of them are designing flow vents into body work for the air to flow out of the engine bay. Maybe a good example of this is the new 197 Clio?
Hope this helps,

[midzt]

I'm not too hot on aero stuff, or any other stuff if I'm honest!

Could it be that its between 60 and 70 mph that the flow separates? This would mean that drag increases fairly dramatically.
You mite be able to work out the Reynolds numbers for the different speeds, and see if this is true, is it that flow turns turbulent at Re = 500,000?
This could be a load of rubbish btw.... I can't remember anything that I supposed to have learnt :S

[chriswood_7]

I dont think its anything that complicated, the car designers at Vauxhall would not design something that induced more drag at 60-70mph, as generally the car will be at this speed most of its life. Calculating the point in which flow seperates is generally different for various different applications and relies on many assumptions.

[kalniel]

Hi again,

The more parts you add to the car, the more you generate downforce and drag (all downforce generates drag). How can you tell that there is a significant increase in drag at 70 to 60? Could there be a flatspot in your engine torque curve, where you notice it more at this speed?

According to the charts for my type of engine there aren't - but no, the way I notice is simply the sharp decrease in fuel economy when I compare smooth motorway travel with much more 'wasteful' country driving - the only thing in favour of the country driving is that I spend most of my time at near 60mph, wheras that's 70-75 on the motorway. This 10-15mph difference seems to directly translate to 4-5 less miles to the gallon, even though there is no energy being lost by other forces on the car.

*shrugs* maybe the increase is purely down to the normal increase in drag at speeds. Just seems weird that it's so high. Perhaps others cars torque peaks later so counter-acting this a little?

Anyone know the equations?

cd of the astra is 0.28, torque is 165Nm (apparently.. feels like more!) flat from 1800-2800rpm (60-75mph is within this range). Don't know what the frontal area is - usual car sized!

[chriswood_7]

Well drag is 1/2*Air Density*Front Area*Velocity^2, and If you know the exact CD then you could technically work out the force on the car at +5mph intervals.

[Zak33]

According to the charts for my type of engine there aren't - but no, the way I notice is simply the sharp decrease in fuel economy when I compare smooth motorway travel with much more 'wasteful' country driving - the only thing in favour of the country driving is that I spend most of my time at near 60mph, wheras that's 70-75 on the motorway. This 10-15mph difference seems to directly translate to 4-5 less miles to the gallon, even though there is no energy being lost by other forces on the car.

*shrugs* maybe the increase is purely down to the normal increase in drag at speeds. Just seems weird that it's so high. Perhaps others cars torque peaks later so counter-acting this a little?

I'm confused.

Are you getting better, or worse, fuel consumption on motorways?

[kalniel]

Worse

[midzt]

Could it not just be down to the efficiency of the engine at the different engine speeds?

[this_is_gav]

Basically, the biggest force acting on a car up until around 60mph-ish is rolling resistance (i.e. tyres and bearings etc) so generally up until this point Aero is ignored. If your wanting to add bits and bobs to your own road going car, my advice would be not to bother. Adding parts will generally add drag to your car, and add no value whatsoever to non-competition driving (and it will eat up your fuel)

If you rode push and even mountain bikes a lot, then you'd know just how wrong that is. Even the slipstream effect makes a significant difference at around 15-20mph. As a kid we used to do a lot of biking, and have slipstreaming races down hills. Rolling resistance does make a difference of course, but at anything beyond 20mph it's totally unrecognisable from the other forces acting upon the object unless you're using equipment to measure those forces.

Amateur aerodynamicist here too.