Drag effects at low speeds?

[aerohead]

Originally Posted by examorph

Hello, I am back, this project has not been started yet and I have been very busy lately so have had no time to do much extra work on it but now I finally have a bit more time to spend thinking about how I should design the car.
I have modelled the car and have ran some wind tunnel tests on it and got some results but dont really know how to read them....the wind tunnel test results show me the drag coeficient (what I wanted) but this is on a graph over time, as time pass's the drag coeficient changes, the velocity of the air going over the vehicle also changes, this is strange because I set the value for the air speed and it should be constant, so I dont understand why the drag coeficient and air speeds are changing, any ideas?

The graph I see is the exact same graph as the one shown at the bottom of this picture:

http://gfxspeak.com/wp-content/uploa...6541711650.jpg

and here is the 2d slice air flow image I see constantly changing with time:

http://gfxspeak.com/wp-content/uploa...6541170698.jpg

Please note, these images were from google and are not of the car I have designed, I will get some images of that soon.

For vehicles the size of an automobile,their length as associated with velocity renders a transition to turbulent boundary layer at around 20-mph (32 km/h).The air around a road vehicle is considered turbulent itself and Reynolds number acts accordingly.
For this reason,coefficients of aerodynamic drag are constant above 20 mph and do not vary.So I'm as confused as you are about some of the results you are getting.
CFD modelling for 3-Dimensional flow requires the complete Navier-Stokes solution which may not be included in your software package.Not long ago,it would require supercomputers to solve these 3-D 'problems.'

[examorph]

Thank you for the reply, I spent some more time playing around with the software and found that I was wrong, the speed the vehicle is going is not constant, it is accelerating or slowing down from its previous speed in real time meaning it takes time for the speed to become the one you tell it.

Now that I have a drag coefficient I am again confused, I am getting low numbers for the car I have modelled and similar ones I modelled in the past, the drag coefficients are ranging from 0.5-0.7, from comparing them to real size vehicles these are very very small numbers but it's understandable because the vehicle size is also a lot smaller, the size of the vehicle is only about 1.5m length x 0.8m width and 0.5m high.

The two questions I have are, do you think these drag coefficients are correct for the vehicle size given and if they are correct wouldn't that mean that the drag coefficient of this vehicle is so low that the motor running it will not be able to get to the speeds required for air resistance to slow the vehicle down allot. If this is also true then wouldn't that mean that I shouldn't really have to worry about aerodynamics and my main goal should be making the vehicle as light as possible?

[Frank Lee]

Coefficients by definition don't consider size.

[examorph]

I am confused now, because I thought that the larger the vehicle the larger the drag therefore the larger the coefficient?

[MetroMPG]

Drag increases with size because the frontal area increases. Total drag = Cd * A (projected / frontal area).

The coefficient of drag is the same for two identical shapes regardless of whether one is larger than the other.

[examorph]

So is a change of about 0.1-0.3 in the coefficient of drag a big change?

[MetroMPG]

Define "big".

(Yes, that probably qualifies as big.)

You can view the impact of changes by comparing results from the aero & rolling drag calculator

[examorph]

thank you for the calculator, very helpful

New question , I asked this before but after researching more I ask again, what shape should I make the car? I ask this again because I made a tear drop shape for a body around the car and tested it in the wind tunnel, the drag coefficient was about 0.5, I read on other websites that it is possible to achieve CD's of as low as 0.07! so I think I could probably do a little better than 0.5 if I change the shape, I saw images of some of the lowest CD vehicles, here is one that looked interesting:

http://www.actiflow.com/images/nuna5.jpg
http://www.techniche-europe.com/files/Nuna5_Car.jpg

What do you guys think of this? do you think I should spend my time modelling it and trying it in the wind tunnel or do you think there are better designs?

Also the car has to have 4 wheels which may cause problems with this design.

[aerohead]

Originally Posted by examorph

thank you for the calculator, very helpful

New question , I asked this before but after researching more I ask again, what shape should I make the car? I ask this again because I made a tear drop shape for a body around the car and tested it in the wind tunnel, the drag coefficient was about 0.5, I read on other websites that it is possible to achieve CD's of as low as 0.07! so I think I could probably do a little better than 0.5 if I change the shape, I saw images of some of the lowest CD vehicles, here is one that looked interesting:

http://www.actiflow.com/images/nuna5.jpg
http://www.techniche-europe.com/files/Nuna5_Car.jpg

What do you guys think of this? do you think I should spend my time modelling it and trying it in the wind tunnel or do you think there are better designs?

Also the car has to have 4 wheels which may cause problems with this design.

I believe that the NUNA cars are at the low end of the drag spectrum.I think they are all 3-wheelers though.
In 1996,the Honda Dream-2 solar car team published a paper for the Japanese Society of Automotive Engineers about some research they did.It was all CFD as they ran out of time before they could put their 'car' in the tunnel.
Anyway,their Cd 0.10, 3-wheel Dream-2 was modified to 4-wheels and I think they were able to maintain around Cd 0.10.
You might check out this paper.Cd 0.10 is pretty good.They show front and side elevation views as well as the plan view.

[examorph]

wow thank you for the great paper!

Now that I am looking at cars such as the NUNA and Honda dream 2 I am starting to wonder, is the large surface area needed if the car is not using solar panel's, would reducing this area give an even lower CD? I ask because although the car I am designing is ran from an electric motor, solar panels or any additional power supply to the car battery is not allowed.