Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Petrochemicals on 29/05/2021 23:58:27

Title: How do I calculate wind resistance for vehicles at different speeds?
Post by: Petrochemicals on 29/05/2021 23:58:27
I am assuming that a vehicle of given shape will have uniform wind resistance over different velocities. I know this to be somewhat incorrect due to Eddie's etc. Discounting the variables, what formula describes wind resistance? Some inverse of boyancy?

I realise I can Google it, but what is the most reliable generalised formulae?
Title: Re: How do I calculate wind resistance for vehicles at different speeds?
Post by: Kryptid on 30/05/2021 00:38:32
Wikipedia actually has a page on this: https://en.wikipedia.org/wiki/Drag_equation

At subsonic speeds, a doubling of speed results in a four-fold increase in drag. This is no longer true once you enter the trans sonic zone. Drag increases faster than this rate once you approach or pass the sound barrier.
Title: Re: How do I calculate wind resistance for vehicles at different speeds?
Post by: Bored chemist on 30/05/2021 11:19:15
On both experimental and theoretical grounds, the air resistance rises as the square of the speed (up to near the speed of sound.
If the car goes n times as fast it hits n times as much air, and  (which  would increase the load by a factor of n) and it also hits it n times faster (which increases the drag by another factor of n) so the overall increase in wind resistance is n squared.
Title: Re: How do I calculate wind resistance for vehicles at different speeds?
Post by: alancalverd on 30/05/2021 11:26:38
In the laminar flow region, drag = ½ρCdv2A where
ρ = air density
Cd = drag coefficient
v = speed and
A =  crosssection area

Cd is best determined experimentally but can be approximated by using weighted tabulated values for all the shapes that make up the vehicle surface. For most road cars it is around 0.25 - 0.5 but a sailplane can achieve less than 0.05.