Quote:
Originally Posted by pascal28
In the equation of drag the drag coefficient (CD) is assumed not to vary and hence indicates the aerodynamic efficiency of an objects shape.
|
In the generalized / simplified version ... yes ... not in underlining science it is based on ... ie Fluid Dynamics , and then bellow that the science fluid dynamics' generalizations are based on allows for even more potential variation.
Quote:
Originally Posted by pascal28
Questions:
1) Is drag coefficient really a constant with speed?
2) If it isn't what would be the typical variation with normal road speeds (say 30 to 70 mph)?
3) Is there an agreed speed that manufacturers quote CD at?
I would like to know this to correctly calibrate a CFD model of the Honda Insight that has a quoted CD of 0.25.
|
1> No... see above.
2> the amount it varies depends on more than the speed ... if you want generalized / 'typical' , just use the Cd... if you are interested enough to explore a degree in fluid dynamics, enjoy.... kind of like F=MA or V=IR are not 100% accurate, but are a fairly good generalizations for most people.
3> No... it is either computer estimated from computer models of the shape ... or they put it in a wind tunnel measure the force at some air speed and run the calculations backwards... no global governmental or scientific standards are imposed on the methods used... it's on the 'honor system' ... if you don't like it, your welcome to pay for your own wind tunnel tests ... although over 99% of people can't afford to do that, so they just use the published number.