What is the effect of velocity on aero design in general? Does the near optimal design for 100kpm look the same/similar as the design for 200kpm? All the test data I've been able to see happens at the static velocity, and is often not even mentioned in the CFD's ,etc that are published.
I would think that airflow at even 50kpm is a great force and should be worthy of harnessing. Just stick your hand out the window .. Also I never see/hear of new undertrays for Monaco, for instance, to try to maximize DF at lower speeds, so that makes me wonder about the role of velocity on design.
I think drag is squared with velocity, so suppose your drag is 5 units at 5km/h, at 10km/h it would be 25 units. the values are not correct of course but thats the idea of it I think.
Murphy's 9th Law of Technology:
Tell a man there are 300 million stars in the universe and he'll believe you. Tell him a bench has wet paint on it and he'll have to touch to be sure.
Thanks for the reply. Yes drag does square with velocity but my question is related to aero design vs velocity.
For instance would you design a difuser or wing the same way for 100kph as you would for 200 kph, for example?
While I cannot find a definative answer to this myself I seem to be getting some confirmations that the answer is yes, the designs would be the same providing the aero goals were the same. Lots of people out there are smarter than myself however so I'd like to hear what others would say on this.
I think there is a huge difference between a design for lower and higher speeds. Take a look at some passenger airplanes for example. Planes designed for higher speeds have different wing configurations and different profiles. This has to do with stall speeds, pressure waves, etc. which are not linear to speed.
The wings of a Formula 1 car would eventually stall of we increase speed. Another thing is mechanical strength. If you design something for high airspeeds then you are forced to use stronger constructions than you can use for lower speeds.
There is two effects here, denoted by the Reynolds and Mach numbers. The Reynolds number is basically the ratio between the drag due to the induced vorticity/turbulence and the drag that is down to direct viscous effects. When the Reynolds number, which itself increases with velocity, is large enough such that the boundary layer dimensions can be neglected with respect to the overall dimensions, then the flow picture will largely indeed be independent of speed, just that all the forces will scale as the square of the speed. For most automotive applications this is indeed the case. This is of course a very much simplified picture as the flow separation may be very sensitive as to what is happening in the boundary layer, but for "well behaved" flows that is the general idea.
At higher velocities, the compressibility effects become another factor to consider. The extent of these effects is largely determined by the Mach number, which is the ratio of the velocity to the speed of sound. A rule of thumb is that at Mach numbers of up to 0.3 or such, the compressibility effects can be neglected. Then again, in very high lift configurations or in general in areas where the (relative) pressure differences are very large, these effects may set in sooner.
In general there should therefore be no large differences in designs for lower and higher speeds, as most cars comfortably fit in between the two speed regimes where such a simplified picture stops being correct. On the other hand, one can come up with designs that will show the various speed dependent effects even in this regime.
Thanks Gecko!!! I guess the beauty of Vortex generators is that they break down boundary layers so they become less of a factor in design. That helps me emmensely.
