Beetle Inspired Aerodynamic Thoughts

[aerohead]

Originally Posted by dmac257

I am just trying to learn not criticize one way or another but I get no sense of speed from this picture. It is my understanding from reading in this forum that speed matters. This picture stops action so well there is almost no bluring of the background or the wheels.

My understanding is that at slow speeds there is less compression of the air in front, air moving around the vehicle will be moving slower, the air will remain attached easier, the air will be able to close behind the vehicle easier .. so less resistance and less drag.

Higher speed would mean higher compression of air in front, faster air flow around the vehicle, and air will be unable to combine behind the vehicle fast enough so detaches with drag behind car.

And my question is this: Is the ideal aero template different for different speeds? If so how do you determine the ideal template for a vehicle that will NEVER see 65mph but will normally be driven at 45mph tops?

Don

For the size and length of road vehicles,the drag coefficient is constant above 20-mph.
As to compressibility,air is considered an incompressible fluid below about 250-mph,as a vehicle would be entering transonic flow in ground-effect.
At really 'slow' speeds flow would actually be less stable and drag actually be higher on a velocity basis do to the weak laminar boundary layer.
As velocity increases to 20-mph,there is a transition to a turbulent boundary layer where momentum of the outer strata of air can be communicated into the boundary layer adjacent the body by violent mixing,increasing the energy near the boundary and allowing it to batter withstand the adverse pressure gradient which occurs downstream of the point of maximum velocity,effectively moving the separation point further back,reducing the wake,and drag.
The 'Template' would be good for anything over 20-mph,up to around 250-mph.
The 'template',at 80% length ( about all we can use due to ground clearance ) would produce a car of Cd 0.13.If you streamlined the wheels and tires like solar racers you could go towards Cd 0.10.
Of course,practicality must be weighed out.Personally,I would only consider the whole length in the context of a trailer with sealed gap.

[SwamiSalami]

That was intense. Awesome.

[JasonG]

If you go back to the Flowillustrator video

http://www.youtube.com/watch?v=_VitTX6b9Jo

you will notice the green transition layer thickens noticible as it goes down the rear of the Beetle.
This may not be full seperation, but is causing significant drag.
A full seperation from a trip edge (ie "Herod's Helper") will cause less drag even though there is more turbulance.
Turbulent seperation provides force at a small magnitude in many directions while a thickened transition layer provides a larger force over an increased area, half of the rear of the car.

[Frank Lee]

Herod's Helper isn't a trip edge; it's a spoiler.