Quote:
Originally Posted by hypermiler01
Reynolds number
When the speed doubles, the Reynolds number also doubles and that is what causes the flow to detach. That is why fast cars need a more sloped shape than slow cars to have attached flow. And why the VW Beetle may have perfectly attached flow at, just for example, 20 mph, but does terribly on the highway.
That comparison picture I made shows the two different shapes with the exact same condition settings.
The major difference at the front is that the extreme wedge shape has almost NO STAGNATION. 100% of the air is scooped over the top and sides, leaving flow underneath completely unmolested. Well, since that is 2D, it is all scooped over the top. The effect is the same for 3D, just integrate to get the effect of a bunch of thin slices stacked together. |
At very low velocity,low Reynolds number,and laminar boundary layer the flow would separate SOONER on the body of the VW Beetle,as the first pressure minimum would occur well ahead of the windshield area.
As velocity increased,Reynolds number would exceed the critical value necessary for the transition to turbulent boundary layer,and the separation point would be moved rearward to the area adjacent to the top of the backlight.
The drag coefficient would be constant for 'normal' driving velocities.
With 'wedge'-shapes,most of the body is in a positive pressure regime and the air is continuously accelerating into an area of decreasing pressure,a situation which is ideal for maintaining a laminar boundary layer.
The downside is the larger wake.
Impressive as is Ford's Probe-IV,Cd 0.154 'wedge',it is eclipsed by aft-body teardrop taper and Cd 0.137 of their Probe-V and its smaller wake.
For sub-sonic flow, drag is governed by flow separation,hence the boat-tail which eliminates separation.
For supersonic flow,drag is governed by shockwaves which are governed by the shape of the nose,something we need not be concerned with.
Below transonic flow ( which begins at about 250-mph with automobiles ) a semi-hemispherical nose offers optimized penetration and high-quality onset flow to the aft-body.