Quote:
Originally Posted by MetroMPG
Phil, excuse the basic question, but does separation imply reversed flow, or might we only see turbulence if the angle is not too far departed from the ideal line?
Reason I ask: I'd be willing to tuft test (& make video) of the flow over the rear of my 1st generation Insight if you thought it would be instructive.
Of course, reversed flow would be easier to spot in a test.
Darin |
At the separation point the velocity of the turbulent boundary layer is already zero.
As the roofline or body sides continue to reduce in cross-section,Bernouli's Theorem dictates that the air must decelerate further,but that's impossible,as it is already at rest against the boundary of the body.
In the presence of this adverse pressure gradient the boundary layer lifts off the surface of the body and the air behind rushes forward into the void beginning the back-flow which will grow into eddies.
If the air can re-attach onto,say,a spoiler,then full-blown turbulence is avoided until after the spoiler.
Without a spoiler it will grow into turbulence with its pressure the same as at the point of separation.
On the Insight,even though the slope is not ideal,the body provides a wake-stuffing function,forbidding air to occupy its 'space,' and in effect,tailors the wake to some extent.
When Carl Breer hung the 3-foot 'stinger' off the back of the 1934 DeSoto Airflow,it was far from ideal in shape,but it altered the wake enough for the car to pull off Cd 0.244.
I think that if the camera angle was better for the photos of cars like the surviving Kamm car,AUDI's A2 3-liter,and Insight,we'd have a more accurate look at the streamline filament over the roof.
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The last point I'd like to make,and then Ill shut up,is that if Hucho is correct about the 'Template',if we streamline short of it,we shoot ourselves in the foot.There won't be any plug-and-play additions,adding length at a later time for even lower drag.The air will be trash,not the robust 'onset' flow which Hucho describes as essential to" moving the point of separation back",the whole premise of streamlining.
Okay,I'm shuttin' up now!