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Old 05-31-2020, 01:51 PM   #16 (permalink)
Cyborg ECU
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Join Date: Mar 2011
Location: Coastal Southern California
Posts: 6,297

Black and Green - '98 Honda Civic DX Coupe
Team Honda
90 day: 66.42 mpg (US)

Black and Red - '00 Nashbar Custom built eBike
90 day: 3671.43 mpg (US)
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Originally Posted by freebeard View Post
And Notchbacks!
So, two serious questions. At the surface of the car body, the flow stream of the boundary layer stalls to zero because of vicosity and friction. The shear effects of that stalled air slows the stream above it, and to a lesser degree the stream above that. The velocity of the stream keeps turbulence down, but nonetheless turbulance develops in the boundary layer as the flow moves downstream. The boundary layer expands because it is becoming more turbulent, and the larger the area of the body, especially length, the more unstable it will become until it actually fully separates due to the turbulance. I think that is mostly correct, and if there are errors I don't know how to judge where I have made them. Maybe it is not turbulance that causes expansion, but something to do with heat and friction that causes both turbulance and expansion.

First question (vocabulary): that stalled flow at the wall is sometimes described as having "separated." I have also seen it claimed that it is capable of localized reversed flow. However, that useage of term "separation" can be confusing because it may not be refering to the same effects as when we talk about "separation" of the entire boundary layer that happens when/if the flow encounters a major change in body contour and pressure, right?

Second question: obviously there is friction drag produced by the stalled flow at the body surface and its interation with the rest of the boundary layer. That is not insignificant drag, either, apparently. What, however, is the relationship between the growing turbulance in the expanding boundary layer and actual separation drag at the rear window, c-pillars, and/or tail of the road vehicle? Is there one?

I hope this makes sense. No time to edit. Daughter and I gotta work on her car.

Thanks for replies.
See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.

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