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Old 02-23-2008, 03:37 PM   #15 (permalink)
aerohead
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turbulent boundary layer

Quote:
Originally Posted by MetroMPG View Post
AWESOME writeup. Thanks for posting that.

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Here's a question: Is there a simple answer as to why introducing a small amount of turbulence in the boundary layer (ahead of the area where flow would normally separate) moves the separation point further downstream & reduces the size of the wake?

Or does that fall into the category of thinking up questions to ask god? :P
The question is a good one and before I try to answer,I'll go home and look it up.It has to do with ideal fluids,invisid flow Daniel Bernoulli's velocity/pressure relationships,momentum interchange,friction,pressure,density,etc..The reason I haven't committed
it to memory,is that from my understanding,our cars live in a world of turbulent boundary layer for their size and velocities whether we like it or not,and there's nothing we can do about it.

Above 20-mph we have the dimpled golfball scenario working for us,and drag coefficients are stable up to transonic flow(about 250-mph).

Submerged in the troposhere,as our cars move and displace larger and larger volumes of atmosphere,air at rest is accelerated until it reaches the point of frontal area for each vehicle.Remember,up to this point,it is in a positive pressure gradient,ramming the air.As the air reaches the frontal area,or area of greatest cross-section,it no longer resides in a region of positive pressure.If air was an ideal fluid,it would have no friction,and its velocity-pressure(Bernouli) would simply be converted back to static-pressure(Bernoulli).

Because of energy loss and heating due to viscous shearing within the laminations of air close to the skin of the car,thermodynamics dictates that entropy will scuttle our attempts to get the air back as it was before we came along.Subsequently,the flow will "stall" and separate from the car,creating a turbulent wake of high drag.

Slick as it is,the smooth polished paint of our cars has enough surface roughness to "trip" the air into tiny burbles,which as members have noted,feed kinetic energy into the layer of air adjacent to the skin of the car which is otherwise at a standstill,and will postpone the point of separation,say for the golfball,well behind the point of maximum diameter,thereby reducing its wake,cutting drag by half,and extending range from 150-yards,to 300-yards with a clubhead exit velocity of 110-mph.

If someone doesn't beat me to it,I'll dig out the details for next time.Main point is, that we couldn't have a laminar boundary layer if we tried.
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