Quote:
Originally Posted by Pendragon
I do not want to get too far afield in the minutia of laminar flow, but true laminar flow is "attached" to the surface over which the fluid (air) is flowing. Even a well designed "laminar flow" wing experiences flow separation sooner than many people realize and the drag associated with the disturbed airflow. For example, there was an experimental aircraft which sucked this layer of disturbed airflow into the engine through a great many small holes. The experiment was successful in demonstrating the drag reduction possible, but the means of doing so was, and remains, impracticable. Certain production F-4s used engine bleed air to maintain lift in certain flight regimes. The drag reduction achieved by the use of flush rivets usefully improved aircraft performance.
Cars, on the other hand operate in a very disturbed airflow. That said, worthwhile improvements (reduction in drag) can be made despite the airflow not being truly laminar. We are seeing some changes in mass produced vehicles. Some of the "low hanging fruit" are more raked windshields and external mirrors that are designed to smooth airflow them.
You may recall that the Shelby Daytona Coupe had the point of maximum height of the roof line positioned well aft of the driver's seat for the specific purpose of delaying flow separation just as a laminar flow wing does.
Anyway, I have probably strayed
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Straying is okay.
And I agree with all the boundary layer considerations you mention.
Aircraft are presumed to have fully attached flow and are primarily dominated by surface friction drag,hence the excitement over 'laminar' wings.
Motor vehicle drag is ruled by separation-induced pressure drag,hence the interest in streamlining the rear,moving the separation point back as far as practical/possible.
I was trying to emphasize what was happening way out away from the body in the flow region un-affected by the separated vortical flow and turbulence.
In road vehicle aerodynamics this region is referred to as laminar and is independent of the boundary layer conditions.
In the thread 'Flow-Images,' photos of smoke traces over vehicles taken in wind tunnels which depict undisturbed curvilinear paths over the vehicle are said to be depicting
'laminar' flow.