Quote:
Originally Posted by freebeard
So could you say that the vortex generator rolls up the boundary layer into tighter streamlines, that pull the top of the boundary layer inward?
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There aren't really streamlines in the boundary layer.Those are all outside the boundary layer,out in the free stream flow (inviscid flow).Turbulent boundary layer us allowing laminar flow outside itself.And the layer of air immediately adjacent to the skin is completely at rest.It has no velocity whatsoever.
Beyond the point of the body's maximum cross-section,the flow is in a very hostile pressure regime.There's very little reason for the flow to be attached.
By definition,if you try to decelerate the flow any more than a streamlined body would,you're asking the air to decelerate,when its already at rest.That's impossible! So at that point the flow will begin to reverse in direction,flow forwards towards the area of minimum pressure,and as it does,if lifts off the surface,rolling up into,first small eddies,then full-blown turbulence.Once you have turbulence you've completely lost the game.All that swirling kinetic energy can never be recovered into any useful pressure.It will eventually be worn down by viscous attrition,just adding heat to the atmosphere.
On an aircraft,the VG induces vorticity (delta-wing lift of the Concorde aircraft ) , kinetic energy, as if it were in a more favorable angle of attack,where energy of the inviscid flow could crash through into the boundary layer (think of a water jet from a garden hose nozzle,pushing water uphill on a driveway,which would otherwise be draining down to the gutter under the influence of gravity) energizing it such that it can adhere to a 'steeper' surface.
As Vortex says,there's a price to pay for the vorticity.If you get it right,the drag reduction from the smaller wake possible with the VGs will overshadow the drag penalty of these excrescences and their frontal area, interrupting to flow.