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Originally Posted by chillsworld
Not sure why everyone keeps bringing up Mythbusters, at no point have I referenced them  I don't even have cable television  
I understand that the TBL shouldn't need to be energized on streamlined surfaces to stay attached, but why are there studies and wind tunnel experiments that show a decrease in surface friction and an improvement of .cd values on already streamlined surfaces (passenger train cars for example, they have no curves or bluff characteristics along their sides). According to your statements, there shouldn't be any improvement upon an already perfect TBL/BL scenario via the use of dimples or any other such "fake vortex generator"?? I guess I'm just confused by the fact that everyone says it won't work, yet I'm the only one providing data/links to anything. So while vortex generators might be "better science", is that true because people simply won't look at new data that didn't exist when Hucho did all of his stuff? Or is it because these new studies are incorrect, or maybe they aren't actually as efficient as they would be if they used "real" vortex generators?
"As the layers of air move over a rough surface, the air particles in the layer closest to the surface collide with the surface. This makes the air particles slow right down (and right at the surface, they completely stop!). These particles then collide with air in layers a bit further out and make them slow down as well. As you move further away from the surface, the speed of the air particles is not affected. This boundary layer is laminar at the beginning of the flow, but it gets thicker as the air moves along the surface and becomes turbulent after a point."
So even on a super slippery, super streamlined surface, eventually friction/drag increases and separation occurs right? Vortex generators are used to mix things up and change the dynamic of this occurrence correct? And more often than not, they focus entirely on a point where separation will naturally occur right? Or on the leading edge of something at an attack angle. You wouldn't just put them down the length of the car, or all over the top of the car for no reason. The research I have been linking to, says it decreases surface friction and thus prevents the friction from increasing and the occurrence of separation (to some extent)... Which is similar but different from what Vortex generators do. I don't claim to know how, which is why I'm providing quotes, links, and statements from other places.
If I would have known it was such a hot button topic, and that I would need to look so much stuff up just to not seem like an idiot who watched some TV episode, I wouldn't have bothered posting in the first place  Anywho, I guess it's not a big deal, I sure don't have the means to test any of it... So I guess I'll move on to something else 
~C |
*Chronologically,it seems like the Mythbusters episode was the catalyst for the concept of dimples and drag reduction entering into the public mind.
*Jamie,or whoever is the 'engineer' on Mythbusters,ought to already understand the difference in the relationship between the Reynolds number of a golf ball and an automobile.It would have been covered in Fluid Mechanics,which he'd have to pass in order to get a degree.
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*Boundary layer aerodynamics of railroad trains would not be germane to a discussion of automobile boundary layers.The volume of the BL of a railroad train can easily exceed its frontal area.
*If you have another research which addresses dimples and automobiles which is peer-reviewed,we could look at that.
*TBL is TBL.Once it's established it works its wonders.
*The only way a dimple could 'work' is if it was acting as a crude VG.
*The 'new' studies must be viewed within a particular context.We have almost 100-years of VG research already documented.I would be very surprised if any stone has gone unturned.Winning world wars have depended upon such information.
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*The layer of air against the smoothest surface is at rest.
*If the TBL is in a unfavorable pressure gradient,due to Bernoulli's Theorem,it must decelerate.
*It can't decelerate since it's already at rest.
*The only reason it can stay attached,is if the TBL is transferring momentum from the inviscid flow outside the TBL.
*If the contour is too 'fast' the TBL will separate.
*If there is a surface downstream within a proper profile,there can be reattachment.
*VGs can help insure reattachment.Their vorticity feeds kinetic energy into a feeble TBL,re-energizing it,holding it against the boundary wall as if it were being machine-gunned from above.
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*On a super-slippery surface there will be about 1-inch of laminar boundary layer,then the rest is TBL.
*Friction drag is a consequence of viscosity,and shear stresses within the fluid as the varying strata laminations,at different velocities shear against one another.
*Attached flow is a function of pressure gradients,which are a function of body geometry.
*If the body is 'streamlined' it cannot produce separation,by definition.
*If it is pseudo-streamlined,like a VW Beetle,or,like a Mitsubishi Lancer,then it will have separation.
*VGs increase surface friction,but reduce overall drag through pressure drag reduction.
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It's not a sin to look and find things and share.The spirit of the whole thing is admirable and it needs to be applauded.
Some research is done by people who haven't done their homework.And because they don't know what they don't know,they don't even realize that they are making contextual comments when they are making them.Then it becomes a matter of damage control,attempting to rebut what is being passed off as true science.It's too dangerous to pass without a test.