Quote:
Originally Posted by aerohead
*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.
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Yes, on multiple comments I have discussed actual studies involving
automobiles. I'll re-post one here, so as to streamline for you and anyone else...
"Use of bionic inspired surfaces for aerodynamic drag reduction on motor vehicle body panels"
http://www.zdrax.de/en/assets/pdf/Bi...d_Surfaces.pdf
Selected pages from the above PDF, to further ease and streamline this discussion...
Look at the dates on that page, one study is more than a decade before the Mythbusters episode. The last paragraph states
"As mentioned above (the part about bluff objects and rynolds numbers)in order to reduce the frequency of burst, the height or depth of the body surface units should be smaller than the distance between the body surface and the logarithmic law layer. To test this, a large range of law layer values have been used (Law layer is based on Reynolds value)."
I'm not a scientist, nor am I an engineer, nor do I know the validity or credentials of those who did this study and published this paper... But you asked for data, and so now you have data. You can feel free to follow the link if you need more sciency stuff than what I have provided with my selected pages
There are also additional studies, they are just hard to find on the googlesphere.
~C