Quote:
Originally Posted by ACEV
@aerohead #46,
Could you please put that into simple language so we all can understand? Your good points are very difficult to visualize. 
Basically, what I have learned about a round shape going thru the air is that the fairly aerodynamic front is offset by they very unaerodynamic back side, therefore negating any beneficial affect. A Borg sphere in space has no such effect upon it since it is in a vacuum. One must assume that the ideas you put forward are having to do within atmospheric situations.
Also, once we have the test done, ie; by MythBusters, then the observable facts have been presented for all to see. The idea presented by Grant-53 must have his definition basis in observation. At this point we do know that dimples in a car body surface do increase fuel economy since we have all observed it.
If we feel that there is something missing in the given test, then it should be pointed out. If we feel that there are other things in the test making the fuel economy better, then they should be pointed out.
Short of that, the average person should be able to rely upon the fact of the fuel mileage improvement as demonstrated. If we overthink everything, we will never move forward.  |
ACEV,if you'll do a search at GOOGLE IMAGES for 'golf ball flow separation' you'll find some pretty nice illustrations.
*as you see for the smooth ball,the separation is occurring ahead of the widest part of the ball's body and the wake is enormous.This is what happens with a laminar boundary layer.
*the next image illustrates the flow with a turbulent boundary layer (triggered by the roughness of the dimples),and you can see that the flow remains attached downstream a bit from the widest part of the ball.By the point where the new wake forms,the air velocity has slowed down some,and from Daniel Bournoulli's research,we know that the static pressure has grown,and this new,smaller wake is at a higher pressure such that the difference in pressure,from front to rear is less (reduced pressure drag) and the overall profile drag is lower,allowing the ball to range further down the fairway.
*On a car,because of it's length,compared to velocity,it will attain this turbulent boundary layer(without any roughness) when it hits about 20 miles per hour,and the flow will remain attached the maximum amount in the aft-body region.
*since the Taurus has kind of a 'fast' roofline,it suffers from flow separation before the air makes it all the way back.
*The dimples appear to behave as turbulators (vortex-generators are a form of turbulator) which,while themselves add additional drag,they are able to transfer enough additional kinetic energy into the boundary layer such that it can remain attached in an otherwise hostile pressure gradient,postponing separation and leading to a smaller wake of higher base pressure behind the car,netting an overall drag reduction (exactly like Mitsubishi's vortex-generators do on their Lancer notch-back).
*if we take MythBusters at their word,then yes,the dimples reduce drag,but not for the same reason they help a golf ball.They might have attached VGs to the Taurus and achieved similar results for a lot less trouble.