I have read that non-laminar flow reduces drag on blunt bodies.
Given that, why is pitting not used on motor vehicles?
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2$\begingroup$ Presumably the answer in the context of airplanes (see here) applies to most other vehicles. $\endgroup$– lemonCommented Jul 5, 2015 at 13:17
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1$\begingroup$ But trucks and cars are much blunter than aircraft? $\endgroup$– 52d6c6afCommented Jul 5, 2015 at 13:21
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4$\begingroup$ I think Mythbusters covered this one a long time ago. $\endgroup$– Kyle KanosCommented Jul 5, 2015 at 13:36
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1$\begingroup$ discovery.com/tv-shows/mythbusters/videos/dimpled-car-minimyth, huffingtonpost.co.uk/2014/06/30/…, newsoffice.mit.edu/2014/… $\endgroup$– ŘídícíCommented Jul 5, 2015 at 15:38
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$\begingroup$ Excuse me if I’m wrong, but aren’t the dimples on a golf ball a form of vortex generator? $\endgroup$– Abdul Moiz QureshiCommented Apr 12, 2020 at 8:41
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3 Answers
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It has to do with size. For small and slow-moving objects, viscous forces have more weight in comparison to inertial forces. Also, the layer of air close to the surface of an object which moves through air is differently structured. The dimples in a golf ball will make this layer resemble that of larger objects and help to make the flow resemble that around larger objects like a car by reducing the area of separated flow at the rear of the object. A car is large enough so it doesn't profit from dimples the way a golf ball does.
Physics Background
The ratio of inertial and viscous forces is expressed by the Reynolds Number. The layer close to the body of the moving object, where air speed varies between the speed of the body and the speed of outer flow is called boundary layer. The speed profile of this boundary layer is almost triangular at low Reynolds numbers (up to 400,000 in non-accelerated flow, so low is rather relative), and no transverse motion can be observed. This is called laminar flow. At higher Reynolds Numbers, oscillations develop and the flow transits to one with transverse motion, which helps to shift motion energy to the layers of flow close to the body. Consequently, the speed profile of the boundary layer becomes much fuller. This is called turbulent flow.
When flowing around a body, the laminar boundary layer will cause flow to separate right after passing the biggest diameter of the body, causing massive pressure drag at the rear-facing part of the body. The re-enrgizing of the turbulent boundary layer delays this separation, causing less drag. The dimples in a golf ball promote an earlier transition to turbulent flow and thus reduce the pressure drag of the golf ball, so it can fly further.
In a car, the size is large enough to develop a turbulent boundary layer early on. Dimples would not help, but increase drag slightly.
answered Jul 12, 2015 at 8:21
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Mythbusters covered this one. They found significant fuel saving at constant speed. Assuming their findings are correct I can only guess that the technique is not used for cosmetic reasons ie few people would want to drive a car that looks like it has been in multiple accidents and covered in dents
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$\begingroup$ According to Adam Savage, one of the big three automakers tried to reproduce that episode with a wind tunnel on one of their own clay cars. The automaker was unable to reproduce Adam's results, though the automaker was using a different type of car. $\endgroup$– BrianCommented Jan 9, 2021 at 1:20
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Major classes of drag:Two main factors of drag are friction and flow separation. This friction drag is dominant in streamlined shape and pressure drag (drag due to flow separation) is dominant in bluff body.
Ways to reduce friction and pressure drag: Turbulent flow has more momentum than laminar flow so they reduces flow separation so pressure drag is reduced that is main cause of drag in bluff body. So such mechanism is used in gulf balls (since gulf ball is spherical there is no way to streamline it!). In streamlined bodies friction drag can be reduced by increasing smoothness of surface.
Why vehicles are not pitted: Car is not a complete bluff body ( length of car is more than height)and not a good streamlined body( length of car not very much than height). Usually pressure drag causes more drag than skin friction drag in most of the flows. If we put pitting in cars that may lead to flow separation and increase the drag. Structure of car can be easily streamlined by avoiding flow separation. That can be archived by increasing steepness of front and back side glasses. We can reduce skin friction drag by using smooth body surface or painted surface. If u consider some part of car where separation can occur, example convex corners of car surface where dimples can reduce drag!. Manufacturing and maintenance cost of the vehicle will increase if vehicles are pitted. But the main reason is dust may accumulated in those holes and increases weight and purpose of those dimples. ( Pls, note that aerodynamics of bluff body and streamlined body is different)
If u want more info about drag redaction techniques in automobiles this review paper is good
http://www.ijens.org/Vol_14_I_02/145302-6868-IJMME-IJENS.pdf
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2$\begingroup$ Your answer seems sound. Please reread and rewrite it, as the sentence structure is hard to comprehend. Thank you. $\endgroup$– WalyKuCommented Jul 6, 2015 at 7:32
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1$\begingroup$ Ty for ur comment. I have modified some thing. Please tell me what part is not good. Shall i have to add more technical contend? $\endgroup$– AGNCommented Jul 6, 2015 at 7:52