DIMPLES FOR AERODYNAMIC ADVANTAGE?

[jimchitas]

Yesterday i was searching about' vehicles from france companies ' and i saw some pretty good desinged especially from citroen then i came across to buggati cars and i show the new buggati bolide a pretty quick car and it make me curious about how drag coefficient and i show some articles about their roof scoop how it can reduce drag coefficient 10 percent and 14 percent lift here is a image

after a little bit research i found this video
https://youtu.be/VUiGhyHC-1A
a expirement this guy claim the car gets better mpg
i look some images of airflow works in golf balls they are pretty close to vortex generators and after this i look their new video which 3 big companies try it and did not find the same results as them so guys is there any way that dimples can be actually beneficial or just a marketing move from buggati to make their cars feel more special

[Piotrsko]

If your application benefits from turbulent flow (say the turbulence diminishes downwind vortices) then it will reduce drag. If the aft boundary layer is mostly attached, then the turbulence will increase drag. It works on golfballs because there's this huge drag vortex behind the golfball that gets dramatically reduced. Aerohead has pointed out that there is an approach angle and a bunch of other criteria involved

[aerohead]

We got into this years ago, when 'Myth Busters' aired their episode with the clay-dimpled Ford Sedan.
1) I believe that the consensus was that, had they dimpled only the rearmost portion of the roof, and C-pillars, that that would have been enough to explain any observed drag reduction.
2) And the explanation had nothing to do with a transition from a laminar boundary-layer ( LBL ), to a turbulent boundary-layer ( TBL ),as is experienced with a dimpled golf ball, but rather that, as you mentioned, the dimples were actually functioning as vortex-generators submerged within a TBL, allowing re-attachment of separated flow onto the boot/ trunklid.
--------------------------------------------------------------------------------------
If you want to 'get into the weeds' for an understanding of what happened, you might look around online for:
* critical roughness
* critical Reynolds number
* LBL to TBL transition point
* tangential force
* normal force
* momentum transfer
* displacement thickness
* pressure distribution
* viscous shearing
* momentum loss
* adverse pressure gradient
* termination of boundary-layer stability
* separation: reduced or negative pressure
* angle of flow @ separation point/line
* adhered & attached longitudinal vortices ( counter-rotating pair )
* turbulence
* Vortex generators ( artificial roughening / vortex-induced low pressure )
* 'Stationary' / 'locked' / 'captured' vortices
* reversal point
* reattachment
* mending
* pressure recovery
* jet-pump mechanism
* base drag
* pressure drag
* Boundary-layer theory by Prof. Dr.-Ing. Hermann Schlichting.

[freebeard]

is there any way that dimples can be actually beneficial

It it possible for small scale localized features to affect air flow. The wickerbill for example. Buggati limit the use to a small area, not the whole car.

Those are pimples, not dimples.

Buggati's innovation is to make them active. To influence the performance of the wing at different speeds.

[aerohead]

1) in fluid mechanics there's a concept of 'dynamic similarity.'
2) the giant world globe of the past New York's World's Fair, a Charters -Thomas cannon ball from a Civil War artillery piece, and a B-B from a B-B gun would have identical drag coefficients, as long as Reynolds number criteria were respected.
3) below a critical Reynolds ( Rn ) number of around 500,000, all spheres have a Cd 0.47.
4) if these spheres are accelerated beyond a velocity capable of producing an Rn above 500,000, by around Rn=1,000,000, their drag drops to Cd 0.10.
--------------------------------------------------------------------------------------
5) 'artificial roughening', via gluing sand to the sphere, or 'dimpling', amplifies the Rn for a given velocity, accelerating the transition from a laminar boundary-layer ( LBL ), to a full turbulent boundary layer ( TBL ).
6) the vast momentum interchange between the local streamline just above a TBL and the surface of the sphere's body enables the sphere's flow to remain attached within the hostile positive ( adverse ) pressure gradient present in the sphere's aft-body, without which, all the flow would attempt to flow 'upstream' towards the highest suction peak, just ahead of the 'equator' of the sphere.
--------------------------------------------------------------------------------------
7) for the 'length' of commercially-produced road vehicles, at, and above around 20-miles per hour, Hucho attributes automobiles with Rn 10,000,000, and they all have this super-critical Rn which ensures a full TBL, capable of maintaining fully-attached laminar aft-body flow; as long as the 'shape', 'form,' 'contour', 'silhouette', of the aft-body is 'streamlined.'
--------------------------------------------------------------------------------------
8) consequently, if we were 'Lilliputians,' driving 'Hot Wheels' or 'Matchbox'-sized vehicles, ' dimples' would be of help, helping facilitate the development of the TBL required to allow fully-attached laminar flow outside the boundary-layer of the tiny cars, as mandated under the conditions of verisimilitude.
9) Short of that, they're essentially 'useless.'