Quote:
Originally Posted by BavarianFanatic
Actually, I don't. Those "little dimples" break up the laminar flow and create turbulence. This of course also increases drag.
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Please think about what you just said and in the context of aerodynamics engineering and fluid dynamics. BMW does extensive wind tunnel testing and whilst most of us agree there is a noise issue, they would not purposely add dimples to induce turbulence where noise is a function of turbulence. These dimples as you call them are actually called longitudinal ridges. If these ridges generate turbulence and noise, in the mirror redesign, they would have removed them altogether rather than only removing one in the mirror redesign. I suggest reading up on trailing-edge noise theory.
"Trailing edge noise from a particular airfoil depends on the shape of the airfoil. Along with the external flow conditions, the airfoil shape determines the development of the turbulent boundary layers that interact with the trailing edge to produce noise. Details of the turbulent boundary layer, such as mean shear and velocity fluctuation distribution normal to the surface, are important factors in the efficiency of noise generation. These can be manipulated through passive airfoil shape changes that change the mean pressure gradient over the foil surface and thus influence evolution of the boundary layer."
Although, I would need to view the modelling results of BMW's wind tunnel testing, I highly doubt BMW added airfoil ridges to induce turbulence.
http://arc.aiaa.org/doi/abs/10.2514/6.2013-489
