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The air flow around the car creates a vortex. There is an image of the air flow around a car: which was used by this earlier answer - that rotating air behind the car is what spins the wheels of the bike.


The role of viscous forces with respect to turbulence is analogue to the one of diffusion for mixtures: it sets a scale below which it smoothes out gradients. So there are no vortices smaller than this. When this scale is much smaller than the scale of the experiment, tubulence can happen. Turbulence is not linked with a peculiarity of the molecules. It can ...


This is something I have looked into a great deal, having an interest in model rotorcraft, including single bladed machines. As with nearly all things related to aviation, and engineering in general, there is no optimum or best answer to number of blades. It is all about trading off cost, performance, reliability, weight, etc. etc. etc. "Best" will depend ...


The idea is all based on scaling laws, which is a very common theme in turbulence. As I mentioned in a comment, this assumes that the energy produced is equal to the energy dissipated. The energy produced is proportional to $u^2$. The turbulence is assumed to dissipate according to some time scale. A reasonable time scale is the eddy-turnover time, or ...

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