Reducing the Aerodynamic loss of a Flywheel in Air I have a hypothetical flywheel spinning at 3000rpm. The steel flywheel is around 800mm in diameter with a "Smooth" finish. I was wondering whether it would be beneficial to implement golf ball dimples on the flywheel surface to reduce the laminar drag. There are many similar questions around golf ball dimples on aeroplane wings but no questions around an object which is statically rotating and therefore has no dominant drag. The surface speed is 125m/s
 A: Ideally, the flywheel would run in a vacuum.
In air, its surface should be as smooth as possible. With aircraft surfaces you have a stagnating point where air "hits" the aircraft straight on and then a flow path with a clear end at the trailing edge. The length of this flow path is used for calculating the Reynolds number, a similarity number which characterizes the ratio of inertial to viscous forces. Golf ball dimples only make sense at low Reynolds numbers (which is 100,000 or less) when a laminar boundary layer needs to be tripped to a turbulent one in order to delay flow separation caused by the pressure rise on the back side of the golf ball.
For the spinning wheel no such flow path length exists, nor does a steep pressure rise. Surface friction will accelerate the air in the boundary layer of the flywheel on a circular path such that centrifugal forces will create a flow towards the rim of the flywheel. In the end, a circulation shaped like a flattened donut will develop on both sides of the flywheel where air close to the surface is being carried away towards the rim and air outside of this boundary layer is flowing back towards the hub of the flywheel. This whole circulation system will also spin with the flywheel, and it is very unlikely that laminar flow can be maintained in that boundary layer. Also, there is no need for a forced transition because no steep pressure rise has to be mastered. Any surface imperfection will only increase the size of the boundary layer and, consequently, the viscous losses from it.
