I know that for the wing, the Reynolds number is based on the MAC. This makes sense to me, as the boundary layer develops along the length of the wing.
However, what kind of Reynolds number needs to be used if there is a flap behind the main wing and there is flow pattern like this:
Some of the air particles will have streamed along the wing before hitting the flap, so I would say then you should use the wing Reynolds number.
But, on the flap, also a new boundary layer will start. Particles coming through the gap will only see the flap, and will thus experience a flow with a lower Reynolds number (based on the flap length).
If I'm talking about flow phenomena on the flap, should I be using the global Reynolds number (based on the wing chord), or the local Reynolds number?