I am slightly confused with this notion of the Reynolds Number. Specifically, I am looking at airfoil performance at very low Reynolds number.
I am reading a lot of papers saying things like because it is a small model aircraft, the Reynolds number will be small (so then it is to do with the shape of the obstruction in the fluid flow?). But then I see something like large turbine performance at low speeds involve low Reynolds numbers (from here I would think it is to do with the 'quality' of the fluid flow itself, whether it is laminar or turbulent, uniform etc).
If it has something to do with the fluid flow, then why do I see Reynolds numbers attributed to specific airfoils etc? Say that an airfoil has $Re=100000$, what does this mean then? If the fluid is flowing at a very low speed with high viscosity, then how can we make use of this number at all?
Do correct me if I'm wrong, I am only beginning to learn these things!