lennyk – Did you ever think that your original post would turn into this?
I'll chime in again for my nickel's worth.
Returning to the Reynolds Number (Re) and scale effect issue, the problem in trying to convey a real understanding in lay terms is that much of the underlying principles get glossed over, to the point where a lay explanation loses too much, and is akin to no explanation.
To grasp the significance of Re requires first getting comfortable with the concepts of viscosity, shear stress, momentum, boundary layers, work and energy, separation, wake, laminar vs. turbulent, etc. Without understanding these concepts, which are themselves based on further sub-concepts, a discussion of Re in the context of scale effect will generate little real understanding. Like trying to explain any other highly conceptual topic on a lay level - Yes, it can be done, but there will be holes in the explanation, and large ones at that.
Consider an everyday concept - Friction. Seems simple enough. We all intuitively "understand" friction and its effects, but could a deeper understanding involving the surface science involved, like the source and dynamics of the intermolecular attractive forces, really be conveyed in lay terms? How about the everyday concepts of light and color perception? We all relate to them in physical terms simply enough, but would a lay discussion attempting to delve deep really shed meaningful insight on spectral absorption, electron transitions, or the neurophysiology of the eye?
Problem is,
every physical phenomena ultimately requires, at some level of understanding, a discussion transcending a lay level in order to accurately describe it. The complexity of the phenomena that Reynolds Number relates to is often underestimated, I believe, simply because the equation for Re is so simple - It gets thrown around and used as a rule of thumb so much that its conceptual foundations aren't fully appreciated.
The level of understanding that one chooses to aspire to, as Ollie stated earlier, is based on that person's motivation to get there - whatever his interests are and his "need to know." As I see it, advanced discussion of a lay topic
may create misunderstandings, but a lay discussion of an advanced topic is
bound to create misunderstandings.