Physical Interpretation of Reynolds Number

swmmr1928

Wikipedia: "Reynolds Number is a dimensionless number that gives a measure of the ratio of inertial forces to viscous forces."

What is meant by the inertial forces?
I have a much better intuition of viscosity. But it seems to be like the Reynolds Number predicts turbulent flow for a viscous fluid like honey if the inertial forces dominate over the viscous forces. I cannot imagine this scenario.

What is the difference between Reynolds Number, inertial forces and viscous forces?

I looked at expressions for the Reynolds Number, but it didn't help.

AlephZero

"Inertia force" means the (mass x acceleration) force needed to change the velocity of the fluid. The inertia force depends on the density of the fluid, and the density is one of the terms in the "formula" for the Reynolds number.

haruspex

Let me say first that I've no expertise in this area and what I write below is just my reading of some web pages.

Suppose you sweep your hand through a bowl of water. The large scale movements that result are considered inertial; they're as you'd expect from basic Newtonian mechanics.

Theory says that these flows create eddies which are unstable and break up to form smaller eddies. Still inertial.
As this cascade (turbulence) progresses, the diminishing scale alters the balance between inertial forces and viscous ones. Viscous forces dissipate the energy as heat and bring the medium back to rest. With no viscosity, the molecules would go on swirling around forever on the smallest scales.

Viscous fluids are more likely to produce laminar flows. The energy gets dissipated as heat early on, preventing the eddy cascade from arising.