A relation between two mechanical systems (often referred to as model and prototype) such that by proportional alterations of the units of length, mass, and time, measured quantities in the one system go identically (or with a constant multiple for each) into those in the other. In particular, this implies constant ratios of forces in the two systems. Dynamic similarity between systems of fluid flow exists when the equations of motion and the boundary conditions of one system can be transformed into those of another by suitable dimensional changes. Two incompressible viscous flows, for example, are dynamically similar if the ratios of the inertial terms to the friction terms in the respective flows are of the same order of magnitude; this ratio is the Reynolds number of the flow, and, in an analogous way, the equality of other non-dimensional numbers (Rayleigh number, Rossby number, etc.) establishes the dynamic similarity of other pairs of flows