Reynold number Symbol R_e. A dimensionless number defined for a flowing incompressible fluid.

R_e = r v D / h (r18)

where r is the density, h the viscosity, v the velocity of the fluid. D is the characteristic length for a particular geometry, like diameter of a cylinder around which the fluid flows. The significance of the Reynolds number is that for any two situation that have the same value of R_e , the characteristic of flow is exactly the same. This allows the study with smaller scale models using wind tunnel and model basin. In fluids that have appreciable compressibility, the Mach number* is also significant near the speed of sound or above. For similar situation, in such fluids, both Reynolds number and Mach number should have the same value