What is Reynolds number and what does it represent?

Reynolds number (Re = rho*V*L/mu) is a dimensionless parameter representing the ratio of inertial forces to viscous forces in a flow. It determines whether flow is laminar or turbulent, with higher Reynolds numbers favoring turbulent flow. In aerospace, Reynolds number affects boundary layer transition, drag characteristics, and is crucial for scaling wind tunnel results to full-scale aircraft. Typical aircraft operate at Reynolds numbers of 10^6 to 10^8.