Reynolds number is the ratio of the fluid's inertia forces to the viscous forces in the boundary layer of the fluid. It is an important parameter in deter- mining the dynamic similarity of flow around models and full-scale aircraft. When the model data are obtained at much lower Reynolds numbers than those encoun- tered at full-scale conditions,the inertia forces of the fluid on the model are much lower in proportion to the viscous forces than those on the full-scale airplane.
As a consequence, the flow conditions are no longer dynamically similar.
The point of transition from laminar to turbulent flow, the thickness of and velocity in the boundary layer at any streamwise station on a surface, and the angle of attack at which the flow field separates from the surface are all functions of Reynolds number. The boundary-layer (viscous flow) conditions on any configuration affect the drag coefficient throughout the angle of attack range and the maximum lift and stall characteristics of the aircraft. The precise effect depends on the particular airfoil and planform used, and on the interference effects of the fuselage and nacelies or pods.
As Reynolds number increases, the point on the surface along the flow line at which the boundary layer changes from laminar to turbulent moves forward. The precise point or locus of transition is affected by the geometry of the surface or body and by the resulting pressure distribution, surface roughness or wavi- ness , and the magnitude of the velocity fluctuations in the airstream. A s a result , it is difficult to extrapolate model test results of natural transition effects obtained in present test facilities to full-scale Reynolds numbers. Efforts are frequently made to simulate flow conditions typical of higher-than-test Reynolds numbers by artificially fixing the transition using strips of roughness particles (grit) or
other flow-tripping devices. The test results at several Mach numbers are then extrapolated to full-scale Reynolds numbers .