What is relation between Reynolds number-turbulent flow-flow separation?
In most commercial aircraft applications the Reynolds number per foot of chord length lies between 1.5 and 2 million per foot. Major operations of takeoff and cruise for turboprop and turbofan airliners fall in this range. Consider the following mean aerodynamic chords:

Turboprop Aircraft = 6.9 ft &lt; cMAC &lt; 10.6 ft
= Reynolds number of 10-20 million

Turbofan Airliners = 13.3 ft &lt; cMAC &lt; 26.9 ft
= Reynolds number of 20-54 million

B747/A380 size Aircraft = 30.6 ft &lt; cMAC &lt; 40.33 ft = Reynolds number of 45-80 million


Note that for highly tapered wings the outboard chords may be considerably smaller than the mean aerodynamic chord and will experience lower Reynolds numbers. During low-speed operations like landing and takeoff, high lift devices such as flaps and slats must be used, because the characteristic lengths of these elements are considerably smaller than the local wing chord they will be operating at lower Reynolds number and therefore more susceptible to flow separation and stalling. The general trend is for cl,max to increase slowly, if at all, with increases in Reynolds number and for that increase to be more substantial for thicker sections. This is understandable because as the Reynolds number increases the boundary layer effects become relatively weaker allowing the flow to remain attached to the airfoil for longer distances along the airfoil surface. The lift of an airfoil depends primarily on keeping the flow attached to the airfoil while friction drag itself weakly influences the lift of an airfoil.
A more detailed explanation is available here