can some explain conection betwen re number -turbulent flow -and flow separation ,in example at wing in the air and water?

1)does high numbers= turbulent flow mean higher resistance to flow separation and low numbers=laminar flow less resitance to flow separation? (becuase we use vortex generator to make trubulent flow which delay flow separation on upper wing surface...)

2)why L distance of chord has something with which type of flow will be over wing? so wide chord will have more turbulent flow but it will delay flow separation?

3)in formula i see density and velocity in numerator,so high density and speed will delay flow separatin compare to lower density and lower speed?

  • 1
    $\begingroup$ I realize English is not your first language, but proper punctuation and capitalization is a thing in most languages, and would help make this question more readable. $\endgroup$ Jul 10, 2020 at 18:50
  • $\begingroup$ so nobody answer? $\endgroup$ Jul 11, 2020 at 12:16
  • $\begingroup$ What is the effect of airflow speed on separation? $\endgroup$
    – mins
    Jul 12, 2020 at 10:49
  • $\begingroup$ @mins,isnt speed increase adrverse pressure gradient which will cause flow separation.so we cant say that with with speed we will dealy flow separation because of higher Re. numbers? $\endgroup$ Jul 14, 2020 at 5:50
  • $\begingroup$ @AeronauticFreek: Separation occurs because of the higher static pressure / lower kinetic pressure encountered while moving downstream within the boundary layer (adverse gradient due to viscosity and leading to deceleration). Lower kinetic pressure happens at lower relative speed / Re. Higher relative speed / Re translates (rather than increases, the gradient is along the chord) the adverse gradient downstream along the chord, delaying flow separation. So higher Re delays separation. $\endgroup$
    – mins
    Jul 14, 2020 at 9:49

1 Answer 1


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 < cMAC < 10.6 ft = Reynolds number of 10-20 million

  • Turbofan Airliners = 13.3 ft < cMAC < 26.9 ft = Reynolds number of 20-54 million

  • B747/A380 size Aircraft = 30.6 ft < cMAC < 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


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