Axial Flow Fans. Design and Practice by R. A. Wallis

By R. A. Wallis

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Drag The drag associated with a two-dimensional aerofoil is known as profile drag while a three-dimensional wing possesses, in addition, induced or secondary drag. 1. Profile drag. This drag arises from skin friction and pressure forces. The air flowing over the top surface of an 58 AEROFOIL DATA FOR BLADE DESIGN aerofoil is initially accelerated but then suffers retardation as the trailing edge is approached. 3). As a result, the pressure forces acting on the aerofoil surfaces downstream of the point of maximum thickness are less than they would be if the boundary layer were absent.

50) ; when substitution is made in eq. 55) 2ik, Substituting in eq. 53), the relation between roughness height and the Reynolds number above which the surface is fully rough is given by \ ~ 8 5 ( ^ - 1 ( 3 . 56) 50 BOUNDARY LAYER A N D SKIN FRICTION RELATIONS Eqs. 56) are graphically presented in Fig. 7 ; the various régimes are indicated. Using the foregoing data, the skin friction coefficient, γ, has been presented as a function of Reynolds number and equivalent roughness height in Fig. 8. The transitional curves are completely arbitrary, especially at the lower Reynolds numbers where they may join either the laminar or turbulent skin friction curves for smooth surfaces.

The laminar layer leaves the surface and becomes a " f r e e " layer. Except for small Reynolds numbers, transition occurs in the " f r e e " layer ; this may be followed by a re-attachment of the flow to the surface provided the-downstream static pressure gradient is not excessively adverse. 44 BOUNDARY LAYER AND SKIN FRICTION RELATIONS The position of laminar separation is independent of Reynolds number but increasing Reynolds number speeds up transition, thus facilitating flow re-attachment.

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