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

By R. A. Wallis

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Roughness density, too, can play an important p a r t ; an increase in "population" may not always lead to an increase in skin friction. <17> I t is fairly obvious, therefore, that test data relating to the actual surface are most desirable. The finished products of certain manufacturing processes often possess a similarity in their surface condition. Hence test results from representative specimens of these products can provide useful data in estimating skin friction. Nevertheless, care must always be exercised when applying this information since, for example, cast steel produced by one foundry may have a surface significantly different from that produced by another.

2) and hence it is more convenient to use Θ, the momentum thickness. 11) which is the most basic Reynolds number to be discussed here. I t is obvious, however, t h a t for flow over aerofoils and along plates, etc. the above Reynolds number will vary with distance and thus some Reynolds number capable of expressing the integrated effect is required. Such numbers will be discussed in subsequent sub-sections but it must be remembered t h a t they can only be justified if they satisfy the requirements of boundary layer theory.

The variation of λ with x follows from eq. 12) which then permits the computation of the local skin friction coefficients, Cf, from eq. 20). The total skin friction force is obtained by integrating eq. 31) This integration can be carried out by simple graphical methods. 4. Turbulent Boundary Layers Because of the mathematical difficulties associated with calculating turbulent layer characteristics from the basic equations of motion, most of the formulae available are empirical. There is, however, a marked similarity between the forms of the laminar and turbulent equations and as full a use as possible will be made of this feature.

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