The minimum Reynolds number for a turbulent boundary layer and the selection of a transition device

In the case of turbulent flow in a pipe there is a lower experimental number to the Reynolds limit for which fully developed turbulent flow occurs. From the similarity and close agreement of the curves showing the coefficient of skin friction c f as a function of the Reynolds number R θ (based on the momentum thickness θ) for the circular pipe and flat plate, it is suggested that there should be a lower limit to R θ for fully developed turbulent flow on a flat plate. Rather limited experimental data confirm this and place the lower limit at R θ = 320. The choice and size of transition device is examined in relation to this minimum R θ and an approximate theory leads to a ‘wire’ Reynolds number in fair agreement with experience.