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Published online by Cambridge University Press: 04 July 2016
In a recent discussion on measurements made by Quarmby in fully-developed turbulent flow in concentric annuli, Quarmby has made several statements which are incorrect and need to be clarified.
Quarmby states that the pitot displacement corrections measured by MacMillan are those solely due to the shear and that they do not take account of the effect due to the presence of the wall. This is not so. Fig. 9 of MacMillan's original paper shows the total displacement effect. It is this total displacement effect which was used in my paper on the calibration of Preston tubes to relate the calibration curve to the inner-law velocity distribution. It is quite obvious that a Preston tube will be influenced both by shear as well as the wall and it would have been pointless to attempt to correlate its calibration with the velocity distribution using pitot displacement corrections unless these corrections included the rather important influence of the wall.