Abstract
The principle of wall similarity expressed by a universal logarithmic inner law relationship is well established for turbulent flow in circular pipes. The validity of Preston's original hypothesis for the measurement of turbulent skin friction depends upon the existence of wall similarity. Although Preston's hypothesis appears to be correct in general it has been found that his calibration for the wall pitot tube was incorrect. Extensive observations by Head and Rechenberg showed some discrepancy. Later work by Patel on the measurement of skin friction in three circular pipes covering a much larger experimental range has led to a modified calibration.
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