Abstract
Preston's method, which uses a simple pilot tube resting on the surface, the so-called Preston tube, provides one of the most convenient ways of measuring turbulent skin friction. It depends on the assumption of the existence of flow similar to the wall layer. If the Preston tube readings are the same in different types of flow, then the similarity between the wall layers in different kinds of flow is substantiated. If not, on the other hand, they would give an indication of different similarity laws between the wall layers of different flow species. The present experiments have been designed to measure the skin friction of turbulent channel and boundary layer flows by using the same Preston tubes in order to study the similarity rules governing them. Four round Pilot tubes, whose sizes are similar to those originally used by Preston, were employed and their diameters were taken as the representative length to determine local skin friction. The experiments show clear differences between the two flows, giving higher values in the channel and lower values in the boundary layer than those which have been reported by earlier authors.