Measuring skin friction with Preston tubes in boundary-layer flows with additional parameters

For determining local skin friction forces near a wall, a surface pitot technique based on the Preston tube method is presented which allows the experimental determination of wall shear stress in boundary layers even when additional parameters are introduced such as an adverse pressure gradient, wall roughness, and heat transfer. The technique is based on the mathematical treatment of stagnation pressure at the Preston tube and allows the computation of calibration curves and the determination of skin friction by means of iteration from known wall laws taking into account the appropriate boundary layer similarity parameters. The technique was experimentally checked in a rough pipe, an intensively heated or cooled flat plate, and with a flat plate erected with adverse pressure gradient.