The Calibration of Preston Tubes in Transpired Turbulent Boundary Layers

The Preston tube calibration of Patel is reproduced for zero pressure gradient and zero mass transfer turbulent flows, by momentum balance and by direct measurement using a floating element with mass transfer capability. The calibration is then extended to blowing and suction conditions, from near blow-off through to relaminarization. The effects of mass transfer are found to be moderate until conditions are encountered approaching relaminarization or blow-off. Transpiration effects are adequately correlated by a transpiration parameter proposed by Baker et al. As either the blow-off or suction relaminarization condition is approached, it is recommended that three or more sizes of Preston tubes be used which satisfy Patel’s middle range criterion. As suction relaminarization is approached the smallest tubes will show progressively greater departure from the wall similarity correlation. If a unique value of shear stress is obtained yet from the larger tubes it is valid. A similar test for consistency is recommended for conditions near blow-off. The study is extended to a boundary layer fence, which shows no significant mass transfer effect upon the zero transpiration calibration.