Evaluation of Preston Tube Calibration Equations in Supersonic Flow

Abstract

Theme S by many investigators have for some time established that the surface pitot tube can be used to measure local turbulent skin friction. Circular surface pitot tubes are generally called Tubes, after the author of Ref. 1. Basically, the Preston Tube technique, as seen in Fig. 1, consists of bringing a tube of appropriate size into contact with the test surface at the location where local skin friction is desired, measuring the pitot pressure sensed by this tube, and combining this pressure with local freestream conditions and tube diameter to calculate local skin friction from an existing calibration equation. The main advantages of the Preston Tube over other skin-friction measuring techniques is its sturdiness, simplicity of use, and the fact that calibration of individual tubes is not required. In previous calibration experiments in supersonic flow,' it was found that data from all Reynolds numbers, Mach numbers, and tube diameters tested collapsed into a single narrow band when the appropriate calibration parameters were used. The calibrations were thus reported to be independent of Mach and Reynolds numbers and tube diameter. The range of tube sizes theoretically usable, however, is very large; much larger than that used by the previous investigators in performing their calibrations. This study was thus initiated to provide calibration data at larger Reynolds-number-based-on-tube-diameter (RD) and higher supersonic Mach numbers (Me) than previously existed. These new data and existing supersonic data are used in this study to evaluate both existing Preston Tube calibration equations and new equations developed in this study in light of the new data. The larger values of RD and Me of the new