The use of pitot-tubes in the measurement of laminar boundary

Abstract

Consideration of experimental results obtained with relatively large pitot-tubes in relatively thin laminar boundary layers on cones and flat plates in supersonic wind tunnels and then analysed using standard pitot equations, shows that the most noticeable distortion of the velocity profile is the appearance of a peak near the outer edge of the boundary layer. The displacement of the main body of the profile, familiar in incompressible-flow tests, may be small in supersonic flow and difficult to detect when making measurements with small-diameter pitot-tubes. Displacement and momentum thicknesses calculated from pitot traverses will be in error because of this distortion and displacement. The simple correction factor obtained by Davies is shown to correlate results obtained by Blue and Low and indicates that if the ratio of tube diameter to boundary-layer thickness is less than 0.2, then measured values may be less than 4 per cent above their true values. Additional profile distortion may occur if flattened pitot-tubes are used to measure boundary layers on slender bodies of revolution and Appendix II describes the method of manufacture of tapered quartz tips of circular crosssection, which have a better response rate than flattened tubes of the same internal height.