A pressure instrument to measure skin friction in turbulent boundary layers on smooth and non-smooth walls

Abstract

SummaryA pressure instrument has been designed for the measurement of local skin friction in turbulent boundary layers on smooth and non-smooth walls. The instrument consists of three pitot tubes placed in the logarithmic mean velocity region of a turbulent boundary layer.An instrument was built and tested in the boundary layer produced on both smooth and rough walls. The rough walls consisted of two sandpaper abrasives of sizes #40 and #24. The experiments were performed over a range of momentum thickness Reynolds numbers, Rθ, from about 5 × 103 to 18 × 103, and in a small favourable pressure gradient given by –0·0005 < (υ/ρU3τ)(dp/dx) <–0·0001. The non-dimensional equivalent sand roughness, ks+ ≡ ksUτ/υ varied from zero to approximately 85, thus spanning all three roughness regimes from smooth to fully rough. Skin friction coefficients calculated using the three tube instrument were compared to those obtained using a skin friction balance. For the smooth wall measurements, skin friction relations and Preston tubes were also used as a basis of comparison. The results were found to agree fairly well for both the smooth and non-smooth walls, and was independent of the instrument's position within the logarithmic region. However, due to measurement difficulties the scatter and uncertainty were larger for the three tube instrument than the other techniques.Numerical smoothing of the results indicated that the agreement is expected to improve with better measurement of the pressure differences between the pitot tubes, or direct measurement of the velocity differences.