Seminars

An accurate measure of the mean wall-shear stress is invaluable when studying turbulent boundary layers and is evidenced by its role in the friction velocity Uτ, a primary scaling parameter of near-wall flow dynamics. Ideally, wall-shear stress is assessed by direct force measurements on frictionless supported elements. Credible measures of local friction coefficients, at discrete momentum-based Reynolds numbers Reθ, are made when the variation of shear over the element is negligible. Historically, small floating elements had to be designed to satisfy this locality condition, but were subject to low signal-to-noise ratios and systematic error. The current work proves the viability of accurate floating element measures of local wall-shear. The element comprises a large surface area of 3m (streamwise) by 1m (spanwise), resulting in a sufficiently strong measurement signal, while error associated with an integral measure of the shear is insignificant in Melbourne’s capacious boundary layer facility. Friction values over a smooth wall follow published trends within ±1% for conditions ranging from Reθ = 15,000 to 48,000. For a rough-wall configuration with P36 grit sandpaper, measurements were performed in the range Reθ = 10,000 to 90,000 and indicate an anticipated trend from the transitionally- to fully-rough regime. Finally, we actively perturb large-scale structures in the log-region of the smooth-wall boundary layer with the aim to affect shear. The feedforward control setup comprises a spanwise array of hot-films to sense the footprint of large-scale structures; these are subsequently perturbed by an array of wall-normal jets. Friction measurements –in essence parasitic drag measures since drag of the embedded control configuration is included– show a promising potential for 10% reduction in skin-friction.