Well-resolved streamwise velocity spectra are reported for smooth-and rough-wall turbulent pipe flow over a large range of Reynolds numbers. The turbulence structure far from the wall is seen to be unaffected by the roughness, in accordance with Townsend's Reynolds number similarity hypothesis. Moreover, the energy spectra within the turbulent wall region follow the classical inner and outer scaling behaviour. While an overlap region between the two scalings and the associated k −1 x law are observed near R + ≈ 3000, the k −1x behaviour is obfuscated at higher Reynolds numbers due to the evolving energy content of the large scales (the very-large-scale motions, or VLSMs). We apply a semi-empirical correction (delÁlamo & Jiménez, J. Fluid Mech., vol. 640, 2009, pp. 5-26) to the experimental data to estimate how Taylor's frozen field hypothesis distorts the pseudo-spatial spectra inferred from time-resolved measurements. While the correction tends to suppress the long wavelength peak in the logarithmic layer spectrum, the peak nonetheless appears to be a robust feature of pipe flow at high Reynolds number. The inertial subrange develops around R + > 2000 where the characteristic k −5/3x region is evident, which, for high Reynolds numbers, persists in the wake and logarithmic regions. In the logarithmic region, the streamwise wavelength of the VLSM peak scales with distance from the wall, which is in contrast to boundary layers, where the superstructures have been shown to scale with boundary layer thickness throughout the entire shear layer. Moreover, the similarity in the streamwise wavelength scaling of the large-and very-large-scale motions supports the notion that the two are physically interdependent.
Results on turbulent skin friction reduction over air- and liquid-impregnated surfaces are presented for aqueous Taylor-Couette flow. The surfaces are fabricated by mechanically texturing the inner cylinder and chemically modifying the features to make them either non-wetting with respect to water (air-infused, or superhydrophobic case), or wetting with respect to an oil that is immiscible with water (liquid-infused case). The drag reduction, which remains fairly constant over the Reynolds number range tested (100 ≤ Reτ ≤ 140), is approximately 10% for the superhydrophobic surface and 14% for the best liquid-infused surface. Our results suggest that liquid-infused surfaces may enable robust drag reduction in high Reynolds number turbulent flows without the shortcomings associated with conventional superhydrophobic surfaces, namely, failure under conditions of high hydrodynamic pressure and turbulent flow fluctuations.
The present results indicate that the use of the antibiotic-impregnated PMMA pin sleeve can prevent the development of pin-tract infection and appears to prevent colonization of the external fixation pins.
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