ORCID Profile
0000-0003-1997-801X
Current Organisation
United States Naval Academy
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Publisher: Cambridge University Press (CUP)
Date: 14-04-2016
DOI: 10.1017/JFM.2016.196
Abstract: Turbulent boundary layer measurements above a smooth wall and sandpaper roughness are presented across a wide range of friction Reynolds numbers, ${\\it\\delta}_{99}^{+}$ , and equivalent sand grain roughness Reynolds numbers, $k_{s}^{+}$ (smooth wall: $2020\\leqslant {\\it\\delta}_{99}^{+}\\leqslant 21\\,430$ , rough wall: $2890\\leqslant {\\it\\delta}_{99}^{+}\\leqslant 29\\,900$ $22\\leqslant k_{s}^{+}\\leqslant 155$ and $28\\leqslant {\\it\\delta}_{99}^{+}/k_{s}^{+}\\leqslant 199$ ). For the rough-wall measurements, the mean wall shear stress is determined using a floating element drag balance. All smooth- and rough-wall data exhibit, over an inertial sublayer, regions of logarithmic dependence in the mean velocity and streamwise velocity variance. These logarithmic slopes are apparently the same between smooth and rough walls, indicating similar dynamics are present in this region. The streamwise mean velocity defect and skewness profiles each show convincing collapse in the outer region of the flow, suggesting that Townsend’s ( The Structure of Turbulent Shear Flow , vol. 1, 1956, Cambridge University Press.) wall-similarity hypothesis is a good approximation for these statistics even at these finite friction Reynolds numbers. Outer-layer collapse is also observed in the rough-wall streamwise velocity variance, but only for flows with ${\\it\\delta}_{99}^{+}\\gtrsim 14\\,000$ . At Reynolds numbers lower than this, profile invariance is only apparent when the flow is fully rough. In transitionally rough flows at low ${\\it\\delta}_{99}^{+}$ , the outer region of the inner-normalised streamwise velocity variance indicates a dependence on $k_{s}^{+}$ for the present rough surface.
Publisher: Informa UK Limited
Date: 26-11-2015
DOI: 10.1080/08927014.2015.1108407
Abstract: Skin-friction results are presented for fouling-release (FR) hull coatings in the unexposed, clean condition and after dynamic exposure to diatomaceous biofilms for 3 and 6 months. The experiments were conducted in a fully developed turbulent channel flow facility spanning a wide Reynolds number range. The results show that the clean FR coatings tested were hydraulically smooth over much of the Reynolds number range. Biofilms, however, resulted in an increase in skin-friction of up to 70%. The roughness functions for the biofilm-covered surfaces did not display universal behavior, but instead varied with the percentage coverage by the biofilm. The effect of the biofilm was observed to scale with its mean thickness and the square root of the percentage coverage. A new effective roughness length scale (keff) for biofilms based on these parameters is proposed. Boundary layer similarity-law scaling is used to predict the impact of these biofilms on the required shaft power for a mid-sized naval surface combatant at cruising speed. The increase in power is estimated to be between 1.5% and 10.1% depending on the biofilm thickness and percentage coverage.
Publisher: Annual Reviews
Date: 05-01-2021
DOI: 10.1146/ANNUREV-FLUID-062520-115127
Abstract: Reliable full-scale prediction of drag due to rough wall-bounded turbulent fluid flow remains a challenge. Currently, the uncertainty is at least 10%, with consequences, for ex le, on energy and transport applications exceeding billions of dollars per year. The crux of the difficulty is the large number of relevant roughness topographies and the high cost of testing each topography, but computational and experimental advances in the last decade or so have been lowering these barriers. In light of these advances, here we review the underpinnings and limits of relationships between roughness topography and drag behavior, focusing on canonical and fully turbulent incompressible flow over rigid roughness. These advances are beginning to spill over into multiphysical areas of roughness, such as heat transfer, and promise broad increases in predictive reliability.
Publisher: American Physical Society (APS)
Date: 26-05-2017
Publisher: Elsevier BV
Date: 06-2206
Publisher: Cambridge University Press (CUP)
Date: 28-12-2016
DOI: 10.1017/JFM.2016.832
Abstract: The spatial structure of smooth- and rough-wall boundary layers is examined spectrally at approximately matched friction Reynolds number ( $\\unicode[STIX]{x1D6FF}^{+}\\approx 12\\,000$ ). For each wall condition, temporal and true spatial descriptions of the same flow are available from hot-wire anemometry and high-spatial-range particle image velocimetry, respectively. The results show that over the resolved flow domain, which is limited to a streamwise length of twice the boundary layer thickness, true spatial spectra of smooth-wall streamwise and wall-normal velocity fluctuations agree, to within experimental uncertainty, with those obtained from time series using Taylor’s frozen turbulence hypothesis ( Proc. R. Soc. Lond. A, vol. 164, 1938, pp. 476–490). The same applies for the streamwise velocity spectra on rough walls. For the wall-normal velocity spectra, however, clear differences are observed between the true spatial and temporally convected spectra. For the rough-wall spectra, a correction is derived to enable accurate prediction of wall-normal velocity length scales from measurements of their time scales, and the implications of this correction are considered. Potential violations to Taylor’s hypothesis in flows above perturbed walls may help to explain conflicting conclusions in the literature regarding the effect of near-wall modifications on outer-region flow. In this regard, all true spatial and corrected spectra presented here indicate structural similarity in the outer region of smooth- and rough-wall flows, providing evidence for Townsend’s wall-similarity hypothesis ( The Structure of Turbulent Shear Flow , vol. 1, 1956).
No related grants have been discovered for Michael Schultz.