Wall-drag measurements of smooth- and rough-wall turbulent boundary layers using a floating element

Autor: M. R. Abbassi, Ivan Marusic, Dougal T. Squire, Woutijn J. Baars, K. M. Talluru, Nicholas Hutchins
Rok vydání: 2016
Předmět:
Zdroj: Experiments in Fluids. 57
ISSN: 1432-1114
0723-4864
Popis: The mean wall shear stress, $$\overline{\tau }_w$$ , is a fundamental variable for characterizing turbulent boundary layers. Ideally, $$\overline{\tau }_w$$ is measured by a direct means and the use of floating elements has long been proposed. However, previous such devices have proven to be problematic due to low signal-to-noise ratios. In this paper, we present new direct measurements of $$\overline{\tau }_w$$ where high signal-to-noise ratios are achieved using a new design of a large-scale floating element with a surface area of 3 m (streamwise) × 1 m (spanwise). These dimensions ensure a strong measurement signal, while any error associated with an integral measurement of $$\overline{\tau }_w$$ is negligible in Melbourne’s large-scale turbulent boundary layer facility. Wall-drag induced by both smooth- and rough-wall zero-pressure-gradient flows are considered. Results for the smooth-wall friction coefficient, $$C_f \equiv \overline{\tau }_w/q_{\infty }$$ , follow a Coles–Fernholz relation $$C_f = \left[ 1/\kappa \ln \left( Re_{\theta }\right) + C\right] ^{-2}$$ to within 3 % ( $$\kappa = 0.38$$ and $$C = 3.7$$ ) for a momentum thickness-based Reynolds number, $$Re_{\theta } > 15{,}000$$ . The agreement improves for higher Reynolds numbers to 38{,}000$$ . This smooth-wall benchmark verification of the experimental apparatus is critical before attempting any rough-wall studies. For a rough-wall configuration with P36 grit sandpaper, measurements were performed for $$10{,}500< Re_{\theta } < 88{,}500$$ , for which the wall-drag indicates the anticipated trend from the transitionally to the fully rough regime .
Databáze: OpenAIRE
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