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

Wall-drag measurements of smooth- and rough-wall turbulent boundary layers using a floating element The mean wall shear stress, $$\overline{\tau }_w$$ τ ¯ w , is a fundamental variable for characterizing turbulent boundary layers. Ideally, $$\overline{\tau }_w$$ τ ¯ 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 $$\overline{\tau }_w$$ τ ¯ 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 $$\overline{\tau }_w$$ τ ¯ 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, $$C_f \equiv \overline{\tau }_w/q_{\infty }$$ C f ≡ τ ¯ w / q ∞ , follow a Coles–Fernholz relation $$C_f = \left[ 1/\kappa \ln \left( Re_{\theta }\right) + C\right] ^{-2}$$ C f = 1 / κ ln R e θ + C - 2 to within 3 % ( $$\kappa = 0.38$$ κ = 0.38 and $$C = 3.7$$ C = 3.7 ) for a momentum thickness-based Reynolds number, $$Re_{\theta } > 15{,}000$$ R e θ > 15 , 000 . The agreement improves for higher Reynolds numbers to <1 % deviation for $$Re_{\theta } > 38{,}000$$ R e θ > 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 $$10{,}500< Re_{\theta } < 88{,}500$$ 10 , 500 < R e θ < 88 , 500 , for which the wall-drag indicates the anticipated trend from the transitionally to the fully rough regime . http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Experiments in Fluids Springer Journals

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

, Volume 57 (5) – May 9, 2016
16 pages

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Publisher
Springer Berlin Heidelberg
Copyright
Copyright © 2016 by Springer-Verlag Berlin Heidelberg
Subject
Engineering; Engineering Fluid Dynamics; Fluid- and Aerodynamics; Engineering Thermodynamics, Heat and Mass Transfer
ISSN
0723-4864
eISSN
1432-1114
D.O.I.
10.1007/s00348-016-2168-y
Publisher site
See Article on Publisher Site

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