Journal of Fluid Mechanics

Papers

Experimental study of the turbulent boundary layer in acceleration-skewed oscillatory flow

Dominic A. van der Aa1 c1, Tom O’Donoghuea1, Alan G. Daviesa2 and Jan S. Ribberinka3

a1 School of Engineering, University of Aberdeen, Aberdeen AB24 3UE, UK

a2 School of Ocean Sciences, Bangor University, Menai Bridge, Anglesey LL59 5AB, UK

a3 Department of Water Engineering and Management, University of Twente, Enschede, PO Box 217, 7500 AE, The Netherlands

Abstract

Experiments have been conducted in a large oscillatory flow tunnel to investigate the effects of acceleration skewness on oscillatory boundary layer flow over fixed beds. As well as enabling experimental investigation of the effects of acceleration skewness, the new experiments add substantially to the relatively few existing detailed experimental datasets for oscillatory boundary layer flow conditions that correspond to full-scale sea wave conditions. Two types of bed roughness and a range of high-Reynolds-number, $\mathit{Re}\ensuremath{\sim} O(1{0}^{6} )$, oscillatory flow conditions, varying from sinusoidal to highly acceleration-skewed, are considered. Results show the structure of the intra-wave velocity profile, the time-averaged residual flow and boundary layer thickness for varying degrees of acceleration skewness, $\ensuremath{\beta} $. Turbulence intensity measurements from particle image velocimetry (PIV) and laser Doppler anemometry (LDA) show very good agreement. Turbulence intensity and Reynolds stress increase as the flow accelerates after flow reversal, are maximum at around maximum free-stream velocity and decay as the flow decelerates. The intra-wave turbulence depends strongly on $\ensuremath{\beta} $ but period-averaged turbulent quantities are largely independent of $\ensuremath{\beta} $. There is generally good agreement between bed shear stress estimates obtained using the log-law and using the momentum integral equation, and flow acceleration skewness leads to high bed shear stress asymmetry between flow half-cycles. Turbulent Reynolds stress is much less than the shear stress obtained from the momentum integral; analysis of the stress contributors shows that significant phase-averaged vertical velocities exist near the bed throughout the flow cycle, which lead to an additional shear stress, $\ensuremath{-} \rho \tilde {u} \tilde {w} $; near the bed this stress is at least as large as the turbulent Reynolds stress.

(Received July 11 2010)

(Reviewed May 26 2011)

(Accepted July 06 2011)

(Online publication September 06 2011)

Key Words:

  • coastal engineering;
  • geophysical and geological flows;
  • turbulent boundary layers

Correspondence:

c1 Email address for correspondence: d.a.vandera@abdn.ac.uk

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