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

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 highReynolds-number, Re ∼ O(10 6 ), oscillatory flow conditions, varying from sinusoidal to highly acceleration-skewed, are considered. Results show the structure of the intrawave velocity profile, the time-averaged residual flow and boundary layer thickness for varying degrees of acceleration skewness, β. 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 β but period-averaged turbulent quantities are largely independent of β. There is generally good agreement between bed shear stress estimates obtained using the loglaw 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, −ρũw; near the bed this stress is at least as large as the turbulent Reynolds stress.

DNS study of particle-bed–turbulence interactions in an oscillatory wall-bounded flow

“…et al (2007) Dixen et al (2008) Van et al (Colour online) Delineation of different flow regimes for oscillatory flow illustrating previous studies(Jonsson & Carlsen 1976;Kemp & Simons 1982;Sleath 1987;Jensen et al 1989;Krstic & Fernando 2001;Chen et al 2007;Dixen et al 2008;van der A et al 2011) and present cases. Symbols represent: u , Re δ = 95; f , Re δ = 150; p , Re δ = 200; q , Re δ = 400.…”

PIV measurements of the bottom boundary layer under nonlinear surface waves

“…To date, uncertainties regarding flow properties of the unsteady wbbl, especially under non-linear surface waves, still exist, for example, the time evolution of the bottom shear stress over a non-linear wave cycle (Abreu et al, 2013;Mirfenderesk and Young, 2003;Nielsen, 1992;van der A et al, 2011). In a way, this is surprising, as the bottom shear stress is the basis for many sediment transport formulations (Ribberink, 1998;van Rijn, 2007).…”

Transition of transient channel flow after a change in Reynolds number

“…There are extensive studies of periodic flows around a non-zero mean, and oscillatory flows around zero mean flows. Readers interested in those topics are referred to recent studies of Scotti & Piomelli (2001), Tardu & Da Costa (2005), He & Jackson (2009) and Manna, Vacca & Verzicco (2012) for pulsating flows and Fornarelli & Vittori (2009) and Van der A et al (2011) for oscillatory flows.…”