ASYMMETRY AND SKEWNESS IN THE BOTTOM BOUNDARY LAYER : SMALL SCALE EXPERIMENTS AND NUMERICAL MODEL

Céline Berni, Leandro Suarez, Hervé Michallet, Eric Barthélemy

Abstract


This study investigates the non-linearities of wave boundary layers in the surf zone. It mainly focuses on the acceleration skewness or asymmetry. Experiments [e.g. Grasso et al., 2011] show that asymmetry influences the sediment transport. Its influence lies in the fact that asymmetry in velocity (acceleration skewness) tends to transform into velocity skewness within the boundary layer. Analysis by Henderson et al. [2004] predicts a linear relation between Skb/Sk∞ and As∞/Sk∞ where Skb is the dimensionless skewness near the bed, Sk∞ the free-stream dimensionless skewness and As∞ the free-stream dimensionless asymmetry. Numerous experiments were carried out in the LEGI wave flume over a mobile bed composed of lightweight sediments. The quasi-random forcing is a repetition of 2 concatenated bichromatic wave packets. Vertical profiles of velocity are measured in the surf zone. A clear linear relation is shown between these two ratios. The experimental results are compared with the numerical results. A linear relation between skewness and asymmetry is also obtained.

Keywords


non linearities, transformation, k-ω model, mobile bed

References


Abreu, T., P. A. Silva, F. Sancho, and A. Temperville (2010). Analytical approximate wave form for asymmetric waves. Coastal Eng., 57(7):656–667.http://dx.doi.org/10.1016/j.coastaleng.2010.02.005

Berni, C., H. Michallet, and E. Barthelemy (submitted), Surf zone cross-shore boundary layer velocity asymmetry ´ and skewness: an experimental study on a mobile bed, J. Geophys. Res.

Grasso, F., H. Michallet, E. Barthelemy and R. Certain (2009), Physical modeling of intermediate ´ cross-shore beach morphology: Transients and equilibrium states, J. Geophys. Res., 114, C09001, Grasso, F., H. Michallet, and E. Barthelemy (2011). Sediment transport associated with morphological beach ´ changes forced by irregular asymmetric, skewed waves. J. Geophys. Res., 116(C3):C03020.http://dx.doi.org/10.1029/2009JC005308

Guizien, K., M. Dohmen-Janssen and G. Vittori (2003), 1DV bottom boundary layer modelling under combined wave and current: suspension ejection events and phase lag effects. J. Geophys. Res., 108(C1):3016.http://dx.doi.org/10.1029/2001JC001292

Henderson, S. M., J. S. Allen, and P. A. Newberger (2004), Nearshore sandbar migration predicted by an eddydiffusive boundary layer model. J. Geophys. Res., 109, C05025, 109.

Hurther, D. (2001), 3-D acoustic Doppler velocimetry and turbulence in open-channel flow, Ph.D. thesis, Ecole ´ Polytechnique Fed´ erale de Lausanne. ´

Hurther, D. and P. D. Thorne (2011), Suspension and near-bed load sediment transport processes above a migrating, sand-rippled bed under shoaling waves, J. Geophys. Res., 116, C07001, doi:10.1029/2010JC006774.http://dx.doi.org/10.1029/2010JC006774

Menter, F. R. (1994), Two-equation eddy-viscosity turbulence models for engineering applications. AIAA Journal, 32 (8), pp 1598-1605.http://dx.doi.org/10.2514/3.12149

O'Donoghue, T. and S. Wright (2006), Flow tunnel measurements of velocities and sand flux in oscillatory sheet flow for well-sorted and graded sands, Coast. Eng., 51, 1163–1184.http://dx.doi.org/10.1016/j.coastaleng.2004.08.001

Ruessink, B. G., H. Michallet, T. Abreu, F. Sancho, D. A. Van der A, J. J. Van der Werf, and P. A. Silva (2011), Observations of velocities, sand concentrations, and fluxes under velocity-asymmetric oscillatory flows. J. Geophys. Res., 116(C3):C03004.http://dx.doi.org/10.1029/2010JC006443

Silva, P. A., Abreu T., H. Michallet, D. Hurther, and F. Sancho (2009), Sheet flow layer structure under oscillatory flow, in River, Coastal and Estuarine Morphodynamics, Santa Fe, Argentina, 21-25 Sept 2009.

Sparrow, K., D. Pokrajac, and D. van der A. (2012). The effect of bed permeability on oscillatory boundary layer flow, Proceedings of the International Conference on Coastal Engineering, 723.

Suntoyo, and H. Tanaka (2009), Effects of bed roughness on turbulent boundary layer and net sediment transport under asymmetric waves, Coast. Eng., 56, 960–969.

Swart, D.H. (1974), A schematization of onshore-offshore transport, paper presented at the 14th International Conference on Coastal Engineering, Coastal Eng. Res. Counc., Copenhagen, Denmark.

Wilcox, D. C. (2006), Turbulence modeling for CFD, 3rd edition. DCW Industries, Inc., La Canada CA.

Wilson, K.C. (1989), Mobile-bed friction at high shear stress, J. Hydraul. Res., 1156, 825–830.

Wilson, K.C. (1989), Analysis of Bed-Load Motion at High Shear Stress, J. Hydraul. Eng., 113, 97–0.13.


Full Text: PDF

Creative Commons License
This work is licensed under a Creative Commons Attribution 3.0 License.