1,721,036 research outputs found
Sea ripple formation: the turbulent boundary layer case
No full textThis study presents a predictive theory of ripple formation beneath sea waves. The theory is based on a linear stability analysis of a flat sandy bottom subject to an oscillatory flow. The flow regime in the bottom boundary layer is assumed to be turbulent; hence previous works on the subject, which considered a laminar flow, are extended in a more relevant range of the Reynolds number. The turbulent stresses are described by means of the constant eddy viscosity model proposed by Sleath in 1991 and a two time-scale approach is used to decouple fluid motion from bottom time development. A closed form solution is found for the turbulent oscillatory how over a fixed wavy wall and the time development of the bottom perturbation is determined by means of sediment continuity equation. The conditions for ripple appearance are determined along with their wavelength as they form. A comparison between theoretical findings and experimental data supports the validity of the present approach
Sea ripple formation: the heterogenous sediment case
No full textRipple formation beneath sea waves is analyzed both by experimental and analytical means when the bottom is made up of a mixture of sands. An oscillatory flow is obtained in a closed duct by the oscillations of two rigidly connected pistons located at the ends of the duct. The amplitude and period of the oscillations can be continuously varied. A fixed tray, located at the bottom of the duct and filled with different types of sediments, allows ripple formation to be observed. The presence of graded sediments is found to have a stabilizing effect and causes longer ripples to appear. Moreover a selective sediment transport is observed and quantified which tends to pile up the coarse grains at ripple crests leaving the fine ones in the troughs. As in the companion paper, the theory is based on a linear stability analysis of a flat sandy bottom subject to an oscillatory flow. Because of the presence of a mixture, a modified version of Exner equation is used and an “hiding” factor should be inserted in the sediment transport rate formula. The flow regime in the bottom boundary layer is assumed to be turbulent. The conditions for ripple appearance are determined along with their wavelengths as they form. Good agreement is found between experimental data and theoretical findings
Numerical investigation of the oscillatory flow around a circular cylinder close to a wall at KC=10 and beta=20 and 50
No full textThe oscillatory flow around a circular cylinder close to a plane wall is investigated numerically, by direct numerical simulation of the Navier–Stokes equations. The main aim of the research is to gain insight into the effect of the wall on the vorticity dynamics and the forces induced by the flow over the cylinder. First, two-dimensional simulations are performed for nine values of the gap-to-diameter ratio e. Successively, three-dimensional simulations are carried out for selected cases to analyse the influence of the gap on the three-dimensional organization of the flow. An attempt to explain the pressure distribution around the cylinder in terms of vorticity time development is presented. Generally, the time development of the hydrodynamic forces is aperiodic (i.e. changes from cycle to cycle). In one case (Re = 200), when the distance of the cylinder from the wall is reduced, the behaviour of the flow changes from aperiodic to periodic. When the cylinder approaches the wall the drag coefficient of the in-line force increases in a qualitative agreement with the results reported in literature. The transverse force is not monotonic with the reduction of the gap: it first decreases down to a minimum, and then increases with a further reduction of the gap. For intermediate values of the gap the decrease of the transverse force is due to the reduction of the angle of ejection of the shedding vortices caused by the closeness of the wall; for small gaps the increase of the transverse force is due to the strong interaction between the vortex system ejected from the cylinder and the shear layer generated on the wall.
Three-dimensional simulations show that the flow is unstable with respect to spanwise perturbations which cause the development of three-dimensional vortices and the distortion of the two-dimensional ones generated by flow separation.
In all the analysed cases, the three-dimensional effects on the hydrodynamic forces are clearly attenuated when the cylinder is placed close to the wall.
The spanwise modulation of the vortex structures induces oscillations of the sectional forces along the axis of the cylinder which in general are larger for the transverse sectional force. In the high-Reynolds-number case (Re = 500), the reduction of the gap produces a large number of three-dimensional vortex structures developing over a wide range of spatial scales. This produces homogenization of the flow field along the spanwise direction and a consequent reduction of the amplitudes of oscillation of the sectional forces
A new shoreline boundary condition for a highly nonlinear 1DH Boussinesq model for breaking waves
No full textIn order to model the wave motion and, in turn, the flow, within the nearshore region, in the last decades the derivation and the application of Boussinesq type of models have been extensively investigated.
Nevertheless, in the framework of such depth integrated numerical models, the problems of modeling wave breaking and moving onshore boundary at the shoreline are not trivial and several approaches have been proposed to overcome these limits.
In the present work an effort toward a more physical based model of the surf and the swash zone has been accomplished. In particular, starting from the work of Musumeci et al. (2005), a new model of the shoreline boundary condition has been implemented. The shoreline boundary condition is developed with a fixed grid method with a wet-dry interface and with a linear extrapolation (Lynett et al. 2002) near the wet-dry boundary has been used and coupled with the shoreline equations (Prasad and Svendsen, 2003). To validate the model a classical test which adopts a monochromatic wave train over a plane beach has been performed. In particular the analytical solution derived by Carrier and Greenspan (1958) has been used for comparison.
The comparison between the analytical and numerical horizontal shoreline movements, gives a fairly good agreement. Other tests on breaking of solitary waves have been performed. The solitary wave shoreline oscillation is investigated by comparison with the experimental tests by Synolakis (1986). The results are in fairly good agreement with the experimental data
Migrating sea ripples
No full textRipple formation under sea waves is investigated by means of a linear stability analysis of a hat sandy bottom subject to the viscous flow which is present in the boundary layer at the bottom of propagating sea waves. Nonlinear terms in the momentum equation are retained to account for the presence of a steady drift. Hence the work by Blondeaux is extended by considering steeper waves and/or less deep waters. Second order effects in the sea wave steepness are found to cause neither destabilizing nor stabilizing effects on the process of ripple formation. However, because of the presence of a steady velocity component in the direction of wave propagation, ripples are found to migrate at a constant rate which is predicted as function of sediment and wave characteristics. The analysis assumes the flow regime in the bottom boundary layer to be laminar and the results are significant for ripples at the initial stage of their formation or for mature ripples of small amplitude (rolling-grain ripples). A comparison of the theoretical findings with laboratory experiments supports the reliability of the approach and of the theoretical results
A theoretical model of asymmetric wave ripples
No full textThe time development of ripples under sea waves is investigated by means of the weakly nonlinear stability analysis of a flat sandy bottom subjected to the viscous oscillatory flow that is present in the boundary layer at the bottom of propagating sea waves. Second-order effects in the wave steepness are considered, to take into account the presence of the steady drift generated by the surface waves. Hence, the work of Vittori & Blondeaux (1990 J. Fluid Mech. 218, 19-39 (doi:10.1017/S002211209000091X)) is extended by considering steeper waves and/or less deep waters. As shown by the linear analysis of Blondeaux et al. (2000 Eur. J. Mech. B 19, 285-301 (doi:10.1016/S0997-7546(90)00106-I)), because of the presence of a steady velocity component in the direction of wave propagation, ripples migrate at a constant rate that depends on sediment and wave characteristics. The weakly nonlinear analysis shows that the ripple profile is no longer symmetric with respect to ripple crests and troughs and the symmetry index is computed as a function of the parameters of the problem. In particular, a relationship is determined between the symmetry index and the strength of the steady drift. A fair agreement between model results and laboratory data is obtained, albeit further data and analyses are necessary to determine the behaviour of vortex ripples and to be conclusive
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