Sandro Longo

Turbulence experiments in the swash zone

Abstract Turbulence and water surface elevation measurements were carried out in the swash zone which was induced by plunging and collapsing breakers in a wave flume. Three different period regular waves were generated on a 1/10 smooth impermeable concrete beach and the water surface elevations were collected by a number of wave gauges and a run-up meter lying on the bottom. Velocity measurements were carried out by a Laser Doppler velocimeter system, at spatial steps of 1 mm from the bottom, along three vertical sections in the swash zone. The data have been used to describe the wave front dynamics, to derive the length and velocity macro and micro scales and the Eulerian frequency spectrum. In some points, it shows a double structure. The results add to and expand on similar results obtained by other researchers in the surf zone.

Turbulence in the swash and surf zones: a review

This paper reviews mainly conceptual models and experimental work, in the field and in the laboratory, dedicated during the last decades to studying turbulence of breaking waves and bores moving in very shallow water and in the swash zone. The phenomena associated with vorticity and turbulence structures measured are summarised, including the measurement techniques and the laboratory generation of breaking waves or of flow fields sharing several characteristics with breaking waves. The effect of air entrapment during breaking is discussed. The limits of the present knowledge, especially in modelling a two- or three-phase system, with air and sediment entrapped at high turbulence level, and perspectives of future research are discussed.

Unsteady Flow of Shear-Thinning Fluids in Porous Media with Pressure-Dependent Properties

In this paper, a model for injection of a power-law shear-thinning fluid in a medium with pressure-dependent properties is developed in a generalized geometry (plane, radial, and spherical). Permeability and porosity are taken to be power functions of the pressure increment with respect to the ambient value. The model mimics the injection of non-Newtonian fluids in fractured systems, in which fractures are already present and are enlarged and eventually extended and opened by the fluid pressure, as typical of fracing technology. Empiric equations are combined with the fundamental mass balance equation. A reduced model is adopted, where the medium permeability resides mainly in the fractures; the fluid and porous medium compressibility coefficients are neglected with respect to the effects induced by pressure variations. At early and intermediate time, the flow interests only the fractures. In these conditions, the problem admits a self-similar solution, derived in closed form for an instantaneous injection (or drop-off) of the fluid, and obtained numerically for a generic monomial law of injection. At late times, the leak-off of the fluid towards the porous matrix is taken into account via a sink term in the mass balance equation. In this case, the original set of governing equations needs to be solved numerically; an approximate self-similar solution valid for a special combination of parameters is developed by rescaling time. An example of application in a radial geometry is provided without and with leak-off. The system behaviour is analysed considering the speed of the pressure front and the variation of the pressure within the domain over time, as influenced by the domain and fluid parameters.

SEDIMENT TRANSPORT UNDER (NON)-LINEAR WAVES AND CURRENTS

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

Non-Newtonian power-law gravity currents propagating in confining boundaries

The propagation of viscous, thin gravity currents of non-Newtonian liquids in horizontal and inclined channels with semicircular and triangular cross-sections is investigated theoretically and experimentally. The liquid rheology is described by a power-law model with flow behaviour index

Propagation of viscous gravity currents inside confining boundaries: the effects of fluid rheology and channel geometry

n

Gravity currents in a linearly stratified ambient fluid created by lock release and influx in semi-circular and rectangular channels

n, and the volume released in the channel is taken to be proportional to