Enrico Foti

The permeability of a random medium: Comparison of simulation with theory

The results of numerical simulations of the lattice‐Boltzmann equation in three‐dimensional porous geometries constructed by the random positioning of penetrable spheres of equal radii are presented. Numerical calculations of the permeability are compared with previously established rigorous variational upper bounds. The numerical calculations approach the variational bounds from below at low solid fractions and are always within one order of magnitude of the best upper bound at high solid fractions ranging up to 0.98. At solid fractions less than 0.2 the calculated permeabilities compare well with the predictions of Brinkman’s effective‐medium theory, whereas at higher solid fractions a good fit is obtained with a Kozeny–Carman equation.

Geometry, migration and evolution of small-scale bedforms generated by regular and irregular waves

Abstract Ripples generated by both regular and irregular waves have been experimentally investigated. In particular, their geometry at the equilibrium (i.e. height, length and steepness) has been analyzed and compared with semi-empirical predictive models. It has been observed that in the investigated range no distinction between ripples generated by regular or irregular waves is detectable, even though the equilibrium configuration is reached through different mechanisms. Moreover, it has been found that ripples migrate as soon as they appear and their velocity of migration qualitatively agrees with theories based on stability analyses recently proposed. Starting from results in literature, new semi-empirical formulae are provided for calculation of equilibrium characteristics of ripples.

Sea ripple formation: the heterogeneous sediment case

Abstract Ripple 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.

Evolution of small scale regular patterns generated by waves propagating over a sandy bottom

In this paper the evolution of a sandy bottom subject to a wave generated flow has been analyzed using an image acquisition technique. In particular, ripple formation was observed starting from a flat bed till a stable configuration was attained. The experimental findings showed that rolling grain ripples never appeared as a stable configuration but only as a transition toward the equilibrium, represented by vortex ripples. The latter stage was reached after about 100 cycles if lightweight sediments were used, or about 400 cycles if quartz sediments were adopted. It was also observed that when the bedforms appear their wavelength is much smaller (about one half) than that at the equilibrium stage, this result being in contrast with most of the analytical models on bedform evolution. Finally, it was observed that ripples migrate as soon as they appear. The measured velocity of migration seems to confirm, from a qualitative point of view, the theoretical findings obtained for rolling grain ripples by Blonde...

Migrating sea ripples

Ripple formation under sea waves is investigated by means of a linear stability analysis of a flat 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.

Backfilling of a Scour Hole around a Pile in Waves and Current

This paper presents the results of an experimental investigation of the backfilling of scour holes around circular piles. Scour holes arounda pile aregenerated either by a current or a wave.Subsequently, the flow climate is changed from current to wave, combined waves and current, or wave to a smaller wave, leading to the backfilling of the scour hole. The investigation has shed light onto the mechanism behind the backfillingprocess.Theresultsshowthatthescourdepthcorrespondingtotheequilibriumstateofbackfillingisthesameasthatcorrespondingto the equilibrium state of scour aroundthepilefor thesame wave (or combined waves and current) climate. The time scale of backfilling hasbeen determinedasafunctionofthreeparameters,namely,(1)theKeulegan-Carpenternumberoftheinitialwaveorcurrent(whichgeneratestheinitial scourhole);(2)thatofthesubsequentwave,whichbackfillsthescourhole;and(3)theShieldsparameterassociatedwiththelatterwave,forlive- bedconditions.Inthecaseofthecombinedwavesandcurrent,thecurrent-to-wave-velocityratioisalsoinvolved.Thetimescaleofthebackfilling process is completely different from that of scour. The time scale of backfilling is much larger than that of scour when the Keulegan-Carpenter numberassociatedwiththebackfillingisKCf ,Oð10Þ(typicalwindfarmapplication),whilethetimescaleofbackfillingcanbesmallerthanthat of scour when KCf ..Oð10Þ. DOI: 10.1061/(ASCE)WW.1943-5460.0000161.

Potential wave energy production by different wave energy converters around Sicily

The performance of several Wave Energy Converter devices is evaluated at three sites located on the west side of Sicily. To select the most energetic site, the average potential wave energy along the coasts of Sicily is evaluated by adopting a third-generation spectral wave propagation model using as boundary conditions the European Centre for Medium-Range Weather Forecasts operational archive wave and wind data. The most energetic sites are on the western side of Sicily. In the three hotspots identified, the mean energy flux is within the range of 5.33–7.52 kW/m. The analysis shows that all considered devices have a low capacity factor in their original configurations (2.19%–5.12%). The main causes of the poor results in terms of energy production are related to the fact that such devices are optimized for high-energy waves. A resizing of the devices on the basis of the local wave climate showed that a capacity factor that exceeds 30% could be obtained.

Measurements of sandy bed scour processes in an oscillating flow by using structured light

In this paper an experimental investigation of the scour process that arises at the base of a vertical cylinder, placed in an erodible sandy bed, in the presence of an oscillating flow, is presented. The choice of the measurement strategy is crucial, as a highly non-invasive approach must be adopted in order to avoid perturbations of the sandy bed due to turbulence induced by the measurement equipment itself. A measurement strategy, based on image acquisition techniques and structured light, has been adopted in this work. In particular, the structured light was applied remotely to measure continuously the evolution of the scour at the base of the pile. The measurement technique has been applied under fixed bed conditions to determine the onset of the horseshoe vortex that arises from the interaction between the flow and the pile. Moreover, measured data gave experimental confirmation that an asymptotic value of the scour is reached and allowed some theoretical results regarding the behavior of the scour process to be assessed. The existence conditions of the horseshoe vortex finally have been derived as a function of the flow characteristics.

Propagation and deposition of stony debris flows at channel confluences

The fluid dynamics of stony debris flows generated in two small tributaries adjacent to each other and flowing into a main receiving channel was analyzed experimentally at a laboratory scale. The analysis on the propagation along the tributaries and deposition in the main channel provide information about sediment-water mobility, dangerous damming, and potential hazard. Debris flows were generated by releasing a preset water discharge over an erodible layer of saturated gravels material. As a consequence, the debris flow sediment concentration varied accordingly to the entrainment rate which, in turn, was strongly controlled by the tributary slope. The data collected by acoustic level sensors, pore fluid pressure transducers, and a load cell were used to characterize the evolution of bulk density and solid concentration of the sediment-water mixture. These two parameters were relevant to assess the stony debris flow mobility which contributes to determine the shape of sediment deposits in the main channel. The detailed bed topography surveys carried out in the main channel at the end of each experiment provided information on the morphology of these deposits and on the interplay of adjacent confluences. The influences of confluence angle, tributary slopes, and triggering conditions have been investigated, for a total of 18 different configurations. Within the investigated range of parameters, the slope angle was the parameter that mainly influences the stony debris flow mobility while, for adjacent confluences, the degree of obstruction within the receiving channel was strongly influenced by the triggering scenario.

Sea ripple formation : the turbulent boundary layer case

Abstract This 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 flow 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.