Pietro Scandura

Steady streaming in a turbulent oscillating boundary layer

The turbulent flow generated by an oscillating pressure gradient close to an infinite plate is studied by means of numerical simulations of the Navier-Stokes equations to analyse the characteristics of the steady streaming generated within the boundary layer. When the pressure gradient that drives the flow is given by a single harmonic component, the time average over a cycle of the flow rate in the boundary layer takes both positive and negative values and the steady streaming computed by averaging the flow over n cycles tends to zero as n tends to infinity. On the other hand, when the pressure gradient is given by the sum of two harmonic components, with angular frequencies ω 1 and ω 2 = 2ω 1 , the time average over a cycle of the flow rate does not change sign. In this case steady streaming is generated within the boundary layer and it persists in the irrotational region. It is shown both theoretically and numerically that in spite of the presence of steady streaming, the time average over n cycles of the hydrodynamic force, acting per unit area of the plate, vanishes as n tends to infinity.

Coherent structures in an oscillatory separated flow: numerical experiments

Numerical experiments are performed to investigate the oscillatory flow over a two-dimensional wavy wall characterized by a large amplitude, such as to induce flow separation. Even though the Reynolds number is moderate, a three-dimensional turbulent flow is observed. The turbulence dynamics is characterized by the presence of coherent ribs superimposed on the main spanwise vortices generated by the roll-up of the free vortex sheets shed at the crests of the wall waviness. The ribs are formed by the stretching of vorticity patches which are generated by the instability of the two-dimensional flow at two different locations. The first are the saddle points of the flow field created, far from the wall, by the vortex pairs generated every half-cycle. The second are the saddle points created, close to the upstream side of the wavy wall, by the combined action of the free-stream flow and of the vortex structures shed by the ripple crests. Later, the ribs wrap around the main spanwise vortices and cause the distortion of these vortices and the alignement of the vortex lines with the free-stream flow, thus inducing large contributions to the coherent helicity. Simultaneously, regions of high dissipation appear which tend to be separated from those characterized by large values of helicity.

Reflection of Sea Waves by Combined Caissons

AbstractThis paper presents the results from several experiments aimed at determining the effectiveness of combined caissons with an internal rubble mound to dissipate incident sea wave energy. Techniques for assessing the reflection coefficient from wave tank data are discussed via a comparison between the established two- and three-probe methods and a new method using four probes. It is shown that obtaining reliable results for the incident and reflected waves requires the determinant of the linear system derived from the Fourier transform to be sufficiently greater than zero. This requirement was always satisfied for the three- and four-probe methods, but it was not always satisfied for the two-probe method. The experiments showed that the calculated reflection coefficient decreased when the ratio of the water depth, D, to the wavelength, L, increased. Increasing the caisson chamber width b and the upper frontal opening h also substantially decreased the reflection coefficient. Plotting the reflection ...

Numerical investigation of the oscillatory flow around a circular cylinder close to a wall at moderate Keulegan–Carpenter and low Reynolds numbers

The 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.

Measurements of wave-induced steady currents outside the surf zone

Experimental measurements of the steady current induced by progressive regular waves outside the surf zone are reported and discussed herein. The experiments have been conducted in a large wave flume to minimize scale effects. The velocity data were acquired using Acoustic Doppler Velocimeters. It has been observed that the mean velocity profiles have a pronounced dependence on the wave period. As it decreases, the mean velocity just below the wave trough increases in the offshore direction while close to the bottom it increases in the onshore direction. The observed qualitative trend is different from that predicted by the theory and also from that reported by other experimental campaigns. The presence of turbulence in the bottom boundary layer along with wave asymmetry partially explain these differences. However, other experimental data in the literature agree with the present results.

Waves Plus Currents Crossing at a Right Angle: Sandpit Case

AbstractThe present paper explores the possibility of exploiting sandpits at intermediate and shallow water depths by analyzing both the hydrodynamic and the morphodynamic response in the proximity of a dredged area. More precisely, a laboratory investigation was carried out to analyze the effects of the wave-current interaction over a borrow area, particularly in terms of the morphodynamic evolution of the pit. The influence of both regular and irregular waves on the current was analyzed. It was found that the flow is significantly influenced by such interaction in the presence of the borrow area. The morphodynamic evolution of the sandpit is mainly affected by the waves, while the superimposition of a current does not introduce qualitative modifications but increases the mobilization of sediments, with repercussions on the speed of the evolution process. The results of the experimental campaign suggested that the analysis of the morphodynamic evolution can be approached at a laboratory scale. Indeed, th...

Bottom Profile Evolution of a Perched Nourished Beach

AbstractThe evolution of a toe-protected nourished beach, located at Belvedere Marittimo (Italy), was investigated by means of both a physical model and a numerical morphological approach. Several surveys, including bottom profiles, sedimentology, and wave climate, were available after the nourishment for this site. Results showed that the physical model is in a fairly good agreement with the field surveys regarding localized phenomena such as the scour around the barrier, whereas it lacks in predicting the slope of the beach profile. The results of the morphological model, carried out on the basis of the field data, well reproduce the beach slope. On the other hand, when it is applied with the same parameters of the physical model, it lacks accuracy in predicting the experimental beach profile.

Experimental investigation on sea ripple evolution over sloping beaches

This paper reports on a wave flume experimental campaign carried out to investigate the appearance, the growth and the migration of small scale bedforms on a sloping sandy bed due to both regular and random waves. A Vectrino Profiler along with a structured light approach were used for velocity and morphodynamic measurements at two positions, one located above the horizontal bed, and the other one above the sloping beach. The velocity was computed by phase averaging the velocity measurements. Several velocity profiles were analyzed, identifying an offshore-directed steady current that extends from few centimeters above the bottom for all the analyzed water column. Ripple geometry was measured by a structured light approach and compared with that predicted by several models to shed light on the effects induced by the sloping beach on the shape and asymmetry. Along the sloping beach, the ripples appeared strongly asymmetric with the onshore half wavelengths smaller than the offshore ones. Finally, ripple geometry and migration triggered by regular waves were compared with those generated by random waves with comparable flow orbital amplitude showing a good agreement.

Waves plus currents crossing at a right angle: near-bed velocity statistics

ABSTRACT Wave–current flow over a bottom covered with different roughness elements was analysed to provide new insights into the statistical properties of the near-bed velocity. Experimental data of three different experimental campaigns, with orthogonal waves and currents over a sandy bed, a gravel bed and a rippled bed were used. Velocity profiles were acquired by means of a micro-ADV. The paper focuses on the effects that the waves have on the statistics of the velocity in the current direction. In particular, in the case of a steady current only, the near-bed velocities closely follow a Gaussian distribution. When waves are added, the distribution becomes double-peaked. In order to get single-peaked velocity distributions the total velocity events in the current direction were split in two classes according to the sign of the wave directed velocities. The nature of the distribution functions is influenced by the mass conservation principle and, in the rippled bed case, by the vorticity dynamics.