Rosaria E. Musumeci

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.

Measurements of the 3-D scour process around a pile in an oscillating flow through a stereo vision approach

Abstract The knowledge of the evolution of the scour around a vertical cylinder placed in a sandy bed and exposed to waves is of fundamental importance in oceanic engineering. Measurements of this phenomenon are very difficult to be performed due to the mandatory constraint of adopting non-invasive techniques. In this paper a novel three-dimensional measurement technique which uses a stereo vision approach has been adopted in order to dynamically recover the bottom characteristics. In particular, the sandy bottom time evolution was here observed from the flat bed initial condition until a stable configuration was attained. Moreover some morphological parameters of the scour such as the maximum scour depth, the extension of the eroded area and the position of the maximum scour toe were measured at equilibrium condition. These quantities were estimated with an average relative accuracy of about 5%, which appears particularly appropriate if compared to the results given by traditional measurement strategies. These latter being not particularly suitable for dynamic analysis of bottom evolution. The experimental investigation performed in this paper allowed to confirm some theoretical hypotheses, such as that of considering the Keulegan–Carpenter number (KC) as the key parameter which controls the phenomenon, to be confirmed. Moreover the experimental measures of the scour depth at equilibrium stage, has been compared with the values given by the relation proposed by Sumer et al. (1992) giving a fairly good agreement. Finally, with regards to other three-dimensional features such as the eroded area, new relations with standard dimensionless parameters have been proposed that result useful for design purposes.

Feasibility of an Offshore Wind Farm in the Gulf of Gela Marine and Structural Issues

Some of the marine and structural issues related to the installation of an offshore wind farm three miles away from the shoreline of Gela (Italy) are discussed. In particular, the characterization of the maritime climate has been carried out by considering the winds, the waves, the mean water level variations and the current regime. The results of such an analysis have been used in order to determine the environmental loads on the foundation structure. A gravity based structural type has been proposed as foundation of the wind turbines and the design of a foundation prototype has been carried out by trying to implement the most updated international standards to the Sicilian case. An original automatic optimization procedure has been developed in order to determine the dimensions of the design foundation, which satisfied the prescribed static verifications and provided the lowest costs. The scour process at the basis of the foundation and the wave run-up on the structure have been also taken into account. Finally, problems related to the laying and the maintenance of electrical connection cables are also discussed.

Stereo vision for noninvasive dynamic measurements of the scour process around a circular cylinder in an oscillating flow

A novel three-dimensional measurement technique which uses a stereo vision approach to recover the morphological evolution of a sandy bottom around a vertical pile is presented. In particular, the bottom time evolution due to the wave action was observed from the initial flat bed condition until a stable configuration was attained. Moreover, some morphological parameters of the scour, such as the maximum scour depth, the extension of the eroded area and the position of the maximum scour toe, were measured at equilibrium condition. These quantities were estimated with an average relative accuracy of about five percent, which appears particularly appropriate if compared to the results given by traditional measurement strategies. The key parameter which controls the phenomenon is the Keulegan-Carpenter number (KC). The experimental measures of the scour depth at equilibrium stage were compared with the empirical relations proposed by Sumer et al. (1992) giving a fairly good agreement, while for other three-dimensional features, such as the eroded area, new relations with typical dimensionless parameters useful for design purposes are proposed.

Influence of Regular Surface Waves on the Propagation of Gravity Currents: Experimental and Numerical Modeling

AbstractThe propagation of gravity currents is analyzed in the presence of regular surface waves, both experimentally and numerically, by using a full-depth lock-exchange configuration. Full-depth ...

Interaction between waves and gravity currents: description of turbulence in a simple numerical model

Effects of surface waves on gravity current propagation are studied by means of a numerical model. The adopted modeling approach couples a Boussinesq-type of model for surface waves and a gravity current model for stratified flows. In particular two different turbulence closure models are introduced which take into account subgrid turbulence and an additional depth-constant eddy-viscosity. The turbulence parameters are calibrated by means of experimental data on the time evolution of the heavy front, obtained both in the absence and in the presence of regular surface waves. Velocity fields, heavy and light front position, shear stresses, vorticity and entrainment calculated by the model are analyzed. The turbulence closure which includes both uniform and Smagorinsky type eddy viscosity allows a better description of the actual gravity current propagation. In particular, the results highlight the fact that the presence of the oscillatory motion causes, simultaneously, a reduction in turbulence and an increase in the mixing of heavy and light fluids. Such a result is in agreement with the experimental observations.

Ferrofluid measurements of bottom velocities and shear stresses

A novel direct measurement strategy of bottom velocities and shear stresses based on the use of ferrofluids is presented. Such a strategy overcomes some of the limits of state-of-the-art instruments. A preliminary experimental campaign has been carried out in the presence of currents in steady flow conditions in order to test the effects of ferrofluid quantity and of the controlling permanent magnetic force. An alternating current (AC) circuit and a direct current (DC) conditioning circuit have been tested. For velocities larger than 0.05 m/s, the near-bottom velocity-output voltage calibration curve has a monotone parabolic shape. The sensitivity of the instrument is increased by a factor of 30 when the DC circuit is used.

Measurement of Wave Near-Bed Velocity and Bottom Shear Stress by Ferrofluids

A novel direct measurement strategy of wave near-bed velocity and bottom shear stress based on the use of ferrofluids is presented. The proposed methodology is aimed at overcoming some of the limits of state-of-the-art instruments, for use in presence of both surface waves and sediments. An inductive readout strategy is proposed to sense displacements of the ferrofluid spike over the inductive circuit due to the flow. Experimental tests have been performed in a wave tank both in hydrostatic conditions and in the presence of regular linear waves corresponding to deep and intermediate water depth cases. Obtained results indicate the applicability of the proposed strategy also in the presence of suspended load (either moderate or intense) and of a moderate bed load transport.

Measurement of bottom velocities and shear stresses by ferrofluids at the sea bottom

A novel direct measurement strategy of bottom velocities and shear stresses based on the use of ferrofluids is presented. The proposed methodology is aimed at overcoming some of the limits of state-of-the-art instruments, for use in presence of both surface waves and sediments. An inductive readout strategy is proposed along with experimental results which indicate the applicability of the proposed strategy.

Measuring Sandy Bottom Dynamics by Exploiting Depth from Stereo Video Sequences

In this paper an imaging system for measuring sandy bottom dynamics is proposed. The system exploits stereo sequences and projected laser beams to build the 3D shape of the sandy bottom during time. The reconstruction is used by experts of the field to perform accurate measurements and analysis in the study of the final equilibrium conditions of sea bottoms in the presence of water flows. Results obtained by processing data acquired in hydraulic laboratory confirm the effectiveness of the system which makes simple and fast the understanding of the sandy bottom dynamics and the related equilibrium phenomena.