1,720,974 research outputs found
Geometrical characterization of sediment deposits at the confluence of mountain streams
Full text linkDebris flow injections from tributaries into a main mountain stream generate deposits of sediments which, in turn, result in obstruction and eventual damming of the river section. This contribution presents the results of a series of flume experiments on the dynamics of these deposits, with reference to three different types of blockage: no blockage, partial blockage, and full blockage. Results show that the shape of the deposit is mainly controlled by the ratio between the debris flow discharge and the main river discharge. The experimental dataset is used to develop a deposit resilience stability index based on the shape of the deposit contour retrieved from photos taken from above. The proposed index is based on the invariant elliptic Fourier coefficients and the dimensionless transverse obstruction parameter. The elliptic Fourier coefficients give information on the symmetry of the deposit contour. High symmetry indicates more stable and resilient deposits. The proposed index is calibrated on the basis of the flume experiments and tested with field data. The results are quite promising and suggest that the index can be appropriate for a fast hazard assessment of multiple debris flow deposits at a regional scale
Computational fluid dynamics for modeling gravity currents in the presence of oscillatory ambient flow
Full text linkGravity currents generated by lock release are studied in the case of initially quiescent ambient fluid and oscillating ambient fluid (regular surface waves). In particular, the dynamics of the density currents are investigated by means of CFD numerical simulations. The aim is to evaluate the influence of the ambient fluid velocity field on the observed mixing and turbulent processes. Results of two different turbulence closure models, namely the standard k # turbulence model and the LES model, are analyzed. Model predictions are validated through comparison with laboratory measurements. Results show that the k # model is able to catch the main current propagation parameters (e. g., front velocity at the different phases of the evolution of the current, gravity current depth, etc.), but that a LES model provides more realistic insights into the turbulent processes (e. g., formation of interfacial Kelvin-Helmholtz billows, vortex stretching and eventual break up into 3D turbulence). The ambient fluid velocity field strongly influences the dynamics of the gravity currents. In particular, the presence of an oscillatory motion induces a relative increase of mixing at the front (up to 25%) in proximity of the bottom layer, and further upstream, an increase of the mixing process (up to 60%) is observed due to the mass transport generated by waves. The observed mixing phenomena observed are also affected by the ratio between the gravity current velocity vf and the horizontal orbital velocity induced by waves uw, which has a stronger impact in the wave dominated regime (vf /uw < 1)
A small scale Pressure Retarded Osmosis power plant: dynamics of the brackish effluent discharge along the coast
No full textThe feasibility of a small scale Pressure Retarded Osmosis (PRO) plant located along the eastern coast of Sicily (Italy) is assessed by considering fresh water intake from a river and salt water intake from the sea. Firstly, PRO design methods are applied in order to estimate potential power production in terms of both the availability of water resources and of local salinity gradients. Secondly, the discharge dynamics of the brackish plant effluent in the coastal waters is investigated using an hydrodynamic model, taking into account wave - current interaction close to the river mouth. The results of the numerical simulations allow to identify the area affected by the brackish discharge, as a function of the nearshore wave climate conditions. Results show that the plume propagation is mainly controlled by the wave direction, whereas significant wave height and peak wave period have a minor influence. Plume hydrodynamics analysis is useful in optimizing the layout of the PRO plant in terms of the placement of the sea water intake and the effluent discharge, both of which should be properly located outside the surf-zone. Plume hydrodynamics analysis is also important for the evaluation of PRO plant impacts on the coastal saline concentration which may affect the local ecosystems
Dynamics of Gravity Currents in the Presence of Surface Waves
No full textThe gravity current dynamics in the presence of regular surface waves is investigated experimentally. The aim is to determine the influence of the wave-induced velocity field on gravity current kinematic parameters and on the concentration distribution. In the presence of surface waves, the front shows a pulsating behavior while periodic oscillations are observed at the interface between lighter and denser fluids. The dynamics of such processes is related to the wave parameters. Present results confirm that the lock-exchange generated front velocity is strongly influenced by the oscillatory motion and it may be increased or decreased depending on the Lagrangian mass transport induced by the surface waves. Compared to the propagation in the absence of waves, the dynamics of the concentration field is governed by an enhancement of mixing process due to the orbital motion
Optical measurements of bottom shear stresses by means of ferrofluids
No full textThe research is focused on the development and on the assessment of a measurement technique for bottom shear stresses. In particular, the wall-fluid interaction is analyzed adopting ferrofluids and using an optical readout system. The principle of operation of this technique is based on the capability of ferrofluids to react to external magnetic field changing their shape and their viscosity. The proposed magneto-rheological sensor, consisting in a magnetized drop of ferrofluid located at the channel bottom, is exposed to different flow conditions and its deformations are video-recorded. Thanks to the application of image analyses processes, the relation between shear stresses and magneto-rheological sensor deformation is investigated. The assessment of the measuring technique is carried out in the presence of different sandy bottoms and by considering several hydraulic (steady current) conditions. The range of measured bottom shear stress is 0.01-0.20 N/ m 2. Tests carried out with different sandy bottoms characterized by different roughness provide insights about the high sensitivity of the sensor, which is able to detect slight changes in the sandy bottom mixture (less than 10% concentration in volume). Statistical analysis on the ferrofluid deformation shows that the sensor deformation is strictly related to the local hydrodynamics. For higher Re number we observed larger mean displacement in the direction of the flow and bigger oscillations. Power spectral densities of the ferrofluid displacement and of the velocity fluctuations measured at the ferrofluid apex point show how the two signals are characterized by the same slope in log-log graph for intermediate and high frequencies (> 0.2 Hz) representative of small-scale eddies
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Boussinesq modelling of breaking waves: description of turbulence
No full textImproved turbulent closures for use in fully nonlinear Boussinesq-type models are
described here. The approach extends previous works in order to give a more flexible and
accurate description of the turbulence due to a breaking wave. Turbulent stresses are
handled by means of the Boussinesq hypothesis, and the eddy viscosity is assumed to vary
over the water depth according to different laws. The model is described in detail, and its
performances are evaluated both against available analytical solutions and against
experimental data of regular waves breaking over a slope. The influence of the vertical
structure of turbulence under a breaking wave is analyzed by means of four different
vertical profiles of eddy viscosity; the differences in terms of hydrodynamic features are
also discussed. Among the four selected profiles, two of them (the uniform one and that
with uniform eddy viscosity over the top half of the water column which linearly decreases
to zero over the lower half ) give better overall performances when compared with
experimental data concerning velocity profiles
New frontiers in the risk assessment of ship collision
Full text linkNavigation is becoming more and more complex over the years. The increase in maritime traffic and vessel size is inducing a global escalation of ship collision accidents, with consequent losses of human lives and economic assets worth billions. This is particularly true for port basins, with maritime authorities struggling worldwide to keep up with the ever-increasing ship traffic. In this respect, the demand for advanced methods to assess and mitigate ship collision risk has never been higher. The interdependency between physical failures, weather conditions, logistics, governance and human factors requires sophisticated frameworks to effectively assist maritime authorities and navigators in decision-making. The present work reviews the most recent advancements in the risk assessment of ship collision. The article focuses on new, rising technologies, identifying the current main trends and discussing future perspectives and challenges. The review revealed a wide and diversified range of methods, including machine learning, clustering techniques, swarm intelligence algorithms and others. To frame the methods in the current literature and compare them with previous efforts, they are categorized according to literature classifications. Advancements of well-established approaches and new promising tools are discussed, considering methods that allow the inclusion of quantitative and qualitative variables in the assessment. Furthermore, a comprehensive analysis of a database of maritime accidents in port areas is carried out to investigate prevailing trends in both worldwide and Mediterranean Sea contexts. Results indicate that ship collision accidents constitute the majority compared with other types of accidents, especially in the Mediterranean
The appearance of turbulence at the bottom of propagating surface waves
No full textDirect numerical simulations of momentum and continuity equations are performed to investigate the transition
process from the laminar regime to turbulence within the boundary layer generated at the bottom of a
propagating surface wave characterized by an amplitude a
∗ and a length L∗
such that the ratio a
∗∕L∗ assumes
small but finite values and the velocity close to the bottom is the sum of an oscillatory component and a
steady one. Perturbations of the laminar flow are found to appear for values of the Reynolds number Reδ =
U
∗
0
δ
∗
ν
∗
slightly smaller than the critical value which leads to the instability of the laminar regime in a Stokes boundary
layer and falls around 100 (U
∗
0
being the amplitude of the velocity oscillations close to the bottom, δ
∗
the Stokes
viscous length and ν
∗
the kinematic viscosity of the fluid). Close to the critical conditions, perturbations of
the Stokes flow appear when the velocity induced by the wave close to the bottom reverses from the onshore
direction to the offshore direction but the flow tends to relaminarize during the other phases of the wave
cycle. When the Reynolds number is increased of a relatively small amount, turbulence is generated also after
the passage of the wave trough, when the velocity induced by the wave close to the bottom reverses from the
offshore to the onshore direction. The obtained results suggest that, in the investigated range of the parameters,
turbulence is present throughout the wave cycle only when the Reynolds number is larger than a value that
depends on the ratio between the water depth and the length of the propagating wave but it falls between
500 and 700
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