1,720,975 research outputs found
Velocity and density measurements in forced fountains with negative buoyancy
Full text linkIn fluid mechanics, fountains take place when a source fluid is driven by its own momentum into a surrounding ambient fluid, and it is counterbalanced by buoyancy. These phenomena are largely encountered in nature and human activities. Despite the numerous studies on the subject, few experimental data are available about the internal structure of turbulent fountains. Here, we present a set of laboratory experiments with the aim to (i) get direct velocity and density measurements of fountains in a controlled environment and (ii) obtain insights about the basic physics of the phenomenon. The results concern the characteristics of the mean and turbulent flow: we report the analysis of the turbulent kinetic energy, the velocity skewness, and the Reynolds stresses, including a quadrant analysis of the fluctuating velocities. For some tests, the correlation between density and vertical velocity is investigated for both mean and fluctuating values. We have quantified the momentum transport, which is mainly out-downward at the nozzle axis with peaks at the mean rise height, where also maximum levels of the buoyancy and mass fluxes are present. The ability of acoustic Doppler current profilers to identify the rise height of the fountain and to measure the velocity field is also discussed
Velocity and stresses of partially-reflected water waves in the presence of opposing wind
No full textIt is known that long waves (or swell) ruffled by an opposing wind tends to be dissipated, while shorter waves develop on top of them and travel in the wind direction. However, the long waves are mostly considered progressive and the effect of reflection is neglected. We present an experiment study of mechanically generated regular waves in partially-reflective conditions and observed how they interact with wind an opposing. We look at how different reflection conditions and spatial variability affect the water flow and the stresses within it. By breaking down the signal through a triple decomposition, we analyse the velocity components and compare the wave-induced Reynolds stresses with a theoretical model which takes into account partial reflection conditions (Addona et al., 2018). This model helps us understand the spatial variability of the wave-induced stresses and avoid misinterpretations of the experimental results, proving the role the key role of reflection. A quadrant analysis of the fluctuating velocities is performed to study the direction of momentum transfer, which always seems to be from the interface to the water below. The work provides novel experimental data of the flow field of partially-reflected water waves in the presence of wind
Interactions between swell and colinear wind short crested waves, following and opposing
No full textWhen wind blows over a water surface during a swell, it generates short-crested, three-dimensional waves that interact with the underlying flow field through a mechanism that ultimately increases the average energy. In the present work, two test cases in which wind is flowing following and opposing a swell are analysed with experiments and are compared with wind-waves-only and swell-only cases. The analysis of the free surface fluctuation and of the flow field, with the three components of fluid velocity measured at the same time through a stereo particle image velocimetry system, leads to an accurate quantification of the energy distribution, of the structure of the oscillating, fluctuating (due to wind-waves) and turbulent kinetic energy, without assumptions on the structure of the flow. The findings demonstrate that the transverse dynamics is a pivotal factor in the transfer of energy in the near-free surface domain, and elucidate the energy transfer between wind-waves and swell. The results also confirm the reduction of oscillating kinetic energy of the swell in the presence of short wind-waves, a process interpreted with different possible mechanisms. There is evidence of the enhancement of wind action in the presence of swell compared to that in the case of wind-waves-only, confirming that energy transfer from the wind to the sea is enhanced when wind flows over a swell. Consequently, when the fetch is influenced by swells generated or propagated from different regions, and during multi-peak sea storms, wave generation models should account for this amplification
Phase-resolved analysis of velocity field structure and vorticity dynamics under colinear swell and wind-waves
No full textThe objective of this study is to analyze the turbulence field generated by the interaction between mechanical waves and colinear wind-waves in the liquid domain just below the free surface. Detailed three-dimensional velocity measurements close to the surface are decomposed into mean, swell, wind-waves, and turbulence contributions. In this work, we treat wind-waves as macroscale turbulence with a pseudo-random velocity field. Advanced data analysis yields phase-resolved and vertical examinations of wind-waves and turbulence stresses, kinetic energy, and vorticity. The results indicate that near the surface, the spanwise energy dominates both the wind-wave and the turbulence kinetic energy. The wind-waves and turbulence stress tensors exhibit a large anisotropy when swell waves are present, as a consequence of the interaction between swell and wind-waves. Furthermore, we present the spatio-temporal distribution of vorticity, and we elucidate the non-trivial interaction between vorticity and the flow field. This interaction results in body forces that contribute to the local variation in inertia, as described by the Navier–Stokes equation. It is observed that in all combinations, a body force acts, on average, downward, modifying the gradient pressure in the vertical direction
AIR-WATER INTERACTION OF PADDLE WAVES UNDER WIND FORCING
No full textThe air-water interaction has been long studied in ocean physics, due to its importance to natural hazard and impact on human activities. Several studies in the literature provided advancements for a better comprehension of gases, mass and momentum exchanges at the interface. However, the subject is wide and many aspects are still unsolved, especially it is quite hard to extract data from the air and water boundary layer. A non-intrusive useful method to acquire data at the air-water interface is Particle Image Velocimetry (PIV), which can extrapolate a huge amount of information in water. We perform an experimental study of regular waves generated by paddles propagating under the action of the wind. We discuss the importance of sea surface boundary layer under waves and winds based on laboratory experiments and scale analysis
An interactive webgis framework for coastal erosion risk management
Full text linkThe Italian coastline stretches over about 8350 km, with 3600 km of beaches, representing a significant resource for the country. Natural processes and anthropic interventions keep threatening its morphology, moulding its shape and triggering soil erosion phenomena. Thus, many scholars have been focusing their work on investigating and monitoring shoreline instability. Outcomes of such activities can be largely widespread and shared with expert and non-expert users through Web mapping. This paper describes the performances of a WebGIS prototype designed to disseminate the results of the Italian project Innovative Strategies for the Monitoring and Analysis of Erosion Risk, known as the STIMARE project. While aiming to include the entire national coastline, three study areas along the regional coasts of Puglia and Emilia Romagna have already been implemented as pilot cases. This WebGIS was generated using Free and Open-Source Software for Geographic information systems (FOSS4G). The platform was designed by combining Apache http server, Geoserver, as open-source server and PostgreSQL (with PostGIS extension) as database. Pure javascript libraries OpenLayers and Cesium were implemented to obtain a hybrid 2D and 3D visualization. A user-friendly interactive interface was programmed to help users visualize and download geospatial data in several formats (pdf, kml and shp), in accordance with the European INSPIRE directives, satisfying both multi-temporal and multi-scale perspectives
Use of a Raspberry-Pi Video Camera for Coastal Flooding Vulnerability Assessment: The Case of Riccione (Italy)
Full text linkCoastal monitoring is strategic for the correct assessment of nearshore morphodynamics, to verify the effects of anthropogenic interventions for the purpose of coastal protection and for the rapid assessment of flooding vulnerability due to severe events. Remote sensing and field surveys are among the main approaches that have been developed to meet these necessities. Key parameters in the assessment and prevision of coastal flooding extensions, beside meteomarine characteristics, are the topography and slope of beaches, which can be extremely dynamic. The use of continuous monitoring through orthorectified video images allows for the rapid detection of the intertidal bathymetry and flooding threshold during severe events. The aim of this work was to present a comparison of different monitoring strategies and methodologies that have been integrated into repeated surveys in order to evaluate the performance of a new camera system. We used a low-cost camera based on Raspberry Pi called VISTAE (Video monitoring Intelligent STAtion for Environmental applications) for long-term remote observations and GNSS-laser tools for field measurements. The case study was a coastal tract in Riccione, Italy (Northern Adriatic Sea), which is the seat of nourishment interventions and of different types of underwater protection structures to combat coastal erosion. We performed data acquisition and analysis of the emerged beach and of the swash zone in terms of the intertidal bathymetry and shoreline. The results show a generally good agreement between the field and remote measurements through image processing, with a small discrepancy of the order of ≈0.05 m in the vertical and ≈1.5 m in the horizontal in terms of the root mean square error (RMSE). These values are comparable with that of current video monitoring instruments, but the VISTAE has the advantages of its low-cost, programmability and automatized analyses. This result, together with the possibility of continuous monitoring during daylight hours, supports the advantages of a combined approach in coastal flooding vulnerability assessment through integrated and complementary techniques
Coastal vulnerability assessment through complementary monitoring technologies: The case of riccione
Full text linkMonitoring is known to be the basis for coastal planning and management studies, for verifying the interventions effects and for forecasting the future state of beaches. There are several proxies that must be considered in the definition of a monitoring plan, such as the spatial and temporal extension of the monitoring activities, the types of data and their required accuracy values, the acquisition frequency of the data and perhaps the available budget to invest are among the most important aspects. The present paper presents the first results of the monitoring activity with integrated and complementary techniques applied to a stretch of coast near the city of Riccione, North Italy, within the framework of the research project STIMARE, financed by the MATTM and of the project POR FESR TAO. The site is protected by non-traditional defense works, and the on-going activities aim to monitor the shoreline evolution by using videocameras, with a low-cost hardware system, based on Raspberry Pi components, and the development of on-site software for the survey, correction and georeferencing of data. The monitoring of the emergence and submerged beach is carried out by means of topographic and bathymetric surveys (terrestrial laser scanner, UAV photogrammetry, ASV multibeam), allowing a very high-resolution digital terrain model (DTM) to be obtained. The comparison between successive surveys acquired with different techniques provides information on the response of the coastal stretch to the local regime of waves and currents and on the effects of the defense structures on the seabed, both in the nearfield and in the farfield, with the possibility of quantifying the volumes of eroded or accumulated sand. Finally, the two monitoring approaches are shown to be complementary in terms of time and precision: direct surveys provide data with high spatial precision but not continuous acquisition due to budget and logistics reasons, while video monitoring provides only information on the shoreline, (used as a proxy for the description of coastal dynamics) but with data continuously acquired. The project aims to verify the complementarity and integration of the different techniques adopted in monitoring, and the possible feasibility in other coastal stretches with the purpose of assessing vulnerability
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