Stima del parametro entropico per correnti stazionarie in presenza di scabrezza intermedia

Nel presente lavoro, una metodologia speditiva, basata sulla distribuzione entropica di velocità, è proposta per ricostruire il campo di moto di correnti a pelo libero in presenza di scabrezza intermedia. Tale metodologia consente di determinare il parametro entropico nota che sia la sommergenza relativa. Un’ampia serie di misure sperimentali è stata acquisita da prove condotte su un canale rettangolare situato presso il Laboratorio di Idraulica dell’Università degli Studi della Basilicata, per differenti valori di pendenza e portata, in condizioni di moto stazionario, con valori intermedi di sommergenza relativa e rapporto di aspetto minore di 7.5

Analysis of river bed dynamic evolution following a landslide dam

Landslides and debris flows can strongly interact with the river network and its mass transport processes, determining modifications of the river pattern with consequent effects on the hydrodynamic phenomena, alterations of the existing morphologies and possible interferences with anthropic works. Modifications of the cross section geometry and channel slope may produce changes in the sediment budget, with consequent repercussions on the stream evolutionary dynamics and its equilibrium configuration, leading to a new river branch arrangement. In this paper, investigations were carried out on a gravel-bed reach in the middle valley of the Noce River in Basilicata (Italy), which in 2007 suffered a progressive morpho-hydrodynamic change caused by a landslide. Because of the phenomenon complexity, mainly due to the mutual interaction between the landslide and the river transport dynamics, an integrated approach that combines field observations and numerical modelling in a spatial scale and natural environment, rarely available in literature, is suggested. The results highlight a satisfying correspondence between the altimetric profiles obtained through the numerical models and those deriving from the field surveys.Peer ReviewedPostprint (published version

Hydrodynamic forces acting on an oscillating structure

The submerged and not completely submerged structures (off-shore platforms, wind farms, floating buildings, marine turbines, oil conduits, buoys) in sea currents are often subjected at strong vibrations that condition the structural stability and give rise also to serious damages to the mechanical parts of the structures, with consequent efficiency loss of the system and above all repercussions in the environmental in which they are located. These vibrations increase when the structures are immersed in bounded flows (presence of the bed and/or the free surface). In order to attenuate and eliminate these problems, analytical and experimental means are used to describe the dynamic behaviour of structures for different flow field configurations and boundary conditions. Recent researches have demonstrated as the integrated numerical modelling (considering both structure and fluid characteristics) is the more suitable way to value the dynamic response of a system, also if the check on the reliability of the resolution methods needs to experimental controls that, for the complexity of structures, requires models with simplified geometries (spheres, cylinders, rectangles). In this context, the present work wants to provide some experimental results obtained analyzing both the oscillations and the forces acting on a structure to simple geometry (sphere) characterized by low values of mass and damping and invested by a free surface steady flow

Analisi dell’influenza della superficie libera sulle vibrazioni di una sfera investita da una corrente stazionaria ed uniforme

Nel presente lavoro si studiano gli effetti della superficie libera sui movimenti di una sfera investita da una corrente fluida stazionaria e uniforme. La sfera è caratterizzata da un basso valore del parametro combinato massa-smorzamento (m*ζ=0.005) ed è libera di muoversi sia nella direzione principale della corrente sia in quella trasversale. In tutti gli esperimenti il coefficiente di bloccaggio γb (rapporto tra la superficie frontale del corpo e la sezione trasversale della corrente) viene mantenuto costante, mentre la sfera viene posizionata a diverse distanze dalla superficie libera. Gli spostamenti nella direzione della corrente e in quella trasversale sono stati misurati contemporaneamente attraverso una telecamera ad alta risoluzione che ha permesso la ricostruzione bidimensionale delle traiettorie e l’analisi in frequenza del segnale. I risultati sottolineano un cambiamento della risposta dinamica della sfera all’aumentare della velocità media della corrente e all’avvicinarsi del corpo alla superficie libera

Boundaries effects on the movements of a sphere immersed in a free surface flow

This work analyses the influence of boundary conditions on the movements of a sphere immersed in a steady free surface flow. The sphere is free to move in both transverse and stream wise directions and it is characterized by values of the mass ratio, m*, equal to 1.34 and of the damping ratio, ζ, equal to 0.006. In all the experiments the blockage coefficient is kept constant, while the sphere is located at different distances from the free surface and from the bottom wall of the channel. The movements of the sphere have been measured by means of the image analysis of CCD camera which provides the 2D (stream wise and transverse) displacements of the sphere with a temporal resolution of 0.02. The experimental data show a significant influence of the boundaries on the sphere movement and highlight a different behaviour of the amplitude response between the three different experimental set-ups considered

Analysis of effects of the free surface on the movement of a tethered sphere immersed in a steady flow

In this work, we study the effects of the free surface on the sphere immersed in a steady flow through the analysis of stream wise and transverse displacements. The range of relative submergence, h*, is between 0 and 0.75 and the tethered sphere is characterized by a low value of the mass ratio m*~1 and a low value of damping ratio (ζ=0.007). The movements of the sphere have been measured using two methods: (a) by means of an analogue laser displacement sensor, and (b) by image analysis of the sphere movement CCD acquisitions. The second method provided the 2D reconstruction of the trajectory. The experimental data have highlighted a significant influence of free surface flow both on the transverse oscillation amplitudes and on the oscillation frequencies of the sphere

Determinazione del parametro entropico nelle correnti regolari a media e bassa sommergenza

La modellistica numerica rappresenta uno degli strumenti più validi nelle attività di studio e progettazione in ambito fluviale, vincolato all’impiego di dati aggiornati e affidabili ottenuti da misure e rilievi diretti e sistematici, non sempre disponibili in tempo reale. In tale ottica, diventa fondamentale la ricerca di base e applicata volta all’ottimizzazione di modelli di calcolo che consentano la stima dei deflussi in maniera speditiva, e che riescano a mantenere la dovuta rigorosità grazie all’introduzione di parametri che tengano conto delle informazioni “geometriche” e di “scabrezza d’insieme” di un corso d’acqua, o più estesamente delle aree di pertinenza fluviale. Studi recenti hanno sottolineato la validità del parametro entropico per la valutazione della portata di corsi d’acqua con morfologie regolari, mostrando altresì significative dipendenze di tale parametro dalle caratteristiche cinematiche e geometriche delle correnti. E’ in quest’ultimo filone di studio che si inserisce il presente lavoro imperniato sull’indagine dettagliata dell’influenza sul parametro entropico di grandezze di facile rilevazione in campo, come la sommergenza relativa e la pendenza del fondo, attraverso un’ampia serie di misure in laboratorio condotte su correnti stazionarie e regolari a superficie libera in presenza di scabrezza omogenea a media e bassa sommergenza

Boundaries influence on the flow field around an oscillating sphere

This work analyzes the effects of the interaction between an oscillating sphere and free surface flows through the reconstruction of the flow field around the body and the analysis of the displacements. The experiments were performed in an open water channel, where the sphere had three different boundary conditions in respect to the flow, defined as h* (the ratio between the distance of the sphere upper surface from the free surface and the sphere diameter). A quasi-symmetric condition at h*=2, with the sphere equally distant from the free surface and the channel bottom, and two conditions of asymmetric bounded flow, one with the sphere located at a distance of 0.003m from the bottom at h*=3.97 and the other with the sphere close to the free surface at h*=0, were considered. The sphere was free to move in two directions, streamwise (x) and transverse to the flow (y), and was characterized by values of mass ratio, m*=1.34 (ratio between the system mass and the displaced fluid mass), and damping ratio, ζ=0.004. The comparison between the results of the analyzed boundary conditions has shown the strong influence of the free surface on the evolution of the vortex structures downstream the obstacle

Influence of the entropic parameter on the flow geometry and morphology

The necessity of updating the numerical models inputs, because of geometrical and resistive variations in rivers subject to solid transport phenomena, requires detailed control and monitoring activities. The human employment and financial resources of these activities moves the research towards the development of expeditive methodologies, able to evaluate the outflows through the measurement of more easily acquirable sizes. Recent studies highlighted the dependence of the entropic parameter on the kinematical and geometrical flow conditions. They showed a meaningful variability according to the section shape, dimension and slope. Such dependences, even if not yet well defined, could reduce the difficulties during the field activities, and also the data elaboration time. On the basis of such evidences, the relationships between the entropic parameter and the geometrical and resistive sizes, obtained through a large and detailed laboratory experience on steady free surface flows in conditions of macro and intermediate homogeneous roughness, are analyzed and discussed

Estimation of Hydrodynamic Loading on an Oscillating Sphere

The design of flexible submerged structures presents issues related to e.g. hydrodynamics loads, induced hydrodynamic motion, effect due to the vortex-induced vibrations, re-sonance phenomena. In this complex scenario results form simple benchmark studies are valuable. Recently, the motion, due to hydrodynamic loading, of an oscillating sphere characterized by low values of mass and damping has been studied by the authors in an free surface flow channel at different values of relative submersion. Knowledge of the mechanical response and of the principal dynamic characteristics of the sphere retaining (mooring) system allows for the reconstruction of the hydrodynamic loading starting form measured displacements. This paper reports on the experimental work carried out to characterize the retaining system and the consequences coming from the estimated structural parameters on non-dimensional hydrodynamic force coefficients