Estudi experimental de l'eficiència de la mescla en dipòsits d'abastament d'aigua

El fenomeno de la mezcla en depósitos de abastecimiento de aguas tiene un efecto directo en la calidad del agua saliente. La creación de zonas de mezcla pobre y la estratificación pueden tener efectos negativos en la salud pública. El diseño de un depósito de abastecimiento debe considerar las condiciones de entrada y salida, así como su orientación (vertical o horizontal) para prevenir la formación de esas zonas. Experimentos realizados en un modelo reducido, de base rectangular i considerando tres entradas diferentes (una por caída libre situada en la mitad del lado largo, otra igual situada en la mitad del lado corto y finalmente una tubería situada en la mitad del lado corto) se han fijado como objetivo decidir cuál de estas entradas conseguía la mejor eficiencia de mezcla. Se consideraron cuatro situaciones distintas en cada una de las entradas, dos situaciones no permanentes: llenado y vaciado, u dos situaciones permanentes con diferente combinación de las salidas. Además, se estudiaron también los efectos que pueden producir las columnas que aguantan el techo del depósito, sobre la eficiencia de la mezcla

PIV applications in vortex rings and oscillatory boundary layers

Particle Image Velocimetry (PIV) is one of the most used experimental techniques in fluid mechanics to obtain the velocity field of a flow. One of its most celebrated characteristics is that it does not have interference on the phenomenon of study which makes it suitable to describe qualitatively and quantitatively many phenomena either micro or macroscopic. This thesis presents the PIV technique applied to two different fluid mechanic problems: vortex rings impinging permeable boundaries and oscillatory boundary layers in the laminar-to-turbulent regime. The first part of this thesis focuses on the impingement of vortex rings towards permeable boundaries and compares the results with the interaction of a vortex ring with a solid boundary. Assuming that a vortex ring is an axisymmetric structure, 2D PIV experiments are performed over boundaries on 4 different permeabilities and a solid boundary. When a vortex approaches a solid boundary, three different phenomena are clearly visible: the vortex ring decelerates when the distance between the core and the wall is in the order of the initial diameter of the ring. At the same time, the diameter starts increasing producing a stretching effect and, finally, secondary vorticity appears after the ring has reached the minimum distance from the wall. Experimental results lead to some interesting conclusions when the permeability of the boundary increases: the deceleration of the vortex ring starts later, the diameter does not increase as much and, finally, secondary vorticity is weaker and has shorter life. The second and third part of this thesis focus on the study of oscillatory boundary layers over smooth and rough walls. Experimental measurements were conducted over smooth and two different rough beds spanning the laminar, transitional and turbulent flow regimes. A multi-camera 2D-PIV system was used in an experimental oscillatory-flow tunnel. Characteristic variables like boundary layer thickness and friction factor were computed using different methods. Results obtained experimentally in smooth wall experiments are consistent with theoretical work. For the rough wall cases different formulations have been compared. Finally, results show how the phase lead between wall velocity and free-stream velocity is better defined when the integral of momentum equation is used to estimate the friction velocity. The observed differences are highly sensitive to the zero level definition. Finally, a detailed analysis of the structures present in such oscillatory boundary layers yield to a description of four different features: vortex tubes present in oscillatory flows over smooth beds, and vortices, turbulent spots and shear layers present in oscillatory flows over rough beds. The inception of vortex tubes is consistent with the state-of-art predictors as a result of the Kelvin-Helmholtz instability. Furthermore, structures present in rough wall experiments are a little bit more complicated because their inception and evolution are clearly influenced by the position of the sediment grains forming the bed. Vortices are created behind a kink in the bed sediment profile during the wall flow reversal and are shed from the wall when flow starts its acceleration cycle. Both the vertical and horizontal evolutions of the vortex position depend on the ratio between the amplitude of oscillation and roughness of the sediment bed. Turbulent spots are defined as structures which are born vortices but lose their shape in an early stage. They follow the same trajectories as vortices but reach lower heights before dissipating. Finally, shear layers were only detected in the larger bed roughness and are described as a sum of vortices that are shed consecutively from the same sediment. These shear layers are linked to vortices during the wall flow reversal when a big vortex is formed in the same place as the shear layer.La tècnica experimental del Particle Image Velocimetry (PIV) és una de les més utilitzades en el món de la mecànica de fluids per obtenir el camp de velocitats en un flux. La seva no interferència en el desenvolupament del fenomen que s’estudia la fa aplicable a tot tipus de fenòmens, ja sigui micro com macroscòpics i permet estudiar qualitativa i quantitativament la dinàmica de fluids d’un fenomen particular. Aquesta tesis presenta l’aplicació del PIV a dos problemes de mecànica de fluids diferents: anells de vorticitat impactant contorns permeables i capes límit oscil•latòries dins del règim transitori. La primera part d’aquesta tesis es centra en l’estudi dels anells de vorticitat impactant contorns permeables i la seva comparació amb l’impacte del mateix anell amb una paret sòlida. Assumint que un anell de vorticitat és una estructura axisimètrica, s’han realitzat experiments PIV en 2D, en quatre contorns permeables diferents i un contorn sòlid. Quan un anell de vorticitat es mou cap a una superfície sòlida es fan presents tres fenòmens principals: la desacceleració de l’anell de vorticitat quan assoleix una distància amb la paret de l’ordre del diàmetre inicial de l’anell. Al mateix temps un augment del diàmetre produint un efecte estirament i finalment l’aparició d’un segon anell de vorticitat quan el primer anell ha assolit la distància mínima de la paret. El treball experimental aporta algunes conclusions interessants sobre l’evolució d’aquestes característiques principals a mesura que la permeabilitat del contorn d’impacte augmenta: la distància de la paret a la qual comença la desacceleració disminueix, el diàmetre de l’anell de vorticitat primari creix amb menys intensitat i finalment el segon anell de vorticitat és més dèbil i té una vida més curta. La segona i tercera part de la tesis es centren en descriure el fenomen de capa límit oscil•latòria en fluxos sobre llits llisos i rugosos. En aquesta tesis es descriuen els experiments centrats en l’estudi de la capa límit oscil•latòria sobre fons llis i dos tipus diferents de fons rugós per a un rang de Rew = 0.4.104 ~ 2.104; els experiments es centren en la transició de règim laminar a règim laminar a turbulent i utilitzen la tècnica de 2D PIV amb multicàmera aplicada a un túnel de flux oscil•latori. Algunes variables característiques com el gruix de la capa límit o el factor de fricció s’analitzen des de diferents perspectives. Els resultats obtinguts en els experiments de llit rugós coincideixen amb les prediccions realitzades per a teoria existent. Per als experiments en llit rugós diferents formulacions es comparen. Finalment els resultats mostren com la diferència de fase entre la velocitat de paret i a la velocitat del pistó que activa l’oscil•lació es defineix més bé quan es calcula a través de la integral de la quantitat de moviment. Les diferències observades amb els resultats obtinguts quan es calcula a través de la suma de tensions de Reynolds i tensions viscoses són degudes principalment a la sensibilitat d’ambdues equacions a la definició del zero d’ordenades. Per altra banda, el gruix màxim observat per la capa límit es produeix just abans de que es produeixi l’instant de velocitat zero, també anomenat fase del revers, independentment del règim del flux. Finalment es realitza un anàlisi detallat de les estructures presents en les capes límit oscil•latòries definint fins a quatre tipus diferents: tubs de vorticitat presents en fluxos oscil•latoris sobre llits llisos, vòrtexs, bursts i capes de tensió detectades en els fluxos sobre llits rugosos. L’evolució dels tubs de vorticitat coincideix amb la teoria ja publicada i són el resultat de l’aparició de la inestabilitat de Kelvin-Helmholtz. Els experiments descrits en aquesta tesis confirmen les mateixes característiques i permeten definir la vida d’aquests tubs de vorticitat. Les estructures presents en llits rugosos són una mica més complicades degut a que la seva formació i evolució està clarament influenciada per la posició dels grans de sediment que formen el fons. Els vòrtexs es creen darrere una punta en el perfil del llit de sediment durant la fase del revers prop de la paret i es separen d’aquesta quan comença el cicle d’acceleració. L’evolució tant de la posició vertical com horitzontal d’aquests vòrtexs depenen del quocient entre l’amplitud de l’oscil•lació i la rugositat del sediment. Els bursts es defineixen com a aquelles estructures que neixen sent un vòrtex però perden la forma de seguida. De fet, aquests bursts són vòrtexs de mida més petita i que apareixen en localitzacions properes en el camp de velocitats instantani. No obstant quan es realitza la mitjana en fase, la seva forma desapareix, mostrant-se com a bursts. La seva trajectòria és semblant a la dels vòrtexs prèviament descrits, però assoleixen una alçada inferior. Finalment les capes de tensió es visualitzaren només en els experiments portats a terme amb la màxima rugositat i es defineixen com una successió de vòrtexs creats en el mateix gra. Aquestes últimes estructures estan relacionades amb els vòrtexs inicialment descrits ja que aquests es formen amb les estructures romanents de la capa de tensió just quan es produeix el revers del flux. El treball presentat en aquesta tesis es basa en dos problemes de mecànica de fluids. No obstant confirma que la tècnica del PIV és aplicable a fenòmens totalment diferents

Rhythmic morphology in a microtidal low-energy beach

Observations of rythmic features along the inner side of the Trabucador barrier beach are coupled to two numerical models to unravel the mechanisms of its formation. The Trabucador is a long (6 Km) narrow (125 m) barrier and microtidal beach at the SW side of the Ebro delta (Catalonia). Its inner side is a low energy beach with a sandy shallow terrace featuring an intricate alongshore rhythmic morphology. Sixteen aerial orthophotos from 1946 to 2014 have been analyzed and complemented with field observations from 1986 to present. This morphology is dynamic but it is usually characterized by: a) long finger transverse bars (LFTB) and b) large scale shoreline undulations (LSSU). The LFTB are thin and elongated with a length of the order of their spacing. They are intertidal and typically attach to the shoreline by a megacusp, commonly opening an anti-clockwise angle of 10°–40° with the shore normal. There can be many, up to 90, with both the mean and the most frequent alongshore spacing in the range 15–25 m. Spectral analysis always shows peaks in this range and sometimes additional peaks in the range 30–65 m that correspond to the spacing between the largest bars with smaller bars in between. The LSSU typically have wavelengths in the range 150–250 m. Their apexes sometimes coincide with the shore attachment of the largest bars but not always. Numerical modelling shows that both features could emerge out of feedbacks between hydrodynamics and morphology during the SW wind events involving a) deflection of the longshore current by the bars combined with the refractive wave focusing and b) gradients in total alongshore sediment transport rate triggering the high-angle wave instability.Peer ReviewedPostprint (author's final draft

Coherent structures in oscillatory flows within the laminar-to-turbulent transition regime for smooth and rough walls

This is an Accepted Manuscript of an article published by Taylor & Francis Group in [Journal of hydraulic research] on [2016], available online at: http://www.tandfonline.com/10.1080/00221686.2016.1174960We investigate coherent structures present in oscillatory boundary layers over smooth and rough beds for Reynolds numbers between 103 and 104, in the transition to turbulence regime. A two-camera 2D- particle image velocimetry (PIV) system was used to visualize coherent structures in an oscillatory-flow tunnel. The obtained results show that smooth-bed flow is populated by vortex-tube structures that are formed due to the Kelvin–Helmholtz instability. Three types of coherent structures are observed in the rough-wall experiments: vortices randomly distributed in space; turbulent bursts – spatial structures without a clear shape compared to vortices but with a longer life than the former; and shear layers of vortices originating due to flow separation from some of the grains on the bed. The reported study contributes to the description of coherent structures in oscillatory flows that are captured with the PIV technique, particularly a new structure in flows over rough beds in the transition to turbulence regime.Peer ReviewedPostprint (author's final draft

The next step on the maritime education and training in the era of autonomous shipping

The development of autonomous shipping is in continuous growth; technology evolution is going on faster steps and new projects are being developed looking for more efficient, more sustainable and safer shipping operations. Autonomous vessels feature a technology that uses a large range of physical sensors to control autonomous functions, as well as a high advance monitoring and control systems, including the use of the newest communications technology. This technological emergence has been driving concerns about the decline in the number of seafarers and jobs, which are expected to be replaced by Artificial Intelligence and the Autonomous Systems. However, the trend will be the opportunity for new businesses and job creation, which will require highly skilled crews and operators. As far as further steps are being developed on higher autonomy, the landscape of jobs related to seafarers will change. For instance, remote operations will transfer most of the seafarers to land-based control centres, causing that the main skills to be developed should drive to more expertise in technology, communication and network administrators without losing the standards as per STCW Convention. This literature review addresses this new and emerging topic to lead to an initial conceptualization of the new Maritime Education and Training (MET) framework and find eliciting information for developing new academic assessment. At the end, this contribution discusses published research in the era of autonomous shipping in order to describe the next step for MET, developing qualifications standards for the new crew and on-shore operators.Peer Reviewe

Numerical investigation of nonlinear diffraction wave loads on a suspended cylinder

Floating offshore wind turbines (FOWTs) interact with waves in complex sea conditions, inparticular wave-induced nonlinear diffraction wave loads can excite the resonant response of FOWTs in the low frequency range. In this paper, a fixed rigid suspended cylinder (considereda simplification of the support columns of a complex structure) is analyzed for nonlinear waveloads under regular waves using OpenFOAM, an open-source Computational fluid dynamics(CFD) software, to obtain nonlinear hydrodynamic load characteristics of a simple structurein regular waves. Results are compared with experimental and potential flow analysis with theCFD model yielding better estimates of first, second and third order wave diffraction loads than the engineering model using potential flow theory. Engineering models based on second-order potential flow theory are not effective in predicting the second and third order wave diffraction loads.Peer Reviewe

Study of the efflux velocity induced by two propellers

Present analysis is related with seabed erosion caused during docking and undocking maneuvering. Twin propellers without rudder were studied using a physical model with a fixed clearance distance and three different rotating velocities. Experimental results were compared to theoretical expressions of the efflux velocity, axial velocity and finally maximum bed velocity. Efflux velocity equations overestimate the experimental results, whereas axial velocity computed using the Dutch method fits reasonably well the experimental data. However, when maximum bed velocity expressions are compared to experimental results, German method behaves better with an over estimation if a quadratic superposition of the single jets is used.Postprint (published version

Dinámica de la morfología rítmica en la playa de baja energía del Trabucador (Delta del Ebro)

Observations of rhythmic morphology along the Trabucador beach are contrasted with two numerical models to unravel the mechanisms of their formation. The Trabucador is a long narrow barrier at the SW side of the Ebro delta (Catalonia). Its inner side is a microtidal low energy beach with a sandy shallow terrace featuring an intricate rhythmic morphology. Sixteen aerial orthophotos from 1946 to 2014 have been analyzed and complemented with field observations from 1986 to present. The morphology is dynamic and it is usually characterized by: a) long transverse finger bars (LTFB) and b) large scale shoreline undulations (LSSU). The LTFB are thin and elongated, commonly opening an anticlockwise angle of 10°–40° with the shore normal. Their alongshore spacing is in the range 15 - 25 m, with a secondary wavelength in the range 30 - 65 m. The LSSU typically have wavelengths in the range 150 - 250 m. Numerical modelling shows that both features could emerge out of feedbacks between hydrodynamics and morphology during the SW wind events involving a) deflection of the longshore current by the bars combined with the refractive wave focusing and b) gradients in total alongshore sediment transport rate triggering the high-angle wave instability.Postprint (author's final draft

Low energy transverse sand bars at el Trabucador beach, Ebro delta: a preliminary study

The complex rhythmic morphology of the inner side of the Trabucador barrier beach is investigated. This system fea-tures transverse bars along with shoreline undulations. From visual inspection, the typical spacing between bars is about 20 m and its length is similar. According to the literature they fall within thePostprint (published version

Erosion caused by propeller jets in a low energy harbour basin

This is an Accepted Manuscript of an article published by Taylor & Francis Group in Journal of Hydraulic Research on January, 2017, available online at: http://www.tandfonline.com/10.1080/00221686.2016.1252801Field data of a harbour basin are compared with analytical formulations for predicting maximum scouring depth due to propeller jets. Spatial data analysis of seven-year biannual bathymetries quantifies the evolution of the scouring hole along with the sedimentation process within a harbour basin. The maximum scouring depth is found to be of the order of the propeller diameter with a maximum scouring rate within the first six months of docking manoeuvring. Three of the analysed expressions yielded realistic results while observed discrepancies between the theoretical predictions and field data are related to scaling factors. The outcomes of this analysis can be extrapolated to other harbours to improve their management. The obtained results highlight the importance of field data in developing combined physical and numerical models.Peer ReviewedPostprint (author's final draft