Tesis sobre el tema "Cohesive bed"

Historically the inter-flood period has been disregarded from investigations as it was deemed that the stability of non cohesive beds could only be altered by above threshold flows capable of sediment transport. However, this is at odds with more recent ‘stress history’ data which provides unequivocal evidence that entrainment thresholds can be delayed to higher shear stresses after being subjected to longer periods of sub threshold flows. The magnitude of this effect appears related to the surface grain size distribution and relative grain size effects, whilst the specific mechanics associated to generating a more resistant bed under sub-threshold flows are merely speculated upon. The aim of the present thesis is therefore to provide a comprehensive and quantitative data set on stress history that specifically address comparative grade effects and provides a detailed mechanistic understanding of the processes responsible for generating a more resistant bed configuration under sub threshold flows. Using a range of grain size distributions, a series of flume based experiments assess two main aspects of the stress history process. Firstly the effects of grain size distribution on the relationship between stress history duration and entrainment threshold is quantified. This is split into two sets of experiments based on the duration of the applied sub threshold antecedent flow, prescribed as 50% of the critical shear stress ( ) of the median grain size (D50). The antecedent durations of first set of experiment ranged from 0 to 60 minutes, whilst the antecedent duration of the second set of experiments ranged from 0 to 960 minutes. To ascertain the effect of the antecedent period on critical entrainment threshold and transported bedload, each experiment is concluded with a stability test composed of incrementally increased flow discharges until critical threshold conditions were reached. Secondly, aspect of stress history investigated uses high resolution laser scanning to assess bed topography and particle repositioning in order to ascertain the granular mechanics underpinning the stability process. The bed is scanned before and after the application of the applied antecedent flow with changes to bed surface structure described using Digital Elevation Models (DEM’s), statistical analysis and 1D and 2D semi-variograms to analyse scaling behaviour. In all experiments, increasing the antecedent flow duration significantly increases river bed stability in that the critical shear stress increases by up to 25% where uniform beds are more responsive to antecedency than bimodal beds. Laser based analysis reveals that vertical settlement, localised changes to bed roughness, pockets of more pronounced development of hiding effects, and particle repositioning are all mechanisms by which the bed reorganises under an applied sub threshold flow. However, the different bed grain size distributions cause significant differences in the importance of each mechanism in determining the magnitude of stress history induced bed stability.

Empirical downstream hydraulic geometry equations for clay-dominated cohesive bed (semi-alluvial) streams were derived using the data from five streams in eastern Ontario and four streams from other regions. The width exponent (0.57) is comparable to the exponents reported for previous studies; however, depth exponent (0.52) was greater for clay-dominated cohesive bed than for typical gravel-bed and sand-bed rivers. Furthermore, the recurrence intervals of bankfull discharges of those streams range from 1 to 2 years. To the author's knowledge, this is the first study which related bankfull hydraulic variables and discharges to derive downstream hydraulic geometry equations specifically for clay-dominated cohesive bed streams. Assessment of width:depth ratio depicted that large channels were deeper and narrower than comparable gravel-bed channels, but small channels were actually wider and shallower than typical gravel-bed rivers. This can likely be attributed to the resistance of stiff and consolidated cohesive-bed to erosive effects of more frequent flows.

An understanding of the behaviour of newly deposited soil is important because of its direct applications in fields such as harbour siltation and storage of dredge slurries. In coastal regions the dominant mode for deposition of fine-grained cohesive material is through flocculation- the electrostatic aggregation of mud particles. The present study focuses on the measurement of both flocculation conditions and bed properties, towards the understanding of the influence of the former on the latter. A computerised laboratory setup, complete from sedimentation and flocculation to the long term consolidation allows each of these processes to be observed in instrumented perspex columns without the many variables present in an estuary. The bed deposition rates resemble those found in natural estuaries, and therefore this technique is a much improved method for examining natural processes than the method of slurry deposition, which is normally employed in soil mechanics. An image acquisition and analysis system has been designed specifically for this work. Image sequences are analysed in real time to give information about the dimensions and concentration of the particles, and the particle velocities. Floc sizes increase as the particle concentration increases in the column. Increasing the concentration further leads to an overall hindering of the sedimentation rate and breakup of the flocs. Images of beds formed through slow sedimentation clearly show aggregate features that are not present in slurry experiments. From low to medium sedimentation rates the bed height, normalised by the mass of sediment in the bed, increases. From medium to high rates of sedimentation, however, the normalised bed heights decrease. Overall slurry experiments have lower bed heights than slowly deposited experiments. Image analysis of the bed surface (top 0.5mm) has revealed that aggregates which are present immediately after deposition are broken down over tens of hours. The ISIS instrument has been modified to gather information about the resistance to erosion of the beds. A bed is more easily eroded after it has had a long period of consolidation. X-ray density measurements and imaging techniques are used to link the strength of the bed to biological factors. X-ray bulk density and pore pressure measurements allow calculations of void ratio, porosity, and effective stress. Bender element apparatus has been constructed to measure shear stiffness of the soil, and bed strength measurements are made using a shear vane. It is found that variations in the sedimentation conditions have significant effects on bed density and on void ratio. These effects continue to be visible in the consolidated bed, even after an order of magnitude increase in the total vertical bed stress (equivalent to the range of metres of overburden pressure). The variations in the bed structure can have profound effects on bed strength. Methods to compare floc measurements to bed measurements are complicated by the inherent variation in floc data. Possible techniques include the use of solids volume fraction, fractal dimension and intrinsic sediment properties. The usefulness of each of these is assessed. It is suggested that it is the frequency at which the bed is bombarded by flocs, and not the properties of the flocs themselves, that largely determines the bed properties.

Ships maneuvering in low bed clearance conditions induce sediment scour due to the high speed jet generated by a rotating propeller. Sediment scour is known to cause instability to quay structures, damage to the bed protection and may create huge accretion areas reaching extremely low depths, preventing the free passage of large ships. This on-going problem is linked to the constant increase in commercial ship sizes and is widely recognized in current harbor management guidelines. This thesis presents in the first place the experimental work carried out to study the effects of two common propeller systems over the sediment bed: single and twin propellers. Next, the analysis of a case study is used to evaluate the impact of ship maneuvers over the seabed in a particular harbor basin. The first part of the thesis presents a set of experiments in propeller jet velocity and bed shear stresses. By measurements performed in a point-by-point grid, with a Pitot-static tube coupled to differential pressure sensors, the axial velocity distribution at the so-called efflux plane is characterized. After that, direct measurements of bed shear stress are performed at several speeds of rotation and two bed-clearance distances with a shear plate. Velocity and bed shear stress measurements are related by an empirical friction coefficient, which is dependent on the bed clearance. A new empirical model is presented to estimate the mean shear stress in case of unconfined ship propeller jets in low bed clearance conditions. The second part of this work shows the results of a new set of experiments in local scour due to confined twin propeller jets. The maximum scour depth is studied as a function of the bed clearance, the wall clearance and the efflux velocity of the jet. Aiming to reproduce more realistic maneuvers, scour due to forward and backward rotation of the propellers is analyzed, showing a different behavior compared to the traditional experiments with only forward rotation. The obtained results are compared to the existing formulae and later used to propose two different empirical models, one for each regime of rotation. A new non-dimensional parameter obtained from the Buckingham Pi analysis, proposed as Wall Froude number, is used to find a threshold triggering the scouring mechanism. The experimental results show that the distance to the vertical wall is the main contributor to the higher scour depths rather than the bed clearance. The last part of the thesis presents a case study that is used to relate the maneuvers of the ship with the scour observed in an inner basin of the port. The study ship characteristics are obtained from the port statistics. The morphological evolution of the seabed is analyzed through hydrographic surveys of the last decade, which allows characterizing the erosion pattern. Through AIS data, the maneuver of the study ship is analyzed and used as an input maneuver in a simulator. The maneuver is reproduced to obtain the behavior of the main engines and bow thrusters. The maneuvering pattern and the behavior of the engines are used to assess the most damaging maneuvering sections, which turns out to be the ship's lateral approach to the berth, during the arrival maneuver, and the early stages of the departure maneuver.
Els vaixells que maniobren en condicions de calat reduït poden provocar erosió local del llit de sediments a causa del raig d'aigua d'alta velocitat generat per les hèlix. L'erosió causa inestabilitat en les estructures dels molls, danys a la protecció del llit i pot crear grans àrees d'acreció, posant en risc la seguretat en la navegació. Aquest fenòmen està relacionat en gran mesura amb l'augment de les dimensions dels vaixells comercials en les darreres dècades i és àmpliament reconegut en les guies actuals de gestió portuària. Aquesta tesi presenta, en primer lloc, el treball experimental realitzat per estudiar els efectes erosius provocats per dos dels sistemes de propulsió més comuns: hèlix convencional i sistema de doble hèlix. A continuació, s'utilitza l'anàlisi d'un cas pràctic per avaluar l'impacte de les maniobres dels vaixells sobre el fons marí en una dàrsena portuària concreta. A la primera part de la tesi es presenten i s'analitzen en un conjunt de mesures experimentals de velocitat del flux d'aigua a la sortida de l'hèlix i de tensions de tall sobre un llit horitzontal. La distribució de velocitats axials es caracteritza a partir de mesures puntuals preses amb un sistema de tub de Pitot-estàtic acoblat a sensors de pressió diferencial. Posteriorment es realitzen mesures directes de les tensions de tall al llit amb una placa de tensions (shear plate) a tres velocitats de rotació de l'hèlix i dues distàncies verticals sobre el llit. Les mesures de la velocitat i la tensió de tall es relacionen amb un coeficient de fricció empíric que és dependent de la distància al llit. A la segona part d'aquest treball es presenta un nou conjunt d'experiments en erosió local per sistemes de doble hèlix confinats. La profunditat màxima d'erosió s'estudia en funció de la distància al sòl, la distància a la paret i la velocitat de sortida del raig d'aigua. Amb l'objectiu de reproduir maniobres de vaixells en entorns portuaris, s'analitza l'erosió generada pels sistemes de doble hèlix en règim combinat, és a dir, avant i invertit. Els resultats mostren diferències respecte els experiments tradicionals en què només es considera la rotació avant. Per estimar l'evolució temporal de la màxima profunditat d'erosió es proposen dos models empírics, un per a cada règim de rotació. De l'anàlisi de Pi de Buckingham s'obté un nou paràmetre adimensional que, proposat com a nombre de Froude del mur, permet establir un llindar d'erosió. Els resultats experimentals també mostren que la distància al mur vertical està més relacionada amb les majors profunditats d'erosió que amb la distancia al sòl. La darrera part de la tesi presenta un cas d'estudi en què les maniobres del vaixell es relacionen amb els canvis en la morfologia portuària observada al llarg del temps. El vaixell d'estudi s'obté a partir de les estadístiques d'ús de les dàrsenes portuàries. L'evolució morfològica del fons marí s'analitza mitjançant estudis hidrogràfics de la darrera dècada, que permet caracteritzar el patró d'erosió. A través de dades AIS, s'analitza la maniobra del vaixell d'estudi i s'utilitza com a model de maniobra en un simulador. La maniobra es reprodueix al simulador, d'on se n'extreu el comportament dels motors principals i de les hèlix de maniobra. El patró de maniobra i el comportament dels motors s'utilitzen per avaluar les seccions de maniobra més perjudicials, que resulten ser l'aproximació lateral del vaixell al moll d'atrac, durant la maniobra d'arribada, i les primeres etapes de la maniobra de sortida.
Enginyeria nàutica, marina i radioelectrònica naval

This study focuses on the interaction of wash load particles with gravel bed rivers. The effects of excess fine sediment loading to streams on general water quality, contaminant transport, and benthic organism mortality has been well examined. A fundamental assumption in fluvial geomorphology and river engineering is that wash load particles ($d<63mu m$) do not deposit to stream beds, but are instead transported downstream until they deposit in reservoirs or estuaries. The goal of this study is to determine if wash load sized particles can deposit to gravel beds, where within the bed substrate deposition occurs, under what hydraulic conditions it occurs, and how the composition of the bed affects the spatial and temporal deposition pattern. Further, this study attempts to quantify the mass flux of wash load to the bed based on a simple mass conservation model using the aforementioned conditions as model parameters. This was accomplished through a series of experiments in which a mixture of pure kaolinite clay was allowed to deposit at constant shear over an acrylic, gravel, or sand-gravel mixture. Discharge was then increased to determine the effects of increased bed shear stress on deposited material and further wash load interaction with the bed. Results indicate that wash load will deposit to acrylic, gravel, and sand-gravel beds during conditions where no bedload movement is occurring. Bed composition is the primary factor controlling the mass flux of wash load from the water column to the bed. Deposition on acrylic beds forms clay ripples which translate downstream, while deposition in porous beds occurs primarily within the bed substrate. Shear stress also affects mass flux and the magnitude of its effects are related to the bed composition. Discharge increases below the threshold of bedload movement only cause large scale entrainment of deposited particles over non-porous beds. Periods of higher discharge over porous beds result in continued deposition within the bed substrates. This research enhances not only our knowledge of sediment processes within fluvial systems, but also allows for the quantification of the wash load portion of those processes given minimal initial condition information. The model developed here may be used within larger hydrologic models when examining contaminant spills or mass loading of stream networks with wash load to estimate the mass deposition to the bed. Instances where wash load is contaminated the mass of contaminated sediment retained by the bed is of great importance to local communities given a reliance of residents on that water source for water, livelihood, and recreation.
Master of Science

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, June 2006.
Includes bibliographical references (leaf 18).
An experiment was performed to study the evolution of a non-cohesive granular bed subject to a succession of erosive turbidity currents to evaluate the applicability of a proposed interface model that prescribes a relationship between the bed shear stress and the local bed elevation and local bed slope. This study considered only the relationship between the local bed elevation and the shear stress by using non-cohesive plastic particles as bed sediment that was laid down in a subaqueous straight channel. The elevation of the plastic bed was measured before and after it was subjected to two erosive salt water turbidity currents using a high resolution displacement laser mapping system. These changes in elevation of the sediment bed are compared to determine whether a correlation exists between local bed elevation and bed erosion. The results from this experiment display a weak correlation between the erosion of the plastic bed and the initial local bed elevation when the initial elevation height is small relative to the thickness of the turbidity currents.
(cont.) However, a strong correlation exists when the initial elevation height is approximately 2% to 6% of the turbidity current thickness. The strength of this correlation also depends upon the window size of the analysis which is shown to be associated with the downstream variability in bed roughness.
by Roberto Carlos Rangel.
S.B.

Cette thèse porte sur la caractérisation du transport sur un lit granulaire cohésif en considérant le cas du sable et de la neige. Une approche numérique pour étudier le processus de l'érosion par impact sur un empilement cohésif a permis de quantifier l'effet de la cohésion sur le processus Splash. Pour quantifier l'effet de la cohésion le transport de particules par un écoulement d'air, nous avons choisi une approche expérimentale en soufflerie. L'étude du transport sur un lit de sable sec a permis de caractériser la transition du régime de saltation vers un régime collisionnel lorsque le nombre de Shields atteint S=0.3. Dans le cas du transport de sable humide, la vitesse seuil d'arrachement des grains du lit augmente avec la teneur en eau tandis que la vitesse seuil d'impact est du même ordre de grandeur que le seuil dynamique obtenu dans le cas sec. Ces résultats suggèrent qu'une fois que les grains sont arrachés au sol, ils ont un comportement similaire au transport sur sol sec. Sur un lit rigide de neige, la longueur caractéristique de décroissance augmente quadratiquement avec la vitesse de friction de l’air mais également avec le diamètre de la particule. Les caractéristiques de transport éolien de neige sur neige cohésive sont proches de celle sur neige rigide. On note une nette diminution, en soufflerie, de la quantité de neige transportée qui peut toutefois être significativement augmentée du fait de l’effet érosif d’un flux entrant. Sur un lit cohésif, dans le cas du sable humide et de la neige, le flux transporté tend vers la saturation mais la longueur de la soufflerie ne permet pas de l'atteindre
This thesis deals with the cohesion effect on the aeolian grain transport. We consider the sand and snow case. A numerical approach to the erosion process by impact allows us to quantify the cohesion effect on the Splash process. Then we adopt wind tunnel experiment to study the effect of the cohesion on the transport. We show over a dry sand bed a transition from the saltation regime to a collision one at a Shields number of 0.3. Over a wet sand bed, the aerodynamic threshold increases with the water content whereas the impact threshold is in the same order of magnitude compare to the dry case. Over a hard snow bed, the characteristic decay height shows a quadratic dependence with the air friction speed and increases with the particle diameter. The characteristics of the aeolian transport over a cohesive snow bed are close to those over a hard one. There is a sharp decrease of the transported snow flux which can increase significantly when the wind tunnel is fed with an incoming snow flux. Our results indicate the mass flow rate aims to a saturation. However the saturation is not reached with the wind-tunnel length we work over wet sand and cohesive snow bed

L’affouillement local autour des piles est considéré comme un des principaux risques de rupture et d’effondrement des ponts. Les expériences en laboratoire sont un élément crucial de l’étude du processus d’affouillement. Pourtant, il n’existe pas d’instrument robuste et standard pour suivre l’évolution de ce phénomène physique dans les études de laboratoire. Les études expérimentales en canal sur l’affouillement autour d’une pile circulaire dans un sol non cohésif sont nombreuses. Cependant, le lit des canaux alluviaux naturels contient souvent des sédiments cohésifs. Comparativement, il existe peu de recherches sur l’affouillement local autour des piles circulaires fondés dans des lits cohésifs. Dans le cadre de cette recherche doctorale, un scanner laser 3D est utilisé pour suivre l’affouillement local autour d’une pile circulaire. Les résultats obtenus avec cette technique, en accord avec les recherches précédentes, notamment celles effectuées sur un lit de sable, démontrent son efficacité. Elle offre des avantages tels qu’une résolution spatio-temporelle continue, mais aussi de nombreuses limitations. Dans une seconde étape, l’étude a porté sur l’effet du type et de la proportion des fines sur la profondeur et la forme de la fosse d’affouillement et sur la propagation temporelle du processus d’affouillement autour du pile circulaire. Les résultats montrent que l’augmentation de l’argile dans la fraction fine réduit significativement l’affouillement. Le mélange avec une gamme d’environ 7.5-10 % de teneur en argile fournit la composition seuil pour un comportement cohérent du sol dans le processus d’affouillement. En outre, la prédiction de la profondeur d’affouillement dans les sols cohésifs dans la conception des fondations de ponts n’est pas encore complètement développée. Dans la pratique, les méthodes d’estimation de la profondeur d’affouillement actuellement utilisées sont celles proposées pour des sols non cohérents. De nombreux chercheurs ont mis au point des méthodes pour optimiser la conception des piles de pont dans le cas d’un sol cohésif. Parmi elles, celle basée sur l’utilisation de l’érodimètre EFA (Erosion Function Apparatus) et appelée SRICOS (Scour Rate In COhesive Soil) a été examinée dans le cadre de ce travail afin de prédire la profondeur d’affouillement maximal dans un lit cohésif
Local scour is considered to be one of the main causes of bridge failure and collapse. Laboratory experiments are a crucial and important approach for the scour process investigation. Yet, there is no robust and standard instrumentation for tracking the evolution of this physical phenomenon in laboratory studies. There are several flume-based studies of scour around a circular bridge pier on cohesionless soil. However, the bed of natural alluvial channels often contains cohesive sediments. Comparatively, there is a limited research on local scour around circular pier founded in cohesive beds that has been documented. In this PhD research, a 3D Laser Scanner is used to monitor scour around a circular pier. The results obtained with this technique, in line with previous research, in particular that carried out on a sandy bed, demonstrate its effectiveness. The technique offers advantages such as continuous spatiotemporal monitoring, but also many limitations. In a next step, the study focused on the effect of the type and proportion of fines in the sediment mixture on the depth and the shape of the scour hole and the temporal propagation of scour process around a circular pier. Results show that increasing the clay in the fine fraction reduce significantly the scour. The mixture with a range around 7.5-10 % of clay fines content provides the threshold composition for coherent soil behavior in scouring process. In addition, scour depth prediction for bridge piers in cohesive soil is not yet fully developed. In engineering practice, scour depth estimation methods currently used are those proposed for sand bed. Many researches aimed to adress a method to optimize the design of bridge piers in the case of cohesive soil. Among the different methods, the one based on the use of the EFA (Erosion Function Apparatus) erodimeter and called SRICOS (Scour Rate In COhesive Soil) was examined as part of this work in order to predict the deepest scour in cohesive bed

Atmospheric freeze drying (AFD) is the lyophilization of a product at atmospheric pressure conditions and temperatures ranging generally between -15 and -5 ◦C (avoiding, thereby, ice melting). The quality of the obtained dried products is quite similar to the quality of products dried by vacuum freeze drying (VFD), but without the need of generating vacuum, maintaining temperatures around -50 ◦C in the condenser, or defrosting it. There are several ways to carry out AFD, such as the use of a fluidized bed or a tunnel conveyor. Nevertheless, AFD involves considerably longer drying times than VFD, and the process must be modified in some way in order to shorten them without loss of product quality at the same time. Moreover, since this process is usually carried out with air at very low temperatures, it can saturate rapidly. This situation leads to a reduction of the gradient of water concentration between air and product surface, and consequently, a diminution of the mass transfer rate. The use of an adsorbent material compatible with the food product (i.e., not toxic for human consumption) in a fluidized bed, could constitute an alternative for using other extra energy supplies (such as IR application, or heat pump). At the same time, the use of the adsorbent medium presents two additional advantages: the first, as the heat of adsorption of water vapour is of the same order of magnitude than sublimation heat of ice, no additional energy supply is necessary; second, it acts as adsorbent medium for generated water vapour, allowing air recirculation, which means an additional reduction of operative costs. In particular, non-food wheat bran is an interesting material to be applied as adsorbent in this process; this adsorbent is not only compatible with foodstuff, but also, since it is a by-product of the cereal processing industry, it is cheap and can be easily discarded (and reused, for example, in compost) without recovering it by means of a thermal treatment. Nonetheless, as it is the hard outer layer of cereals consisting of combined aleurone and pericarp, its particles exhibit a very irregular plane shape, with rests of grain brush and, in some cases, broken pericarp. These characteristics confer to the particle a rough surface and, as undesired consequence, the possibility of mechanical interaction during fluidization. However, when two different materials are fluidized in a fluidized bed, the mixture may undergo segregation, causing a poor contact between the adsorbent and the food particles. Thus, instead of using a traditional fluidized bed, a spout-fluid bed (an apparatus similar to the spouted bed, with lateral air injectors beside the main jet) may be utilized, and thereby enhancing mixing. On the other hand, CFD simulation would offer a great potential for simulating the AFD process, its optimization from the fluid dynamic point of view, and the design of new equipment. Various investigators have been working on the application of CFD models for simulating AFD in fluidized bed. However, in general, they simulated a single piece of foodstuff, but not the complete system with air, food material, and adsorbent (when it is applied). The general objectives of the PhD work are to determine the hydrodynamic conditions under which AFD in adsorbent fluidized bed is feasible, and to obtain a first approach to a CFD model of the process. Particularly, the study of the hydrodynamics of the process (non-food wheat bran fluidization behaviour, and mixing of binary mixtures) in a fluidized bed as well as in a spout-fluid bed is aimed from the experimental point of view, while the evaluation of the possibility of simulation by means of a CFD code of the AFD process by immersion in adsorbent medium in a fluidized bed is intended in the theoretical field. Unlike sand or other materials in which regular bubbles are formed, non-food wheat bran exhibits canalization or preferential air paths formation, and bed does not expand after overcoming minimum fluidization velocity. In addition, bran particle diameter is represented by a population distribution whose majority is Geldart B. Therefore, considering other bran particles physical characteristics such as rough surface and rest of grain brushes, it can be said that this "pseudo-cohesive" behaviour is caused principally by mechanical interactions rather than electrostatic forces as occurs in cohesive powders. In general terms, it can be observed a cyclical behaviour of channels generation and collapse. The number of channels and their shape depend on air superficial velocity as well as the bed position where they are formed. Anyway, in general they follow the Channel Generation and Collapse Cycle where two main stages are represented: I, generation, and II, collapse. Experiments emulating different stages of the AFD process with adsorbent application (using fresh food, partially lyophilized material, and completely lyophilized foodstuff) were done employing different food particles (peas, carrot discs, and potato slabs). Experiments were carried out in a 35 cm squared base fluidized bed and in a 20x10 rectangle base fluid-spout bed. The effects on segregation of air superficial velocity, product volumetric fraction, and particle shape were evaluated. For evaluating the segregation, segregation indexes form literature were evaluated, but some difficulties were found using them, besides it is not possible to obtain information about the segregation profiles with them. Thus, a novel way for characterizing segregation was proposed (the Three Thirds Segregation Indexes Set, TTSIS), consisting of three numbers that evaluate the distribution profile of a material of interest (food product, for the current case) and a fourth one that gives an idea of the segregation level. TTSIS was found the best tool for quantifying the segregation phenomenon, as it allows not only to measure the segregation level, but also classify the segregation pattern. As it was expected from the theory, it was evidenced that, even for a binary mixture composed by a pseudo-cohesive powder and a solid whose particles are considerably greater than the powder ones, the air superficial velocity plays a very important role in mixing. Particularly, at high air flows (2.6 umf-adsorbent for the analysed cases) uniform distribution of the material of interest are reached when dried foodstuff is used. Nonetheless, product density plays a fundamental role, since disuniform segregation profiles were obtained when fresh or partially lyophilized food material was used. Uniform mixing profiles were reached in the fluid-spout bed with a good circulation of the food particles along the bed during the fluidization. These results shown to be independent of the product density. Thus, this kind of bed should be used if an uniform mixing between adsorbent and food product is desired. Segregation phenomenon in channelling fluidized beds and the mixing process in fluid-spout beds might be explained by means of two food particle transport mechanisms (passive and active) and two movement blocking effects (floor and roof effects) observed during experimentation (video analysis), and the Channel Generation and Collapse Cycle. Regarding to CFD simulations, relatively reasonable results were obtained from the hydrodynamic point of view only at high air superficial velocity. However, specific models for cohesive or pseudo-cohesive powders are required if an accurate simulation of this kind of solids or binary mixtures integrated by them is intended. In sum, from the experimental results, emerged that the fresh product completely segregates toward the bed bottom in fluidized bed when mixtures containing food material without drying were used. Thus, a good contact between the material to be dried and the adsorbent (desirable for utilizing the adsorption heat for ice sublimation) would be not possible for AFD with use of adsorbent in fluidized bed applications. In contrast, using a fluid-spout bed maintains a very good mixing even if fresh food particles are used. Thus, beside the already known applications of this kind of beds for catalytic processes, its utilization for AFD with adsorbent medium seems to be an interesting and novel option for this process. Nevertheless, CFD simulation might be performed only for non-cohesive powders since the simulation of pseudo-cohesive materials fluidization is currently limited because of the lack of hydrodynamic models for this kind of solids.

Bridge scour, which is the removal of bed materials from near the bridge foundations, is observed to be the most predominant cause of bridge failures in the United States. Scour in cohesive soils is greatly different from scour in cohesionless soils owing to the differences in critical shear stresses, scour extents and the time taken to reach the maximum scour depth in the scour process. The present solutions available for the cohesionless soils cannot be applied to cohesive soils because of the above crucial reasons. Also, a compound channel model with main channel and flood plain arrangement represents more closely the field stream conditions rather than a simple rectangular prismatic model. In this study, a systematic investigation of the scour process due to flow contractions in a compound channel with cohesive soil bed is made by conducting a series of flume tests representing typical field conditions. The effect of the most crucial factors causing contraction scour namely flow velocity, depth of flow and the shape of the abutment is examined. Correction factors are developed for changes in flow geometries incorporating simulation results from the one dimensional flow simulation model HEC RAS. Most importantly, a methodology to predict the depth of the deepest scour hole and its location in the vicinity of the contraction structure is developed for compound channels through an extension of the presently available methodology to predict maximum scour depths in simple rectangular channels. A prediction method to identify the extent of the uniform scour depth is also developed. Finally, an investigation of precision of the proposed methodology has been carried out on the field data from a number of real life contraction scour cases. The results obtained from this study indicate that depth of flow and geometry of the contraction section significantly influence final scour depth in cohesive soils with deeper flows and harsh contractions resulting in increased scour depths. However, corrections for different contraction inlet skew angles and long contractions need to be further explored in future studies.

Most previous studies on the genesis and evolution of bedforms have focused on aggradational bedforms within cohesionless sediments, with very few investigations that concern either erosive bedform genesis and evolution or bedrock channel abrasion processes. The study presented here details experiments that involve the genesis and formation of erosional bedform features within natural (soft clay) cohesive sediment beds and analogue bedrock substrates by modelling clay under the effect of both open-channel plain water flows, and sediment-laden flows. A new approach without using plaster-of-Paris or real bedrock developed provides a feasible method to simulate the genesis and evolution of the erosional bedforms in cohesive sediment beds and sculpted forms in bedrock channels on relatively short time-scales in the laboratory by using a realistic substrate substitute. A series of flume experiments are presented herein where the undrained shear strength of two different kinds of substrate material is systematically varied under constant flow conditions. Experiments using plain water flow indicated that erosive bedforms in cohesive sediment substrate cannot be produced only under the effect of sediment-free flow. Particulate-laden flows do form erosional bedforms in both kinds of clay beds and the shear strength of the bed material plays a key role in determining the diversity of erosional features forming on such substrates. Optimisation of modelling clay beds has enabled us to successfully replicate a suite of bedrock bedforms, including potholes, flutes, longitudinal furrows, etc., that have clear equivalents to those observed in bedrock rivers and contributed to investigate the genesis and evolution process of them and explore the flow structures within and above them in experimental analogue bedrock substrate for the first time.

Siekiant nustatyti iš misijos Afganistane grįžusių karių psichologinę būklę, patiriamus stresogeninius įvykius, kokios stresinės situacijos įtakoja potrauminio streso pasireiškimą, šiame darbe analizuojami jų patirti stresogeniniai įvykiai, potrauminio streso išsivystymas bei grupės sutelktumas. „Karių patiriamo streso klausimynas“, Potrauminio streso Misisipės skalė“, „Grupės sutelktumo“ klausimynais. Apklausta 111 iš misijos Afganistane grįžusių karių. Rezultatai parodė, kad kariams, patyrusiems daugiau stresogeninių įvykių bei blogiau vertinantiems grupės sutelktumą, yra labiau išreikšti potrauminio streso simptomai, nei tiek pat patyrusiems stresogeninių įvykių, bet geriau vertinantiems grupės sutelktumą. Nustatyta, kad nepriklausomai nuo patiriamų stresogeninių įvykių, kariai patiria daugiau potrauminio streso simptomų, esant blogesniam grupės sutelktumui. Tiek aukštojo, tiek neturinčių aukštojo išsilavinimo, tiek vyresnių, tiek jaunesnių ir patyrusių stresogeninių įvykių karių potrauminio streso simptomų išreikštumas nesiskiria.
Aiming to assess the psychological condition of soldiers, who have returned from an international mission in Afghanistan, the incurred stressogenic events, to find out what stressful situations cause post-traumatic stress, the stressogenic events they have experienced as well as development of post-traumatic stress and unit cohesion are the scope of analysis of this paper. 111 soldiers, who had returned from an international mission, were interviewed using the following surveys: Soldiers' Stress Survey, Mississippi Scale for Post-traumatic Stress, Unit Cohesion. The results demonstrated that the soldiers, who had experienced more stressogenic events and evaluated the unit cohesion with a lower grade, had a deeper post-traumatic stress symptoms than those, who had experienced the same number of stressogenic events but gave a better evaluation of the unit cohesion. It has been established that in the event of lower unit cohesion, soldiers experience deeper post-traumatic stress symptoms, regardless of incurred stressogenic events. The depth of post-traumatic stress symptoms did not vary between groups of soldiers having higher education degree and those who did not, as well as between older and younger or the ones, with bigger stressogenic event experience.

With continued feature size reduction in microelectronics and with more than a billion transistors on a single integrated circuit (IC), on-chip interconnection has become a challenge in terms of processing-, electrical-, thermal-, and mechanical perspective. Today’s high-performance ICs have on-chip back-end-of-line (BEOL) layers that consist of copper traces and vias interspersed with low-k dielectric materials. These layers have thicknesses in the range of 100 nm near the transistors and 1000 nm away from the transistors close to the solder bumps. In such BEOL layered stacks, cracking and/or delamination is a common failure mode due to the low mechanical and adhesive strength of the dielectric materials as well as due to high thermally-induced stresses. However, there are no available cohesive zone models and parameters to study such interfacial cracks in sub-micron thick microelectronic layers. This work focuses on developing framework based on cohesive zone modeling approach to study interfacial delamination in sub-micron thick layers. Such a framework is then successfully applied to predict microelectronic device reliability. As intentionally creating pre-fabricated cracks in such interfaces is difficult, this work examines a combination of four-point bend and double-cantilever beam tests to create initial cracks and to develop cohesive zone parameters over a range of mode-mixity. Similarly, a combination of four-point bend and end-notch flexure tests is used to cover additional range of mode-mixity. In these tests, silicon wafers obtained from wafer foundry are used for experimental characterization. The developed parameters are then used in actual microelectronic device to predict the onset and propagation of crack, and the results from such predictions are successfully validated with experimental data. In addition, nanoindenter-based shear test technique designed specifically for this study is demonstrated. The new test technique can address different mode mixities compared to the other interfacial fracture characterization tests, is sensitive to capture the change in fracture parameter due to changes in local trace pattern variations around the vicinity of bump and the test mimics the forces experienced by the bump during flip-chip assembly reflow process. Through this experimental and theoretical modeling research, guidelines are also developed for the reliable design of BEOL stacks for current and next-generation microelectronic devices.

This research is part of the extension of the SRICOS-EFA method for predicting the maximum scour depth history around the bridge abutment. The basic objective is to establish the equation for predicting the maximum bed shear stress around the abutment at the initial condition of scouring. CHEN3D (Computerized Hydraulic ENgineering program for 3D flow) program is utilized to perform numerical simulations and predict bed shear stress before scouring. The Chimera technique incorporated in CHEN3D makes the program capable of simulating all kinds of complex geometry and moving boundary. CHEN3D program has been proven to be an accurate method to predict flow field and boundary shear stress in many fields and used in bridge scour study in cohesive soils for more than ten years. The maximum bed shear stress around abutment in open rectangular channel is studied numerically and the equation is proposed. Reynolds number is the dominant parameter, and the parametric studies have been performed based on the dimensional analysis. The influence of channel contraction ratio, abutment aspect ratio, water depth, abutment shape, and skew angle has been investigated, and the corresponding correction This research is part of the extension of the SRICOS-EFA method for predicting the maximum scour depth history around the bridge abutment. The basic objective is to establish the equation for predicting the maximum bed shear stress around the abutment at the initial condition of scouring. CHEN3D (Computerized Hydraulic ENgineering program for 3D flow) program is utilized to perform numerical simulations and predict bed shear stress before scouring. The Chimera technique incorporated in CHEN3D makes the program capable of simulating all kinds of complex geometry and moving boundary. CHEN3D program has been proven to be an accurate method to predict flow field and boundary shear stress in many fields and used in bridge scour study in cohesive soils for more than ten years. The maximum bed shear stress around abutment in open rectangular channel is studied numerically and the equation is proposed. Reynolds number is the dominant parameter, and the parametric studies have been performed based on the dimensional analysis. The influence of channel contraction ratio, abutment aspect ratio, water depth, abutment shape, and skew angle has been investigated, and the corresponding correction factors have been proposed. The study of the compound channel configuration is conducted further to extend the application of the proposed equation. Numerical simulations of overtopping flow in straight rectangular channel, straight compound channel and channel bend have been conducted. The bridge deck is found to be able to change the flow distribution and the bed shear stress will increase significantly once overtopping. The influence of the channel bend curvature, abutment location in the channel bend, and the abutment shape is also investigated. The corresponding variation of the bed shear stress has been concluded. The scour models, including the erosion rate function, roughness effect, and the turbulence kinetic energy, have been proposed and incorporated into the CHEN3D program. One flume test case in NCHRP 24-15(2) has been simulated to determine the parameters for the roughness and the turbulence kinetic energy. The prediction of the maximum scour depth history with the proposed model is in good agreement with the measurement for most cases. The influence of overtopping flow on the abutment scour development is also studied and the corresponding correction factor is proposed.

博士
國立中央大學
機械工程研究所
89
This thesis examines the dynamic behaviors of granular materials subjected to external vertical vibration. The cohesionless and cohesive materials are considered in this thesis. Two-dimensional discrete element computer simulation is used to study the flow behaviors, self-diffusion and mixing of vibration granular flows. Experiment was also performed to supplement the simulation results of the current study. The flow behaviors of convection cells of cohesionless materials under vertical vibration are first investigated by simulation. The flow pattern and velocity vectors are consistent with the former experimental results. The profiles of solid fractions and the granular temperatures with the altitude of granular bed are studied with different vibration conditions. A power law relation exists between the convection flow rate and the dimensionless vibration velocity. The influences of flow parameters on self-diffusion in the vibrated granular bed are studied by simulation and experiment. Employing the image processing technology and particle tracking method, the local displacements and velocities of particles are measured. The self-diffusion coefficients are determined from the history of particles’ diffusive displacements. The DEM simulation is performed to calculate the particles’ self-diffusive displacements with the same flow parameters and material properties as the experiment. The simulation results are compared to the experimental tests. The flow behaviors of convection cell are strongly related to the self-diffusion of particles induced by the energy input from the vertical external vibration. The velocity fluctuations, granular temperature and self-diffusions are anisotropic with greatest components in the vertical direction. The dependence of the diffusion coefficients on the dimensionless acceleration, vibration amplitude, vibration frequency, solid fraction, velocity fluctuations, restitution coefficient and granular temperature are discussed carefully. The wet particles with the effect of liquid bridge are used as the cohesive granular materials. Three types of viscous liquids with different tension and viscosity are used in this thesis. A simplified model of dynamic bridge strength based on the superposition of lubrication and circular capillary force approximation is incorporated in the DEM model. The energy dissipations during vertical vibration are generated from the friction and inelasticity between particles, viscous resistance and liquid bridge bond rupture due to the liquid bridge. For cohesive granular materials, the energy dissipation is mainly associated with the viscous force, the interparticle friction and the inelasticity of collision, rather than with the capillary force of liquid bridge. The liquid bridge force due to surface tension and viscosity interacts with frictional force mutually to determine the strength of the convection flow rate. The energy dissipation increases monotonously with the increase of the dimensionless interstitial liquid volume, and the distributions of the energy dissipation are strongly influenced by the properties of viscous liquids. The self-diffusion motions for cohesive materials are faster than those of cohesionless materials. The mixing of vibrated granular flow is strongly dependent on the self-diffusivity of particles and is related to the magnitude of interstitial liquid volume between particles.

The amount and type of cohesive sediment found in gravel river beds can have important implications for the health of aquatic biota, surface/groundwater interactions and water quality. Due to landscape disturbances in the Elbow River watershed, increased sediment fluxes have negatively impacted fish habitat, water quality and water supply to the City of Calgary. However, little is known about the source of cohesive sediment and its interaction with gravel deposits in the Elbow River. This research was designed to: 1) quantify the transport properties (critical shear stress for erosion, deposition, porosity, settling velocity, density) of cohesive sediment and 2) evaluate the potential for coarse gravel to entrap cohesive sediment in the Elbow River. A 5m annular flume was used to conduct erosion and deposition experiments using plane and coarse bed conditions. The critical shear stress for deposition and erosion of the Elbow River cohesive sediments was 0.115Pa and 0.212Pa, respectively. The settling velocity of the cohesive sediment had an inverse relationship between floc size and settling velocity for larger flocs, due to a decrease in floc density with increased size. Cohesive sediment moved from the water column into the gravel bed via the coupling of surface and pore water flow. Once in the gravel bed, cohesive sediments were not mobilized from the bed because the shear produced by the flume was less than the critical shear to mobilize the gravel bed. Using a model developed by Krishnappan and Engel (2006), an entrapment coefficient of 0.2 was determined for the gravel bed. Entrapment coefficients were plotted against substrate size, porosity and hydraulic conductivity, demonstrating a relationship between entrapment coefficient and these variables. It was estimated that 864kg of cohesive sediment is stored in the upper 0.08m of a partially submerged point bar in the Elbow River. Accordingly, when flow conditions are sufficient to mobilize the gravel bed and disturb the amour layer, cohesive materials may be entrained and transported into the Glenmore Reservoir, where it will reduce reservoir capacity and may pose treatment challenges to the drinking water supply.

Hydraulic models, using either cohesive or non-cohesive beds, have traditionally been employed to gauge scour below a spillway. Empirical equations, based on world-wide prototype and model experience, have been developed for estimating scour depth depth. In this study both the hydraulic modelling and analytical approach were used to assess the maximum potential scour in the rock lined channel below Ruskin Dam. Ruskin Dam, built in 1928, has an overflow spillway with seven bays discharging into a rock-lined channel. The spillway has a complex arrangement of concrete benches lining the sides and a large concrete bridge pier in directly downstream of the spillway. A non-cohesive bed, hydraulic model was employed to understand how the degree of scour is affected by the presence of the bridge pier and redirection of the spillway discharge by the concrete benches, and to locate areas of maximum scour intensity. Five scour equations were used to determine theoretical scour depths in both the prototype and model. Data on scour depths gathered from the model study was compared to the theoretical values calculated from the equations. From the model study it was shown that if the bridge pier was removed from the spillway channel the point of maximum scour moves downstream and the intensity is reduced. The location of the point of maximum scour was also influenced by gate openings. High scour intensities were recorded for gate openings where flow was deflected to one side of the channel and localized by the pier armature. The results of the scour depth comparisons using theoretical and measured model depths showed that 2 of the 5 equations reasonably represented the model scour. This result was generalized to the scour calculations using prototype data and an estimate of scour depth was made for the rock lined spillway channel below Ruskin Dam.

Dissertação de Mestrado apresentada no ISPA – Instituto Universitário de Ciências Psicológicas, Sociais e da Vida, para obtenção do grau de Mestre na especialidade de Psicologia Social e das Organizações
O presente estudo tem como objetivo principal compreender se a liderança do treinador tem influência no desempenho e bem-estar da equipa, e ainda compreender se essa relação é mediada pela coesão. A amostra é constituída por 110 jogadores de futebol e rugby, em que 90 pertenciam ao sexo masculino e 20 ao sexo feminino, com idades compreendidas entre os 14 e superior ou igual a 25 anos (M=19,3; DP=3,8). Para avaliar as variáveis em estudo foram utilizados quatro instrumentos: a escala Multidimensional de Liderança no Desporto, de Gomes e Resende (2014); o questionário de Perceção de Rendimento Desportivo, de Gomes (2016); a Warwick-Edinburgh Mental Well-being Scale (WEMWBS), de Kammann e Flett (1980), e ainda, a escala Group Environment Questionnaire, de Carron, Brawley e Widmeyer (1985). Os resultados obtidos demonstram que as hipóteses de estudo foram suportadas, constatando que existe uma relação significativa entre a liderança transformacional e o desempenho percebido e bem-estar da equipa. Além disso, testemunhou-se que existe impacto significativo da liderança transformacional no desempenho percebido, tal como no bem-estar da equipa, por meio da coesão.
The main objective of this study is to understand if coach leadership has influence on team performance and well-being, and also to understand if this relationship is mediated by cohesion. The sample is composed by110 soccer and rugby players, in which 90 were male and 20 female, with a range of age14 to over 25 years old (M = 19.3; SD = 3.8). In order to evaluate the variables in this study, four instruments were used: the Multidimensional Sport Leadership scale, by Gomes and Resende (2014); the Sports Perception Questionnaire, by Gomes (2016); the Warwick-Edinburgh Mental Well-being Scale (WEMWBS) by Kammann and Flett (1980); and the Group Environment Questionnaire, by Carron, Brawley and Widmeyer (1985). The results reveal that the study hypotheses were supported, showing that there is a significant relationship between transformational leadership and perceived performance and well-being of the team. Besides that, it has been witnessed that there is a significant impact of transformational leadership on perceived performance as well as team well-being, through cohesion.

O mundo organizacional está em permanente mudança, quer devido à globalização, ao aumento da concorrência, às crises económicas que se sucedem, o que faz com que as organizações e os seus colaboradores estejam sujeitos a exigências extremas. É neste sentido que ganham cada vez mais pertinência estudos no âmbito do stress e bemestar no trabalho, sendo que o presente estudo propõe-se a analisar a relação entre as exigências laborais e o stress profissional percebido e o bem-estar psicológico, assim como verificar o papel moderador que a coesão grupal percebida possui na relação entre exigências laborais e stress profissional percebido e bem-estar psicológico. A amostra utilizada é constituída por 158 indivíduos que pertencem a um grupo hoteleiro, sendo que a aplicação dos questionários ocorreu via electrónica e pessoalmente. Os resultados obtidos permitiram comprovar que existe uma forte relação entre as exigências laborais de tempo com, quer o stress profissional percebido, quer o bem-estar psicológico, sendo neste segundo caso a correlação mais forte. Por outro lado, a coesão grupal percebida não teve um papel moderador entre as exigências laborais e o stress profissional percebido e o bem-estar psicológico, sendo no entanto um forte preditor do bem-estar psicológico.
The organizational world is constantly changing, whether due to globalization, increased competition, economic crises that follow, which makes it possible for organizations and their employees are subject to extreme demands. In this sense, is of increasing relevance in studies of stress and well-being at work, and that this study proposes to examine the relationship between job demands and occupational stress and perceived psychological well-being, as well as check the moderating role that perceived group cohesion has on the relationship between perceived job demands and perceived occupational stress and psychological well-being. The sample consists of 158 individuals belonging to a hotel group, and the questionnaires occurred electronically and personally. The results allowed to demonstrate that there is a strong relationship between the job demands of time with either perceived occupational stress, either psychological well-being, and in this second case the strongest correlation. Furthermore, perceived group cohesion had a moderating role between job demands and occupational stress and perceived psychological well-being, and yet a strong predictor of psychological well-being.