Research on video-based face recognition has started getting increased attention in the past few years. Algorithms developed for video have an advantage from the availability of plentitude of frames in videos to extract information from. Despite this fact, most research in this direction has limited the scope of the problem to the application of still image-based approaches to some selected frames on which 2D algorithms are expected to perform well. It can be realized that such an approach only uses the spatial information contained in video and does not incorporate the temporal structure.Only recently has the intelligence community begun to approach the problem in this direction. Video-based face recognition algorithms in the last couple of years attempt to simultaneously use the spatial and temporal information for the recognition of moving faces. A new face recognition method that falls into the category of algorithms that adopt spatio-temporal representation and utilizes dynamic information extracted from video is presented. The method was designed based on the hypothesis that the strain pattern exhibited during facial expression provides a unique "fingerprint" for recognition. First, a dense motion field is obtained with an optical flow algorithm. A strain pattern is then derived from the motion field. In experiments with 30 subjects, results indicate that strain pattern is an useful biometric, especially when dealing with extreme conditions such as shadow light and face camouflage, for which conventional face recognition methods are expected to fail. The ability to characterize the face using the elastic properties of facial skin opens up newer avenues to the face recognition community in the context of modeling a face using features beyond visible cues.

This dissertation presents an experimental investigation of fluid flow, proppant settling and horizontal proppant transport in hydraulic fractures. The work is divided into two major sections: investigation of proppant settling in polymer-free surfactant-based viscoelastic (VES) fluids and development of a new method of proppant injection, referred to as Alternate-Slug fracturing. VES fluid systems have been used to eliminate polymer-based damage and to efficiently transport proppant into the fracture. Current models and correlations neglect the important influence of fracture walls and fluid elasticity on proppant settling. Experimental data is presented to show that elastic effects can increase or decrease the settling velocity of particles, even in the creeping flow regime. Experimental data shows that significant drag reduction occurs at low Weissenberg number, followed by a transition to drag enhancement at higher Weissenberg numbers. A new correlation is presented for the sphere settling velocity in unbounded viscoelastic fluids as a function of the fluid rheology and the proppant properties. The wall factors for sphere settling velocities in viscoelastic fluids confined between solid parallel plates (fracture walls) are calculated from experimental measurements made on these fluids over a range of Weissenberg numbers. Results indicate that elasticity reduces the retardation effect of the confining walls and this reduction is more pronounced at higher ratios of the particle diameter to spacing between the walls. Shear thinning behavior of fluids is also observed to reduce the retardation effect of the confining walls. A new empirical correlation for wall factors for spheres settling in a viscoelastic fluid confined between two parallel walls is presented. An experimental study on proppant placement using a new method of fracturing referred to as Alternate-Slug fracturing is presented. This method involves alternate injection of low viscosity and high viscosity fluids into the fracture, with proppant pumped in the low viscosity fluid. Experiments are conducted in Hele-Shaw cells to study the growth of viscous fingers over a wide range of viscosity ratios. Data is presented to show that the viscous finger velocities and mixing zone velocities increase with viscosity ratio up to viscosity ratios of about 350 and the trend is consistent with Koval’s theory. However, at higher viscosity ratios the mixing zone velocity values plateau signifying no further effect of viscosity contrast on the growth of fingers and mixing zone. The plateau in the velocities at high viscosity ratios is caused by an increase in the thickness of the displacing fluid and a reduction in the thin film of the displaced fluid on the walls of the Hele-Shaw cell. Fluid elasticity is observed to retard the growth of fingers and leads to growth of multiple thin fingers as compared to a single thick dominant finger in less elastic fluids. Observations show the shielding effect is reduced by fluid elasticity. Elastic effects are observed to reduce the thickness of thin film of displaced fluid on the walls of Hele-Shaw cell. The dominant wave number for the growth of instabilities is observed to be higher in more elastic fluids. At the onset of instability, the interface breaks down into a greater number of fingers in more elastic fluids. Experiments are performed in simulated fractures (slot cells) to show the proppant distribution using alternate-slug fracturing. Observations show alternate-slug fracturing ensures deeper placement of proppant through two primary mechanisms: (a) proppant transport in viscous fingers formed by the low viscosity fluid and (b) an increase in drag force in the polymer slug leading to better entrainment and displacement of any proppant banks that may have formed. The method offers advantages of lower polymer costs, lower pumping horsepower, smaller fracture widths, better control of fluid leak-off and less gel damage compared to conventional gel fracs. / text

Södra DuraPulp is a relatively new eco-composite, made from natural wood fibers and polylactic acid (PLA), which comes from corn starch. Until now, there are only few applications for DuraPulp, mainly in the area of design. To find new fields of application, more knowledge about its mechanical material properties are of great interest.This study deals with characterizing the mechanical properties of DuraPulp in an analytical way by means of micromechanical modelling and evaluation with help of Matlab. The mechanical properties for PLA were taken from scientific literature. Not all properties of the wood fibers could be found in literature (particularly Poisson’s ratios were unavailable). Therefore, they partly had to be assumed within reasonable boundaries. These assumptions are later validated regarding their influence on the final product.Figures and tables were used to present and compare the in- and out-of-plane E-Moduli, shear moduli and Poisson’s ratios of DuraPulp. The calculated in-plane E-Moduli were then compared to those obtained from an earlier study, where DuraPulp was tested in tension. The results showed that experimental and analytical values are very similar to each other. / Södra DuraPulp är en relativt ny eco-komposit, tillverkat av naturliga trä fibrer och polylactic syra som kommer från majsstärkelser. I dagsläget finns det få användningsområden för DuraPulp, huvudsakligen används det inom design. För att expandera användningsområdet behövs det mer kunskaper angående de mekaniska egenskaperna för materialet. Studien handlar om att karakterisera de mekaniska egenskaperna för DuraPulp på ett analytiskt sätt i form av mikro-mekanisk modellering och evaluering med hjälp av Matlab. De huvudsakliga mekaniska egenskaperna för PLA kunde hämtas från flera vetenskapliga källor, men de motsvarande mekaniska egenskaperna för fibrer kunde inte alla valideras. Delvis antogs dem i rimliga gränser och deras inverkan validerades med hjälp av en parameter studie.Figurer och tabeller användes för att presentera och jämföra in- och ut-plan E-Moduler, skjuvmoduler och tvärkontraktionstalen av DuraPulp. De beräknade in-plan E-modulerna för DuraPulp jämfördes med motsvarande E-moduler från en tidigare studie där DuraPulp genomgick dragtest. Resultatet visade att analytiska och experimentella värden överensstämmer bra med varandra.

Slender structures are elegant aesthetically. The insufficiency in knowledge of the real resistance to buckling of very slender reinforced concrete columns leads to an exaggeration of the sizes of the columns. _The examples of concrete compression members cited and constructed in Industry on a global basis suggest that very slender columns have inherent safety both from the point of view of the ultimate strength and stability. The strengths of columns given. by the British codes would seem to be exceeded by many of the long slender reinforced concrete columns and struts which have been used Internationally. Both the theoretical and the experimental short term investigations have been carried out to establish the behaviour of hinged, very slender reinforced concrete columns at various stages'of axial loading. Forty three very slender reinforced concrete columns of two different square cross sections with two sizes of longitudinal reinforcements with lateral ties were cast. Slenderness rates, L A, were varied from 30 to 79. Special factors were obtained to relate the actual modulus of elasticity of concrete in columns at buckling failure to a knowledge of the initial modulus of elasticity of concrete in control cylinder specimens. Both theoretical and experimental graphs of load against moment, made dimensionless for critical sections of columns have been obtained. Dimensionless load-moment interaction diagrams using material failure as the criterion have been superimposed on these graphs to show considerable inherent material strength of the tested columns near buckling collapse failures. A theory using the fundamental approach has, been developed to predict the deflected shape and moments along the, heights of the columns at various stages of loading. The proposed theory predicts with good correlations the experimental deflections and moments of any loading stages of the columns. The theory has been used to obtain the required variables, to arrive at the initial predicted design loads of the investigated columns. Good correlations of the moments derived from observed strains have also been obtained. The developed theory predicts satisfactorily the buckling collapse loads of the columns. Although the theory has been derived for axially I loaded very slender reinforced concrete-columns, it seems to accept satisfactorily eccentricities of up to about 10 mm. This was confirmed after extensive comparisons of the theoretical buckling collapse loads with the applicable tests of other authors. Creep In the columns investigated was discovered to be one of the major factors for serious consideration. This was conclusively revealed from the observations on the last two very long term creep tests on columns. The actual safe sustained loads for these very slender columns of slenderness ratios, L/H, between 40 and 79 seem to be between 33% and 19% of the short term buckling collapse loads. The reduced modulus approach to predict the safe long term sustained loads seems to give reasonable values for L/H ratios of 40 and 50. The recommendations given for the proposed design of very slender reinforced concrete columns seem to be adequate and simple to use in practice. They are further simplified by the derivation of two equations for the reduction factors, R, for the slenderness ratios between 36 and 40 and between 40 and 79 respectively. The investigation has proved that very slender reinforced concrete columns are very dangerous structural members, as they tend to have violent buckling failures. Nevertheless, It must be prudent not to design against disaster at any cost. This Investigation seemed to have enhanced considerably knowledge of the design of very slender reinforced concrete columns.

With the significant rise in immigration to the U.S. over the last few decades, fully understanding the economic impact of immigration is paramount for policy makers. As such, this dissertation consists of three empirical essays contributing to the literature on the impact of immigration. In my first essay, I re-examine the impact of immigration on housing rents and completely controlling for endogenous location choices of immigrants. I model rents as a function of both contemporaneous and initial economic and housing market conditions. I show that existing estimates of the impact of immigration on rents are biased and the source of the bias is the instrumental variable strategy common in much of the immigration literature. In my second essay, I present a new approach to estimating the effect of immigration on native wages. Noting the imperfect substitutability of immigrants and natives within education groups, I posit an empirical framework where labor markets are stratified by occupations. Using occupation-specific skill to define homogeneous skill groups, I estimate the partial equilibrium (within skill group) effect of immigration. The results suggest that when one defines labor market cohorts that directly compete in the labor market, the effect of immigration on native wages is roughly twice as large as previous estimates in the literature. In my third essay, I return to the housing market and examine the effects of immigration within metropolitan areas. Specifically, I investigate the relationship between immigrant inflows, native outflows, and rents. Taking advantage of the unique settlement patterns of immigrants, I show that the effect of immigration on rents is lower in both high-immigrant neighborhoods and portions of the rent distribution where immigrants cluster. Contrary to the existing belief in the literature, the results suggest that the preferences of natives, not immigrants, bid up rents in response to an immigrant inflow.

This thesis presents four investigations into mechanical aspects of soft thin structures, focusing on the effects of stochastic and thermal fluctuations and of material inhomogeneities. First, we study the self-organization of arrays of high-aspect ratio elastic micropillars into highly regular patterns via capillary forces. We develop a model of capillary mediated clustering of the micropillars, characterize the model using computer simulations, and quantitatively compare it to experimental realizations of the self-organized patterns. The extent of spatial regularity of the patterns depends on the interplay between cooperative enhancement and history-dependent stochastic disruption of order during the clustering process. Next, we investigate the influence of thermal fluctuations on the mechanics of homogeneous, elastic spherical shells. We show that thermal fluctuations give rise to temperature- and size-dependent corrections to shell theory predictions for the mechanical response of spherical shells. These corrections diverge as the ratio of shell radius to shell thickness becomes large, pointing to a drastic breakdown of classical shell theory due to thermal fluctuations for extremely thin shells. Finally, we present two studies of the mechanical properties of thin spherical shells with structural inhomogeneities in their walls. The first study investigates the effect of a localized reduction in shell thickness—a soft spot—whereas the second studies shells with a smoothly varying thickness. In both cases, the inhomogeneity significantly alters the response of the shell to a uniform external pressure, revealing new ways to control the strength and shape of initially spherical elastic capsules. / Engineering and Applied Sciences

No description available.

Σκοπός της παρούσας διδακτορικής διατριβής είναι η ανάπτυξη αριθμητικής μεθόδου, η οποία επιλύει προβλήματα δισδιάστατης στατικής ελαστικότητας, καθώς και δυναμικής ελαστικότητας στο πεδίο των συχνοτήτων και στο πεδίο του χρόνου. Το κύριο χαρακτηριστικό της είναι ότι η προσέγγιση του άγνωστου πεδίου γίνεται με την τοποθέτηση σημείων και όχι με τη χρήση κάποιου πλέγματος όπως γίνεται στις μέχρι τώρα κλασικές μεθοδολογίες των πεπερασμένων ή συνοριακών στοιχείων. Μέρος της παρούσας διατριβής αποτελεί και η ανάπτυξη προγράμματος ηλεκτρονικού υπολογιστή, ο οποίος υποστηρίζει πλήρως τα όσα αναφέρονται στην παρούσα εργασία. Η παρούσα διατριβή αποτελείται από δύο ενότητες. Στην πρώτη ενότητα, η οποία περιλαμβάνει τα πρώτα τρία κεφάλαια, παρατίθεται το θεωρητικό υπόβαθρο της μεθοδολογίας. Στη δεύτερη ενότητα περιγράφονται διάφορες τεχνικές λεπτομέρειες, όπως ολοκληρώσεις και προσέγγιση πεδίου και δίνονται αρκετά παραδείγματα, τα οποία πιστοποιούν την ακρίβεια και την αξιοπιστία της. / -

Σκοπός της παρούσας διδακτορικής διατριβής είναι η ανάπτυξη μεθοδολογίας συνοριακών στοιχείων για την αριθμητική επίλυση τρισδιάστατων (3-D) στατικών προβλημάτων στα πλαίσια μιας θεωρίας βαθμοελαστικότητας, που στηρίζεται σε μια απλουστευμένης μορφής της θεωρίας του Mindlin και διατυπώθηκε από τους Vardoulakis and Sulem, η οποία λαμβάνει υπόψη και την επιφανειακή ενέργεια, και από τους Aifantis και συνεργάτες. Η διδακτορική διατριβή αποτελείται από δύο ενότητες. Στην πρώτη ενότητα (κεφάλαια 1 και 2) γίνεται μία πλήρης ανασκόπηση της βιβλιογραφίας ως προς τις θεωρίες βαθμοελαστικότητας και στη συνέχεια, περιγράφεται διεξοδικά η παρούσα θεωρία βαθμοελαστικότητας με επιφανειακή ενέργεια. Στη δεύτερη ενότητα παρουσιάζεται η μέθοδος των Συνοριακών Στοιχείων (ΜΣΣ) όπως αυτή εφαρμόζεται για την επίλυση τρισδιάστατων και αξονοσυμμετρικών βαθμοελαστικών προβλημάτων, αντίστοιχα. Η ΜΣΣ βασίζεται στη διατύπωση των ολοκληρωτικών εξισώσεων των βαθμοελαστικών προβλημάτων. Οι άγνωστοι των ολοκληρωτικών εξισώσεων είναι οι συνοριακές τιμές του βασικού πεδίου των μεταβλητών και οι παράγωγοί τους, που για τη βαθμοελαστικότητα είναι τα διανύσματα των μετατοπίσεων, των βαθμίδων τω μετατοπίσεων και τα διανύσματα των επιφανειακών τάσεων. Η προσέγγιση των συναρτήσεων αυτών πάνω στο σύνορο γίνεται με τη βοήθεια συναρτήσεων παρεμβολής από τις αντίστοιχες τιμές τους σε έναν επιλεγμένο αριθμό κόμβων. Η ταχύτητα και η ακρίβεια της ΜΣΣ κατά την εφαρμογή της επηρεάζεται σημαντικά από την ταχύτητα και την ακρίβεια του υπολογισμού των ιδιόμορφων και υπερ-ιδιόμορφων ολοκληρωμάτων. Στην παρούσα διατριβή τα ιδιόμορφα και υπερ-ιδιόμορφα ολοκληρώματα υπολογίζονται με τη χρήση τεχνικών ιδιόμορφης και υπερ-ιδιόμορφης ολοκλήρωσης (Guiggiani (1992) και Huber et al. (1993)) αντίστοιχα. Στα πλαίσια της παρούσας διδακτορικής διατριβής κατασκευάστηκε αλγόριθμος που επιλύει τρισδιάστατα στατικά προβλήματα βαθμοελαστικότητας καθώς και αλγόριθμος που επιλύει στατικά βαθμοελαστικά προβλήματα με αξονική συμμετρία. Στο τέλος κάθε κεφαλαίου, επιλύονται αντίστοιχα στατικά βαθμοελαστικά προβλήματα με ή χωρίς να λαμβάνεται υπόψη η επιφανειακή ενέργεια και με γνωστές αναλυτικές λύσεις. Τα αριθμητικά αποτελέσματα των παραπάνω προβλημάτων συγκρίνονται με τα αντίστοιχα αναλυτικά. Τέλος, γίνεται μία ανακεφαλαίωση της διδακτορικής διατριβής και διατυπώνονται προτάσεις για μελλοντική έρευνα. / In the present Doctoral Thesis a boundary element methodology (BEM) is developed in order to solve numerically 3-D and axis-symmetric static gradient elastic problems. Microstructural effects on the macroscopic behavior of the considered materials have been taken into account by means of a simple strain gradient theory with surface energy obtained as a special case of the general one due to Mindlin, proposed by Vardoulakis and Sulem. All possible boundary conditions (classical and non-classical) have been determined with the aid of a variational statement of the problem. The fundamental solution of the gradient elastic with surface energy has been explicitly determined and used to establish the boundary integral representation of the solution of the problem with the aid of the reciprocal identity, specifically constructed for this gradient elastic with surface energy case. The boundary integral representation consists of one equation for the dispalcement and another one for its normal derivative. Also, the integral forms of the gradient of displacement as well as the Cauchy, relative, double and total stresses in the interior of the gradient elastic body have been derived and presented. The numerical implementation of the integral equations is accomplished with the aid of quadratic isoparametric line (axis-symmetry case) and surface (3-D case) boundary elements. The computation of the singular and hyper-singular integrals involved is done with the aid of highly accurate advanced algorithms.

Posterior tibial tendon dysfunction (PTTD) is a common degenerative condition leading to a severe impairment of gait. There is currently no effective method to determine whether a patient with advanced PTTD would benefit from several months of bracing and physical therapy or ultimately require surgery. Tendon degeneration is closely associated with irreversible degradation of its collagen structure, leading to changes to its mechanical properties. If these properties could be monitored in vivo, it could be used to quantify the severity of tendonosis and help determine the appropriate treatment. Ultrasound elasticity imaging (UEI) is a real-time, noninvasive technique to objectively measure mechanical properties in soft tissue. It consists of acquiring a sequence of ultrasound frames and applying speckle tracking to estimate displacement and strain at each pixel. The goals of my dissertation were to 1) use acoustic simulations to investigate the performance of UEI during tendon deformation with different geometries; 2) develop and validate UEI as a potentially noninvasive technique for quantifying tendon mechanical properties in human cadaver experiments; 3) design a platform for UEI to measure mechanical properties of the PTT in vivo and determine whether there are detectable and quantifiable differences between healthy and diseased tendons. First, ultrasound simulations of tendon deformation were performed using an acoustic modeling program. The effects of different tendon geometries (cylinder and curved cylinder) on the performance of UEI were investigated. Modeling results indicated that UEI accurately estimated the strain in the cylinder geometry, but underestimated in the curved cylinder. The simulation also predicted that the out-of-the-plane motion of the PTT would cause a non-uniform strain pattern within incompressible homogeneous isotropic material. However, to average within a small region of interest determined by principal component analysis (PCA) would improve the estimation. Next, UEI was performed on five human cadaver feet mounted in a materials testing system (MTS) while the PTT was attached to a force actuator. A portable ultrasound scanner collected 2D data during loading cycles. Young's modulus was calculated from the strain, loading force and cross sectional area of the PTT. Average Young's modulus for the five tendons was (0.45±0.16GPa) using UEI. This was consistent with simultaneous measurements made by the MTS across the whole tendon (0.52±0.18GPa). We also calculated the scaling factor (0.12±0.01) between the load on the PTT and the inversion force at the forefoot, a measurable quantity in vivo. This study suggests that UEI could be a reliable in vivo technique for estimating the mechanical properties of the human PTT. Finally, we built a custom ankle inversion platform for in vivo imaging of human subjects (eight healthy volunteers and nine advanced PTTD patients). We found non-linear elastic properties of the PTTD, which could be quantified by the slope between the elastic modulus (E) and the inversion force (F). This slope (ΔE/ΔF), or Non-linear Elasticity Parameter (NEP), was significantly different for the two groups: 0.16±0.20 MPa/N for healthy tendons and 0.45±0.43 MPa/N for PTTD tendons. A receiver operating characteristic (ROC) curve revealed an area under the curve (AUC) of 0.83±0.07, which indicated that the classifier system is valid. In summary, the acoustic modeling, cadaveric studies, and in vivo experiments together demonstrated that UEI accurately quantifies tendon mechanical properties. As a valuable clinical tool, UEI also has the potential to help guide treatment decisions for advanced PTTD and other tendinopathies.