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Papers by Dimitrios Fafalis

Στο πρώτο μέρος της εργασίας γίνεται γνωριμία με το πρόβλημα που προκαλείται από την παραγωγή Επι... more Στο πρώτο μέρος της εργασίας γίνεται γνωριμία με το πρόβλημα που προκαλείται από την παραγωγή Επικίνδυνων Ιατρικών Αποβλήτων (Ε.Ι.Α.) από υγειονομικές μονάδες και τονίζεται η αναγκαιότητα αντιμετώπισής του, μέσω συντονισμένου προγραμματισμού. Ακολουθεί μια σύντομη περιγραφή της κατάστασης που επικρατεί στην Ελλάδα και παρουσιάζεται το νομοθετικό πλαίσιο αντιμετώπισης, εθνικό και κοινοτικό-ευρωπαϊκό. Στη συνέχεια περιγράφονται συνοπτικα οι κυριότερες εφαρμοσμένες τεχνικές αντιμετώπισης των ιατρικών αποβλήτων -ενδονοσοκομειακά και εξωνοσοκομειακά- καθώς και η αναδυόμενη τεχνολογία πλάσματος. Έμφαση δίνεται στην αποτέφρωση, η οποία είναι και η επικρατέστερη και χαρακτηριζόμενη ως βέλτιστη διαθέσιμη τεχνική –BAT- από την ευρωπαϊκή ένωση. Ιδιαίτερο βάρος δίδεται στα γενικά χαρακτηριστικά του αποτεφρωτήρα περιστρεφόμενης κλιβάνου, ο οποίος ενδείκνυται για την αντιμετώπιση ιατρικών αποβλήτων. Λόγω της ευρείας εφαρμογής της μεθόδου της αποτέφρωσης, συμπληρωματικά στο πρώτο μέρος αυτής της ε...

Dynamics, 2021

In this study, a dynamic Mindlin–Reissner-type plate is developed based on a simplified version o... more In this study, a dynamic Mindlin–Reissner-type plate is developed based on a simplified version of Mindlin’s form-II first-strain gradient elasticity theory. The governing equations of motion and the corresponding boundary conditions are derived using the general virtual work variational principle. The presented model contains, apart from the two classical Lame constants, one additional microstructure material parameter g for the static case and one micro-inertia parameter h for the dynamic case. The formal reduction of this model to a Kirchhoff-type plate model is also presented. Upon diminishing the microstructure parameters g and h, the classical Mindlin–Reissner and Kirchhoff plate theories are derived. Three points distinguish the present work from other similar published in the literature. First, the plane stress assumption, fundamental for the development of plate theories, is expressed by the vanishing of the z-component of the generalized true traction vector and not merely...

Acta Biomaterialia, 2021

The round window membrane (RWM) covers an opening between the perilymph fluid-filled inner ear sp... more The round window membrane (RWM) covers an opening between the perilymph fluid-filled inner ear space and the air-filled middle ear space. As the only non-osseous barrier between these two spaces, the RWM is an ideal candidate for aspiration of perilymph for diagnostics purposes and delivery of medication for treatment of inner ear disorders. Routine access across the RWM requires the development of new surgical tools whose design can only be optimized with a thorough understanding of the RWM's structure and properties. The RWM possesses a layer of collagen and elastic fibers so characterization of the distribution and orientation of these fibers is essential. Confocal and two-photon microscopy were conducted on intact RWMs in a guinea pig model to characterize the distribution of collagen and elastic fibers. The fibers were imaged via second-harmonic-generation, autofluorescence, and Rhodamine B staining. Quantitative analyses of both fiber orientation and geometrical properties of the RWM uncovered a significant correlation between mean fiber orientations and directions of zero curvature in some portions of the RWM, with an even more significant correlation between the mean fiber orientations and linear distance along the RWM in a direction approximately parallel to the cochlear axis. The measured mean fiber directions and dispersions can be incorporated into a generalized structure tensor for use in the development of continuum anisotropic mechanical constitutive models that in turn will enable optimization of surgical tools to access the cochlea.

Colloids and Surfaces B: Biointerfaces, 2019

Microperforations in the round window membrane have been suggested for enhancing the rate and rel... more Microperforations in the round window membrane have been suggested for enhancing the rate and reliability of drug delivery into the cochlea. Intratympanic injection, the most common delivery method, involves injecting therapy into the middle ear to establish a reservoir from which drug diffuses across the round window membrane into the cochlea. This process is highly variable because (i) the reservoir, if liquid, can lose contact with the membrane and (ii) diffusion across the membrane is intrinsically variable even with a stable reservoir. To address these respective sources of variability, we compared the thermoreversible hydrogel poloxamer 407 (P407) to saline as a drug carrier and studied the effect of membrane microperforations on drug diffusion rate. We used Rhodamine B as a drug proxy to measure permeance across an artificial membrane in a horizontal diffusion cell. We found that permeance of Rhodamine B from a saline reservoir was an order of magnitude higher than that from a P407 reservoir across unperforated membranes. Moreover, permeance increased with total perforation cross-sectional area regardless of number of perforations (p < 0.05 for all saline-based experiments), but the same association was not found with P407. Rather, for a P407 reservoir, only a large perforation increased permeance (p < 0.001), while multiple small perforations did not (p = 0.749). These results confirm that for drug dissolved in saline, multiple small perforations can effectively enhance diffusion. However, for drug dissolved in P407, larger perforations are necessary.

International Journal for Numerical Methods in Engineering, 2018

The computational continua (C 2) framework, which is a variant of higher order computational homo... more The computational continua (C 2) framework, which is a variant of higher order computational homogenization theories that is free of scale separation, does not require higher order finite element continuity and is free of higher order boundary conditions, has been generalized to unstructured meshes. The salient features of the proposed generalization are: (i) nonlocal quadrature scheme for distorted elements that accounts for unit cell distortion in the parent element domain, and (ii) an approximate variant of the nonlocal quadrature that eliminates the cost of computing positions of the quadrature points in the pre-processing stage. The performance of the computational continua (C 2) framework on unstructured meshes has been compared to the first-order homogenization theory and the direct numerical simulation (DNS).

Computational Mechanics, 2015

The present manuscript focusses on computational aspects of dispersive computational continua ($$... more The present manuscript focusses on computational aspects of dispersive computational continua ($$C^2$$C2) formulation previously introduced by the authors. The dispersive C^2$$C2 formulation is a multiscale approach that showed strikingly accurate dispersion curves. However, the seemingly theoretical advantage may be inconsequential due to tremendous computational cost involved. Unlike classical dispersive methods pioneered more than a half a century ago where the unit cell is quasi-static and provides effective mechanical and dispersive properties to the coarse-scale problem, the dispersive C^2$$C2 gives rise to transient problems at all scales and for all microphases involved. An efficient block time-integration scheme is proposed that takes advantage of the fact that the transient unit cell problems are not coupled to each other, but rather to a single coarse-scale finite element they are positioned in. We show that the computational cost of the method is comparable to the classical dispersive methods for short load durations.

Computer Methods in Applied Mechanics and Engineering, 2016

The two primary objectives of the present manuscript are: (i) to develop a variant of the computa... more The two primary objectives of the present manuscript are: (i) to develop a variant of the computational continua formulation (C 2) with outstanding dispersive properties, and (ii) to conduct a rigorous dispersion analysis of it. The ability of the C 2 formulation to capture dispersive behavior stems from its underlying formulation, which does not explicitly assume scale separation and accounts for microstructures of finite size. The dispersion study in heterogeneous elastic media with periodic microstructure has been conducted using both analytical and numerical approaches. The so-called analytical dispersion analysis is based on the Floquet-Bloch wave solution, while the numerical dispersion analysis is based on the modal analysis of the discrete coupled fine-coarse-scale equations. The dispersive curves obtained from the dispersive C 2 formulations were compared with the classical exact Floquet-Bloch wave solution, hereafter referred to as the reference dispersive curve. It has been observed that in the case of the unit cell sizes being either half of the coarse-scale element size or equal to it, the dispersive curves obtained by the dispersive C 2 formulation are practically identical to the reference solution. For other cases, the dispersive C 2 solution is in good agreement with the reference solution provided that the wavelength is resolved by at least two coarse-scale quadratic elements. The dispersion analysis results have been further verified by the wave propagation problem in a periodic heterogeneous medium with a wavelength comparable to the microstructural size.

International Journal for Numerical Methods in Engineering, 2014

In the recent paper, Fish and Kuznetsov introduced the so-called computational continua .C 2 / ap... more In the recent paper, Fish and Kuznetsov introduced the so-called computational continua .C 2 / approach, which is a variant of the higher order computational homogenization that does not require higher order continuity, introduces no new degrees of freedom, and is free of higher order boundary conditions. In a follow-up paper on reduced order computational continua, the C 2 formulation has been enhanced with a model reduction scheme based on construction of residual-free fields to yield a computationally efficient framework coined as RC 2. The original C 2 formulations were limited to rectangular and box elements. The primary objectives of the present manuscript is to revisit the original formulation in three respects: (i) consistent formulation of boundary conditions for unit cells subjected to higher order coarse scale fields, (ii) effective solution of the unit cell problem for lower order approximation of eigenstrains, and (iii) development of nonlocal quadrature schemes for general two-dimensional (quad and triangle) and three-dimensional (hexahedral and tetrahedral) elements.

European Journal of Mechanics - A/Solids, 2012

Generalized theories of continuum mechanics, such as gradient and nonlocal elasticity, have been ... more Generalized theories of continuum mechanics, such as gradient and nonlocal elasticity, have been widely used to account for the small scale effects on materials' behavior when dealing with structures at the micro-or nano-scale. It has been demonstrated that these enhanced theories provide better approximations that are closer to experimental observations than classical ones for problems in the field of fracture mechanics, dislocations, and wave propagation. The present work investigates the capability of one-dimensional elastic models-gradient, nonlocal and mixed-to predict the dispersive behavior of traveling waves, in comparison with the BorneKarman model of lattice dynamics. The linear theories adopted herein are limited to Mindlin's first (grade 2) and second (grade 3) strain gradient theories in elasticity with two and three intrinsic parameters and Eringen's nonlocal elasticity theory with one and two intrinsic parameters. Mixed models of nonlocal and gradient theories with up to three intrinsic parameters are also considered. More specifically, seven 1D models are considered: one grade 2 elastic bar with micro-inertia, one grade 3 elastic 1D model, three nonlocal elastic bars-two with Helmholtz operator, and one with bi-Helmholtz operator after Lazar et al. (2006), one mixed nonlocal/grade 2 elastic bar with Helmholtz operator, and the mixed nonlocal model after Challamel et al. (2009). Only three models, under specific assumptions for their intrinsic parameters, result in matching satisfactorily the dispersion curve of BorneKarman's atomic model. The rest suffer violation of their fundamental thermodynamic restrictions. This violation is naturally explained by further analyzing the mathematical structure of the obtained dispersion relations, via Padé approximants, whose coefficients are directly related to each model's intrinsic parameters.

Computational Continua for Heterogeneous Solids: Studies on Unstructured Finite Element Meshes an... more Computational Continua for Heterogeneous Solids: Studies on Unstructured Finite Element Meshes and on Wave Propagation Dimitrios Fafalis The computational continua (C2) framework, which is the focus of the present thesis, is a coarsescale continuum description coupled with an underlying fine-scale description of material heterogeneity of finite size. It is intended to account for a variation of the coarse-scale stresses (strains) over a unit cell (UC) domain. It was originally developed to overcome the theoretical and computational limitations of higher order homogenization models and generalized continuum theories, namely the need for higher order finite element continuity, additional degrees-of-freedom, and nonclassical boundary conditions. The key feature of the C2 is so-called nonlocal quadrature scheme (NLQS) defined over a computational continua domain consisting of a disjoint union of so-called computational unit cells (CUC). The CUCs, which are merely computational entities,...