45 research outputs found
A fully explicit fluid-structure interaction approach based on the PFEM
No full textThe efficient numerical simulation of fluid-structure interaction (FSI) problems is of growing interest in many engineering fields. In the present work, a staggered approach for the solution of the FSI problem is proposed. The fluid domain is discretized with an explicit Particle Finite Element Method (PFEM) while the solid domain with a standard finite element method. The weakly compressible formulation of fluid flow, originally proposed in for the PFEM, is here used for the fluid domain. The PFEM has shown its capability in simulation of free surface flows in many applications. Thanks to the Lagrangian formulation, the free surface is directly defined by the current position of the particles, while the governing equations are imposed like in standard FEM. When the mesh becomes too distorted, a fast remeshing algorithm is used to redefine the connectivities. SIMULIA AbaqusExplicit has been used for the solution of the structural domain.
The GC Domain Decomposition method is here used for the coupling: the problem is solved independently on each subdomain and then linked at the interface using a Lagrange multiplier technique. The proposed method allows for different time-steps in the two subdomains and for non-conforming meshes at the interfaces between the solid and fluid domains. Moreover, this approach guarantees an explicit coupling at the interfaces.
2D test-cases will be presented to validate the proposed coupling technique. The explicit scheme for both the fluid and solid subdomains, together with the explicit treatment of the coupling, makes this method appealing for applications in a variety of engineering problems with fast dynamics and/or a high degree of non-linearity
An explicit Lagrangian finite element method for free-surface weakly compressible flows
No full textIn the present work, an explicit finite element approach to the solution of the Lagrangian formulation of the Navier-Stokes equations for weakly compressible fluids or fluid-like materials is investigated. The introduction of a small amount of compressibility is shown to allow for the formulation of a fast and robust explicit solver based on a particle finite element method. Newtonian and Non-Newtonian Bingham laws are considered. A barotropic equation of state completes the model relating pressure and density fields. The approach has been validated through comparison with experimental tests and numerical simulations of free surface fluid problems involving water and waterâsoil mixtures
A fully explicit fluid-structure interaction approach based on PFEM and FEM
No full textThe numerical simulation of fluid-structure interaction (FSI) problems involving free-surfaces is of great interest in many engineering applications. Particle-based methods, like PFEM, are particularly suited for the analysis of free-surface flows and fluid-structure interaction with large displacements of the interface. In the current work, a staggered approach for the solution of the FSI problem is proposed. The fluid domain is discretized with an explicit Particle Finite Element Method (PFEM) while the solid domain with a standard Finite Element Method (FEM). The weakly compressible formulation of fluid flow, originally proposed in for the PFEM, allows for a fully explicit solution scheme. Thanks to the Lagrangian formulation, the free surface is directly defined by the current position of the particles, while the governing equations are imposed like in standard FEM. When the mesh becomes too distorted, a fast remeshing algorithm is used to redefine the connectivities. The structural domain is instead analyzed with a standard commercial explicit FEM (SIMULIA Abaqus\Explicit).
The coupling between the fluid and solid domains is treated with the GC Domain Decomposition approach. On each subdomain the problem is solved independently and then the two solutions are linked at the interface using a Lagrange multiplier technique. The proposed method allows for different time-steps in the two subdomains and for non-conforming meshes at the interfaces between the solid and fluid domains. Moreover, this approach allows for an explicit coupling, without iterations, between the two subdomains.
2D test-cases will be presented to validate the proposed coupling technique. The explicit scheme for both the fluid and solid subdomains, together with the explicit treatment of the coupling, makes this method appealing for applications in a variety of engineering problems with fast dynamics and/or a high degree of non-linearity
Effect of the combination of basic fibroblast growth factor and cysteine on corneal epithelial healing after photorefractive keratectomy in patients affected by myopia
No full textBackground: This study sought to evaluate the effect of basic fibroblast growth factor eye drops and cysteine oral supplements on corneal healing in patients treated with photorefractive keratectomy (PRK). Materials and Methods: One hundred and twenty patients treated bilaterally with PRK for myopia were enrolled at one of two eye centers (Clinica Santa Lucia, Bologna, Italy and Department of Ophthalmology, University of Magna Graecia, Catanzaro, Italy) and were treated at the former center. Sixty patients included in the study group (Group 1) were treated postoperatively with topical basic fibroblast growth factor plus oral L-cysteine supplements, whereas 60 subjects included in the control group (Group 2) received basic fibroblast growth factor eye drops. We recorded the rate of corneal re-epithelialization and patients were followed-up every 30 days for 6 months. Statistical analyses were performed on the collected data. Results: The eyes in Group 1 demonstrated complete re-epithelialization at Day 5, whereas the eyes in Group 2 achieved this status on Day 6. No side-effects were reported. Conclusions : Patients treated with basic fibroblast growth factor eye drops and L-cysteine oral supplements benefit from more rapid corneal re-epithelialization. In human eyes, this combination treatment appeared to be safe and effective in accelerating corneal surfacing after surgery. Financial Disclosure: No author has any financial or proprietary interest in any material or method used in this study. Trial Registration: Current Controlled Trials ISRCTN73824458. © 2005 - Indian Journal of Ophthalmology
Accelerated 15 mW pulsed-light crosslinking to treat progressive keratoconus: Two-year clinical results
No full textPurpose To assess the clinical and microstructural results of accelerated 15 mW pulsed-light corneal crosslinking (CXL) to treat progressive keratoconus. Setting Siena Crosslinking Center, Siena, Italy. Design Prospective case series. Methods After epithelium removal (with Epi-Clear) and 10 minutes stromal soaking with riboflavin 0.1% hydroxypropyl methylcellulose solution, all eyes had 15 mW/cm2pulsed-light epithelium-off accelerated CXL for 6 minutes of ultraviolet-A (UVA) irradiation (1 second on/1 second off), maintaining a total UVA exposure of 12 minutes at a fluence of 5.4 J/cm2. The 2-year follow-up examination included uncorrected (UDVA) and corrected (CDVA) distance visual acuities, Scheimpflug tomography, in vivo confocal microscopy (IVCM), and spectral-domain optical coherence tomography (SD-OCT). Results The study comprised 132 eyes of 96 patients (mean age 23.7 years ± 4.3 [SD]) with stage II keratoconus. The change in UDVA and CDVA was statistically significant, from 0.51 ± 0.106 logarithm of the minimum angle of resolution (logMAR) at baseline to 0.309 ± 0.074 logMAR (P =.0001) and 0.271 ± 0.144 logMAR at baseline to 0.135 ± 0.100 logMAR (P =.0023), respectively. Coma values measured by Scheimpflug analysis showed a statistically significant improvement beginning with the first postoperative month (P =.0004). The IVCM scans documented basal epithelial healing occurring 72 hours after treatment associated with the presence of subepithelial nerves. The SD-OCT scans performed in the central 6.0 mm of corneal diameter documented a demarcation line at a mean depth of 280 ± 32 μm. Conclusion The 15 mW/cm2pulsed-light epithelium-off accelerated CXL was effective and safe, stabilizing keratoconus progression through 2 years of follow-up
Reduced time of arrival on brain perfusion ct in a patient with recurrent cryptogenic stroke: an indirect sign of a patent foramen ovale.
No full textN/
Explicit Lagrangian simulation of landslide runouts
No full textLAUREA MAGISTRALELe frane sono uno dei fenomeni con capacità distruttive maggiori in natura, e costituiscono
un rischio potenzialmente molto elevato per l’incolumità dell’uomo e
dell’ambiente costruito. Tra i vari tipi di frane, una categoria di particolare interesse
per l’Ingegneria Strutturale è rappresentata da frane che impattano un bacino
idrico di origine naturale o artificiale, inducendo la creazione e propagazione di
grandi onde.
La rilevanza di questi eventi catastrofici ha dato un forte impulso alla ricerca
in questo campo, in modo da acquisire competenze per prevederne l’innesco e
l’intensità e valutare la pericolosità delle varie zone. In particolare, nella presente
tesi l’attenzione verrà concentrata su quei tipi di frane, come le colate detritiche,
il cui comportamento può essere avvicinato a quello di un fluido viscoso non lineare,
modellabile attraverso un legame di tipo Bingham.
Un Metodo agli Elementi Finiti Particellari Lagrangiano è stato sviluppato per
simulare lo sviluppo di frane sotto l’ipotesi di materiale comprimibile. Con tale
approccio si è ottenuto un risolutore esplicito efficiente, con equazioni disaccoppiate
e, quindi, senza necessità di affrontare la soluzione di sistemi linearizzati,
come nell’approccio incomprimibile, generalmente impiegato in questi casi.
L’algoritmo è stato validato attraverso la simulazione di alcuni casi test, per un
confronto con risultati sperimentali e provenienti da altri approcci numerici. Tale
confronto ha dato risultati molto soddisfacenti sia in termini di accuratezza nella
riproduzione dell’evoluzione del flusso, sia in termini di conservazione della massa,
che costituisce una delle problematiche più limitanti dell’approccio incomprimibile.
Successivamente, l’algoritmo è stato modificato al fine di poter trattare casi di interazione
tra flussi di materiali diversi. In particolare la modifica ha permesso che
i diversi flussi fossero trattati separatemente, introducendo allo stesso tempo un algoritmo
di contatto per evitare la compenetrazione tra essi. Il codice così ottenuto
permette quindi di simulare gli eventi franosi in bacini idrici che generano la formazione
di onde impulsive. L’accuratezza del metodo è stata verificata attraverso
il confronto con prove sperimentali svolte al Swiss Institute of Technology (ETH)
e con le simulazioni delle stesse prove, eseguite con altri approcci numerici.Infine è stata eseguita la simulazione dell’onda tsunami che si è generata a Lituya
Bay, dopo il celebre evento franoso nel 1958. I risultati sono stati nuovamente
confrontati con risultati sperimentali ottenuti da prove su di un modello in scala
e con simulazioni ottenute con altri approcci. Il modello ha dimostrato di essere
adatto a rappresentare i complessi fenomeni che avvengono nei casi di onde generate
da eventi franosi; da tutti i confronti infatti è emerso come i risultati siano
in accordo con quelli sperimentali e numerici.Landslides are one of the most destructive hazards in nature, representing a potentially
high risk for human life and built environment. Among them, a category
of particular interest for Structural Engineering is represented by landslides impacting
water bodies such as artificial or natural reservoirs, fiords or estuaries,
inducing the creation and propagation of large waves.
The catastrophic relevance of these events has stimulated intensive investigations
in order to be able to predict the occurrences and the magnitude of these events
and to develop site hazard evaluation. In particular, in this thesis the attention will
be focused on those landslides (e.g. debris flow) which can be described as a non
linear viscous fluid, reasonably modeled by a Bingham Law.
A Lagrangian Particle Finite Element Method has been developed to simulate
the landslide evolution, under the general assumption of compressible flow. This
framework has lead to an efficient explicit solution scheme with decoupled equations,
with no need for solving linearized system as in the incompressible approach,
which is usually employed in these cases. The solution scheme has been
validated through several test cases, where its results have been compared with
experimental tests and numerical simulation obtained with other approaches. In
all the considered test cases, the present method showed satisfactory results in
terms of accuracy both in the representation of the evolution of the flow and in
the mass conservation, which on the contrary constitutes a remarkable issue in the
incompressible flow approach.
Then, the solution scheme has been modified to deal with flows of different materials.
The main modification consists essentially in treating separately the two
material flows, introducing a contact algorithm at the interface to guarantee the
non-penetration condition.
The multi-material solution scheme allows to simulate the events of landslides impacting
with water reservoirs with the subsequent generation and propagation of
water waves. The accuracy of the method has been verified through comparison
with results obtained from experimental tests performed at the Swiss Institute of
Technology (ETH) and with other numerical approaches.
Finally, the simulation of the 1958 Lituya-Bay landslide-generated impulse wave has been performed, comparing the results against a scaled-down experiment and
other published numerical results. In both cases the proposed approach proves to
be suitable to capture the complex phenomena that occur during the whole process
of the landslide-generated waves. Moreover, comparisons with experimental and
numerical data show a good agreement for all the tested cases
A fully explicit Lagrangian Finite Element Method for highly nonlinear Fluid-Structure Interaction problems
No full textLa simulazione numerica di problemi di Interazione Fluido-Struttura (FSI) è un argomento di grande rilevanza a causa della vasta gamma di applicazioni in molti campi ingegneristici. In questa tesi, viene presentato un Metodo agli Elementi Finiti (FEM) esplicito e Lagrangiano per la simulazione di problemi di FSI. Una nuova versione esplicita e Lagrangiana del Metodo degli Elementi Finiti Particellari (PFEM) è impiegata per la modellazione del fluido.
Una caratteristica distintiva del metodo proposto è rappresentata dal fatto che il dominio solido è modellato utilizzando il solutore FEM esplicito del software commerciale SIMULIA Abaqus/Explicit di Dassault Systémes. Ciò consente di eseguire simulazioni con una descrizione avanzata del dominio strutturale, includendo legami costitutivi avanzati e interazioni di contatto. L'accoppiamento tra struttura e fluido viene eseguito tramite il motore di Co- Simulazione integrato nel software SIMULIA e si basa su una tecnica derivante dai metodi di decomposizione dei domini. Il metodo garantisce un accoppiamento forte e la stabilità del risolutore partizionato, mantenendo allo stesso tempo un sistema globale di equazioni esplicite completamente disaccoppiate. Inoltre, consente l'utilizzo di mesh non conformi e passi di integrazione temporale differenti nei due sottodomini fluido e solido. La natura esplicita del risolutore accoppiato è vantaggiosa per problemi ingegneristici reali caratterizzati da una dinamica veloce o da un elevato grado di non linearità. La descrizione pienamente Lagrangiana è particolarmente efficace nella simulazione di problemi di FSI che coinvolgono flussi a superficie libera e grandi spostamenti strutturali, poiché i contorni del fluido sono automaticamente definiti dalla posizione dei nodi della mesh, senza necessità di algoritmi di tracciamento dell'interfaccia.
Viene inoltre proposta una tecnica innovativa per semplificare l'imposizione all’interno di metodi Lagrangiani di condizioni al contorno non omogenee di interesse pratico in diverse applicazioni ingegneristiche, ad esempio le condizioni di inflow/outflow, lo slip del fluido alle pareti di contorno o le superfici di simmetria. Il metodo si basa su una descrizione mista Lagrangiana-Euleriana, che introduce nodi Euleriani fissi solo sui bordi dove devono essere applicate condizioni non omogenee, portando a un'implementazione semplice e computazionalmente conveniente.
Viene infine proposto un nuovo ed efficiente algoritmo di regolarizzazione della mesh applicabile runtime durante le simulazioni con PFEM lagrangiano ed esplicito. La stabilità condizionata degli schemi di integrazione temporale espliciti richiede l'uso di piccoli passi di avanzamento temporale, proporzionali alla dimensione dell'elemento più distorto della mesh. D'altra parte, nelle applicazioni 3D, la tassellazione di Delaunay impiegata nel PFEM perde alcune delle sue proprietà ottimali possedute nelle applicazioni 2D, così che nella triangolazione vengono frequentemente aggiunti tetraedri di forma irregolare. Ciò causa dimensioni estremamente ridotte dei passi temporali stabili che comportano tempi computazionali inaccettabili per le analisi di PFEM esplicito 3D. La nuova tecnica di regolarizzazione della mesh proposta è in grado di correggere gli elementi eccessivamente distorti ad un costo computazionale accettabile, in modo da poter essere applicata runtime nel framework di frequente remeshing del PFEM. Più in generale, potrebbe essere convenientemente applicata anche in altri metodi Lagrangiani per la regolarizzazione della mesh e il miglioramento della qualità della soluzione. Ciò è ottenuto sfruttando un'analogia elastica che consente lo sfruttamento della stessa architettura esplicita e parallelizzabile del risolutore di fluidi.
Dopo un'approfondita validazione dell'approccio FSI PFEM-FEM attraverso il confronto con risultati analitici, sperimentali e numerici presentati in letteratura, viene affrontata un’applicazione di ingegneria reale rappresentata dalla simulazione del dispiegamento degli airbag negli autoveicoli. In tale applicazione, si osservano le elevate potenzialità di questo approccio pienamente esplicito e Lagrangiano in questa classe di complessi problemi industriali.The numerical simulation of Fluid-Structure Interaction (FSI) problems is a topic of great relevance because of the wide range of applications in many engineering fields. In this thesis, a partitioned fully explicit and fully Lagrangian Finite Element Method (FEM) for FSI problems is presented. A novel explicit version of the Lagrangian Particle Finite Element Method (PFEM) is employed for the fluid modelling.
A distinctive feature of the proposed FSI strategy is that the solid domain is modelled using the explicit FEM of the commercial software SIMULIA Abaqus/Explicit from Dassault Systémes. This allows to perform simulations with an advanced description on the structural domain, including advanced structural material models and contact interactions. The structure-to-fluid coupling is performed through the SIMULIA built-in Co-Simulation engine and it is based on a technique derived by the Domain Decomposition methods. The method ensures strong coupling and stability of the partitioned solver, retaining at the same time an overall system of fully decoupled explicit equations. Moreover, it allows for the use of different time integration steps and nonconforming meshes in the two subdomains. The fully explicit nature of the coupled solver is appealing
for large-scale engineering problems characterized by fast dynamics or high degree of non-linearity. The fully Lagrangian description is particularly effective in the simulation of FSI problems with free surface flows and large structural displacements, since the fluid boundaries are automatically defined by the position of the mesh nodes with no need for interface tracking
algorithms.
A novel technique is proposed to simplify the imposition in Lagrangian methods of non- homogeneous boundary conditions which are of practical interest in several engineering applications, e.g., inflow/outflow conditions, fluid slip at boundary walls and symmetry surfaces. The method is based on a mixed Lagrangian-Eulerian description, which introduces fixed Eulerian
nodes only on the boundaries where non homogeneous conditions have to be applied, leading to a simple and computationally convenient implementation.
A novel efficient runtime mesh smoothing algorithm for explicit Lagrangian PFEM simulations is proposed. The conditional stability of explicit time integration schemes requires the use of small time increments, proportional to the size of the element in the mesh with the worst geometrical
quality. On the other hand, in the 3D framework the Delaunay tessellation employed in the PFEM loses some of its optimality properties holding in 2D, so that badly shaped tetrahedra are frequently added in the triangulation. This leads to unacceptably small stable time step size for explicit
solvers. The novel mesh smoothing technique is able to correct overly distorted elements at an acceptable computational cost, so that it can be applied runtime in the frequent remeshing framework of the PFEM. More in general, it could be conveniently applied to regularize the mesh and improve the solution of other Lagrangian methods. This is achieved exploiting an elastic analogy that allows for the use of the same explicit and parallelizable architecture of the fluid solver.
After an extensive validation of the proposed PFEM-FEM FSI approach against analytical, experimental and numerical results presented in the literature, the real engineering application of the automotive airbag deployment is addressed, showing the great potentialities of the fully explicit and Lagrangian approach in this class of challenging industrial problems.DIPARTIMENTO DI INGEGNERIA CIVILE E AMBIENTALE31DI PRISCO, CLAUDIO GIULIOPEREGO, UMBERT
Multi-stakeholder Participation in Disaster Recovery: A Case Study
No full textAbstractEver since the Indian Ocean Tsunami in 2004 and a series of disasters that followed in the last decade, triggered the nations around the world to focus on emergency relief operations. A well prepared and coordinated relief operation is required to reduce the vulnerability of the affected and bring back normalcy in the minimum possible time. The present article discusses the relief and recovery operation during the Hudhud cyclone that hit the coast of Bay of Bengal in October, 2014. The paper brings together the information collected through focus groups, personal interviews with personnel involved in the relief operations when the disaster hit the city. The article describes the different activities carried out at the different phases of disaster management process and concluding the different actions taken by the state to reduce the impact of such disasters in the future. Also, key learnings and focus areas are listed for reference in future disasters
In vivo confocal microscopy: qualitative investigation of the conjunctival and corneal surface in open angle glaucomatous patients undergoing the XEN-Gel implant, trabeculectomy or medical therapy
No full textPurpose Assessing the quality of the ocular surface by in vivo scanning laser confocal microscopy (IVCM) in primary open angle glaucoma (POAG) patients treated by Xen 45 Gel Stent, medical therapy and trabeculectomy. Methods Retrospective, single-center, single-masked, comparative study including 60 eyes of 30 patients (mean age 61.16 +/- 10 years) affected by POAG. Eyes were divided into 3 groups: Group 1 eyes underwent the Xen 45 Gel Stent procedure, Group 2 eyes were under medical therapy, Group 3 eyes were surgically treated by trabeculectomy. All patients underwent HRT II IVCM analysis of cornea, limbus, conjunctiva, sub-tenionian space and sclera. Results The Xen 45 Gel stent, if properly positioned in the sub-conjunctival space preserves goblet cells and limits ocular surface inflammation. Regular corneal epithelial cells with micro-cysts, and normo-reflective sub-epithelial nerve plexus are documented by IVCM. In sub Tenon's implants an alternative lamellar intra-scleral filtration is detectable. Combined surgical procedures show a noticeable number of inflammatory cells with rare micro-cysts. Post-trabeculectomy inflammatory reaction is more evident than Xen 45 Gel Stent associated surgical procedures, but less than medical therapy where a conspicuous presence of Langerhans cells, peri-neural infiltrates, marked loss of goblet cells and fibrosis is visible. Conclusion Ocular surface inflammation was more notable in topical therapy than after trabeculectomy, which itself causes more inflammation than XEN Gel stents
