A computational neuromuscular model of the human upper airway with application to the study of obstructive sleep apnoea

Includes bibliographical references.Numerous challenges are faced in investigations aimed at developing a better understanding of the pathophysiology of obstructive sleep apnoea. The anatomy of the tongue and other upper airway tissues, and the ability to model their behaviour, is central to such investigations. In this thesis, details of the construction and development of a three-dimensional finite element model of soft tissues of the human upper airway, as well as a simplified fluid model of the airway, are provided. The anatomical data was obtained from the Visible Human Project, and its underlying micro-histological data describing tongue musculature were also extracted from the same source and incorporated into the model. An overview of the mathematical models used to describe tissue behaviour, both at a macro- and microscopic level, is given. Hyperelastic constitutive models were used to describe the material behaviour, and material incompressibility was accounted for. An active Hill three-element muscle model was used to represent the muscular tissue of the tongue. The neural stimulus for each muscle group to a priori unknown external forces was determined through the use of a genetic algorithm-based neural control model. The fundamental behaviour of the tongue under gravitational and breathing-induced loading is investigated. The response of the various muscles of the tongue to the complex loading developed during breathing is determined, with a particular focus being placed to that of the genioglossus. It is demonstrated that, when a time-dependent loading is applied to the tongue, the neural model is able to control the position of the tongue and produce a physiologically realistic response for the genioglossus. A comparison is then made to the response determined under quasi-static conditions using the pressure distribution extracted from computational fluid-dynamics results. An analytical model describing the time-dependent response of the components of the tongue musculature most active during oral breathing is developed and validated. It is then modified to simulate the activity of the tongue during sleep and under conditions relating to various possible neural and physiological pathologies. The retroglossal movement of the tongue resulting from the pathologies is quantified and their role in the potential to induce airway collapse is discussed

A CFD investigation of cavitation and associated deposit formation in modern diesel fuel injectors

Includes bibliographical references (leaves 78-81).Reducing the pollution of new vehicles has become a priority to vehicle manufacturers, particularly given the fact that emissions requirements that must be achieved by diesel vehicles are becoming more stringent. Modem fuel injectors on common-rail diesel vehicles use very high rail pressures to aid atomisation and increase combustion efficiency. However, associated with the high injections pressures is the issue of nozzle cavitation. Cavitation leads to pockets of diesel vapour forming in the nozzle and it is hypothesised that this causes the formation of deposits in the nozzle. It is also suggested that the collapse of the cavitation vapour space results in extremely high temperatures within the nozzle, resulting in thermal cracking of the fuel and eventually the formation of carbon deposits. A two-dimensional axisymmetric CFD model with dimensions representative of an injector nozzle was constructed using a fully structured grid

I’m stuck! How to efficiently debug computational solid mechanics models so you can enjoy the beauty of simulations

A substantial fraction of the time that computational modellers dedicate to developing their models is actually spent trouble-shooting and debugging their code. However, how this process unfolds is seldom spoken about, maybe because it is hard to articulate as it relies mostly on the mental catalogues we have built with the experience of past failures. To help newcomers to the field of material modelling, here we attempt to fill this gap and provide a perspective on how to identify and fix mistakes in computational solid mechanics models. To this aim, we describe the components that make up such a model and then identify possible sources of errors. In practice, finding mistakes is often better done by considering the symptoms of what is going wrong. As a consequence, we provide strategies to narrow down where in the model the problem may be, based on observation and a catalogue of frequent causes of observed errors. In a final section, we also discuss how one-time bug-free models can be kept bug-free in view of the fact that computational models are typically under continual development. We hope that this collection of approaches and suggestions serves as a “road map” to find and fix mistakes in computational models, and more importantly, keep the problems solved so that modellers can enjoy the beauty of material modelling and simulation.EC and JPP wish to thank their former supervisor Paul Steinmann for the inspiration to write this paper, which can be traced back to the talk we prepared for the ECCM-ECFD conference held in Glasgow in 2018. EC’s work was partially supported by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 841047. WB’s work was partially supported by the National Science Foundation under award OAC-1835673; by award DMS-1821210; by award EAR-1925595; and by the Computational Infrastructure in Geodynamics initiative (CIG), through the National Science Foundation under Award EAR-1550901 and The University of California – Davis .Peer ReviewedPostprint (published version

The deal.II Library, Version 8.5

This paper provides an overview of the new features of the finite element library deal.II version 8.5

The deal.II Library, Version 9.0

This paper provides an overview of the new features of the finite element library deal.II version 9.0

The deal.II code gallery: Quasi-Static Finite-Strain Quasi-incompressible Visco-elasticity

Quasi-static finite-strain quasi-incompressible rate-dependent elasticity computing the displacement history of a thin viscoelastic strip with a hole

Application of metaheuristic algorithms to the identification of nonlinear magneto-viscoelastic constitutive parameters

Metaheuristic algorithms offer a robust and convenient method to solve highly nonlinear optimisation problems in engineering applications. In this work we evaluate the effectiveness of a collection of canonical algorithms at performing parameter identification for nonlinear constitutive laws that describe coupled, magnetic-field responsive materials. To achieve this, we define an objective function that captures the influence of many physical measurements recorded during experimental analysis of a coupled material, and incorporates the influence of experimental uncertainty. A benchmark of the algorithms is conducted through the evaluation of a magneto-visco-elastic material by means of numerically-derived parallel-plate rotational rheometry. The effectiveness of each algorithm at matching the fictitious, but representative, experimental data was considered using two different metrics. In addition to the ranking based on a non-parametric statistical test, we consider an ad hoc criterion that accounts for only the top performing candidate solutions. It is determined that the continuous real and discrete bitstring genetic algorithm provide the best overall performance in terms of the accuracy of the predicted parameters, while globally-elitist simulated annealing provides the best compromise between accuracy and computational efficiency. When experimental uncertainties exist (which is always the case for data determined within a laboratory setting), it has been observed that the strong link between constitutive parameters and physical material properties, which is typically assumed, no longer holds

The deal.II tutorial step-44: Three-field formulation for non-linear solid mechanics

A three-field formulation for quasi-static finite-strain quasi-incompressible elasticity solving the indentation problem considering a near incompressible Neo-Hookean material

Numerical modelling of nonlinear thermo-electro-elasticity

This work presents the numerical modelling of nonlinear thermo-electro-elasticity in the context of electro-active polymers (EAPs). EAPs are characterised by their electro-mechanical coupling behaviour that converts electrical into mechanical energy. As polymeric materials in general are sensitive to the influence of temperature, thermal effects play an important role in the material behaviour of EAPs. Based on a thermo-electro-mechanically coupled constitutive framework presented in an earlier contribution, a variational formulation is developed and the finite-element method is employed to solve the nonlinear thermo-electro-mechanical coupling problem. The numerical implementation is studied by means of several examples