Quantum gravity and the renormalisation group: theoretical advances and applications

It is well known that quantisation of gravity within the conventional framework of quantum field theory faces challenges. An intriguing novel prospect was put forward by S. Weinberg in 1979 who suggested that the metric degrees of freedom of gravity could be quantised nonpertubatively provided that the theory becomes asymptotically safe (AS) at high energies. In this thesis we put forward a systematic search strategy to test the AS conjecture in four dimensional quantum gravity. Using modern renormalisation group (RG) methods and heat kernel techniques we derive the RG equations for gravitational actions that are formed from powers of the Ricci scalar and powers of the Ricci tensor. The non-linear fixed point equations are solved iteratively and exactly. We develop a sophisticated algorithm to express the fixed point iteratively, and to high order, in terms of its lower order couplings. We also evaluate universal scaling exponents and find that the relevancy of invariants at an asymptotically safe fixed point is governed by their classical mass dimension, providing structural support for the asymptotic safety conjecture. We also apply our findings to the physics of higher dimensional black holes. Most notably, we find that the seminal ultra-spinning Myers-Perry black holes cease to exist as soon as asymptotically safe RG corrections are taken into account. Further results and implications of our findings are discussed

Detection of Tectonic and Crustal Deformation using GNSS Data Processing: The Case of PPGnet

Aitolo-Akarnania prefecture, western Greece, is an area with strong earthquakes and large active fault systems. The most prominent are the Katouna sinistral strike slip fault and the Trichonis Lake normal fault system. Their proximity to large cities, and the lack of detailed information on their seismogenic potential, calls for multiparametric research. Since 2013, the area’s crustal deformation has been monitored by a dense GNSS Network (PPGNet), consisting of five stations, equipped with Leica and Septentrio receivers. The objective of this network is to define the rate of deformation across these two main fault systems. Data is recorded using two sampling frequencies, 1 Hz and 10Hz, producing hourly and daily files. Daily data is processed using Bernese GNSS Processing Software using final orbits of International GNSS Service. Double-difference solution is computed using phase measurements from the PPGNet network complemented by four stations from Athens’ National Observatory GNSS network and six stations from METRICA network. First results show a NNE movement at PVOG station of 12 mm/y and a similar movement at RETS station of about 9 mm/y. This means that the Trichonis Lake normal fault system, located between these two stations, depicts a slip rate of 3 mm/y. KTCH and RGNI stations move eastwards at a velocity of about 5 mm/y due to the Katouna-Stamna fault system. Data from PPGNet has provided important results on crustal deformation in the area, i.e. slip rates have been attributed to specific fault systems. The comparison and links of these data with broader geodynamic models is now possible and we expect, in a later phase that will provide a more detailed image of the associated seismic hazard for Aitolo-Akarnania. Doi: 10.28991/cej-2021-03091633 Full Text: PD

Thermal-hydrodynamic behaviour of coated pivoted pad thrust bearings: Comparison between Babbitt, PTFE and DLC

The hydrodynamic lubrication and thermal analysis of tilting pad thrust bearings has been a major subject for many studies in the field of tribology. There is only a limited number of studies regarding thrust bearings with coated surfaces. The purpose of this study is to build a parametric, iterative algorithm in order to perform a complete thermal and hydrodynamic lubrication analysis for pivoted pad thrust bearings with coatings. The analytical model is mainly based on the energy, continuity and Navier-Stokes equations, which are solved numerically with the Semi-Implicit Method for Pressure Linked Equations Consistent (SIMPLEC) method. The analysis focuses on a single pivoted pad of the thrust bearing. The thermal properties of the coating material are taken into account and the resulting thermal and flow fields are solved. The basic hydrodynamic and tribological characteristics are calculated for an uncoated, a Babbitt coated, a PTFE coated and a diamond like carbon (DLC) coated pivoted pad thrust bearing. The pressure and the film thickness distribution, as well as the load capacity and the frictional forces, are determined for several pad positions and velocities of the rotor. A mineral oil lubricant is used to estimate the shear thinning or thickening effects on the pad tribological performance. The results indicate that pads coated with PTFE and DLC show lower friction forces compared to the common steel and Babbitt applications. At the same time, the DLC coating seems to affect the bearing's flow and thermal fields less than the PTFE, making it more suitable for thrust bearings applications

Ruptured abdominal aortic aneurysm: Endovascular treatment

Abstract Aim The elective endovascular treatment of abdominal aortic aneurysm (AAA) is nowadays a daily routine practice in selected patients. The traditional treatment of ruptured abdominal aortic aneurysm (rAAA) has a peri-operative mortality of 40–50% that has not changed in the last 20 years. Nowadays, the endovascular repair may reduce mortality, hospitalization and sanitary costs. Methods The study included 14 patients affected by AAA who came to the Emergency Department because of hemodynamic shock (nine patients) or back pain (five patients). All patients underwent a CT angiography before surgery. Forty-two percent of the patients presented with shock (systolic pressure ≤70 mm Hg) in the operating room, and they underwent an endovascular aortic repair (EVAR) as an emergency procedure. Five bifurcated endoprotesis and nine uniliac protesis making a femoro-femoral bypass to revascularize the excluded limb were made. Patients underwent a follow up with CT angiography one month and then six months after surgery and if no problems were detected, patients underwent a follow-up every year. Results Two cases were immediately converted to open surgery because of failed EVAR. Four patients (28%) died after surgery because of multi-organ failure (MOF). The mean hospitalization was 12 days (range 3–21 days). We observed only one case of first-type endoleak at the 1-month follow up and we successfully treated it with a proximal cuff. Conclusion In our experience, the intention-to-treat protocol for rAAA offered acceptable results in terms of mortality rates. Multicenter studies are necessary to establish the role of endovascular treatment in patients with rAAA

Autonomous inspection and repair of aircraft composite structures

This paper deals with the development of an innovative approach for inspection and repair of damage in aeronautical composites that took place in the first two years of the H2020 CompInnova project which. The aim is a newly designed robotic platform for autonomous inspection using combined infrared thermography (IRT) and phased array (PA) non-destructive investigation for damage detection and characterization, while integrated with laser repair capabilities. This will affect the increasing societal need for safer aircraft in the lowest possible cost, while new and effective techniques of inspection are needed because of the rapidly expanding use of composites in the aerospace industry

Applicability and added value of novel methods to improve drug development in rare diseases

The ASTERIX project developed a number of novel methods suited to study small populations. The objective of this exercise was to evaluate the applicability and added value of novel methods to improve drug development in small populations, using real world drug development programmes as reported in European Public Assessment Reports. The applicability and added value of thirteen novel methods developed within ASTERIX were evaluated using data from 26 European Public Assessment Reports (EPARs) for orphan medicinal products, representative of rare medical conditions as predefined through six clusters. The novel methods included were 'innovative trial designs' (six methods), 'level of evidence' (one method), 'study endpoints and statistical analysis' (four methods), and 'meta-analysis' (two methods) and they were selected from the methods developed within ASTERIX based on their novelty; methods that discussed already available and applied strategies were not included for the purpose of this validation exercise. Pre-requisites for application in a study were systematized for each method, and for each main study in the selected EPARs it was assessed if all pre-requisites were met. This direct applicability using the actual study design was firstly assessed. Secondary, applicability and added value were explored allowing changes to study objectives and design, but without deviating from the context of the drug development plan. We evaluated whether differences in applicability and added value could be observed between the six predefined condition clusters. Direct applicability of novel methods appeared to be limited to specific selected cases. The applicability and added value of novel methods increased substantially when changes to the study setting within the context of drug development were allowed. In this setting, novel methods for extrapolation, sample size re-assessment, multi-armed trials, optimal sequential design for small sample sizes, Bayesian sample size re-estimation, dynamic borrowing through power priors and fall-back tests for co-primary endpoints showed most promise - applicable in more than 40% of evaluated EPARs in all clusters. Most of the novel methods were applicable to conditions in the cluster of chronic and progressive conditions, involving multiple systems/organs. Relatively fewer methods were applicable to acute conditions with single episodes. For the chronic clusters, Goal Attainment Scaling was found to be particularly applicable as opposed to other (non-chronic) clusters. Novel methods as developed in ASTERIX can improve drug development programs. Achieving optimal added value of these novel methods often requires consideration of the entire drug development program, rather than reconsideration of methods for a specific trial. The novel methods tested were mostly applicable in chronic conditions, and acute conditions with recurrent episodes. The online version of this article (10.1186/s13023-018-0925-0) contains supplementary material, which is available to authorized users

Quantum gravity effects in Myers-Perry space-times

We study quantum gravity effects for Myers-Perry black holes assuming that the leading contributions arise from the renormalization group evolution of Newton's coupling. Provided that gravity weakens following the asymptotic safety conjecture, we find that quantum effects lift a degeneracy of higher-dimensional black holes, and dominate over kinematical ones induced by rotation, particularly for small black hole mass, large angular momentum, and higher space-time dimensionality. Quantum-corrected space-times display inner and outer horizons, and show the existence of a black hole of smallest mass in any dimension. Ultra-spinning solutions no longer persist. Thermodynamic properties including temperature, specific heat, the Komar integrals, and aspects of black hole mechanics are studied as well. Observing a softening of the ring singularity, we also discuss the validity of classical energy conditions

Numerical modelling of high strength structures under severe loadings

Le projet dans lequel s’inscrit ce travail de thèse porte sur la détermination de la durabilité des structures métalliques de grandes dimensions lorsqu’elles sont soumises à des surcharges accidentelles (collision, explosion, impact d’oiseau, etc). À cette fin a été développée une méthodologie numérique unifiée tri-dimensionnelle capable de reproduire qualitativement et quantitativement les étapes successives de dégradation menant à la rupture ultime des structures constituées de métaux et alliages ductiles (aciers, alliages d’aluminium et de titane, etc.). Ces étapes comprennent généralement (i) l’endommagement induit par la croissance de microcavités, (ii) la localisation de la déformation /endommagement dans une bande étroite, et (iii) la formation et la propagation de macro-fissures. Dans la première étape (i), le matériau est supposé obéir au modèle de plasticité microporeuse de Gurson en utilisant la méthode standard des éléments finis (FEM), tandis que dans la troisième étape (iii) les conséquences cinématiques de l’ouverture de la fissure sont décrites par la méthode des éléments finis étendus (XFEM). L’étape intermédiaire (ii) de localisation, qui est probablement la plus complexe à reproduire d’un point de vue physique et numérique, est décrite via un modèle de zone cohésive (CZM) couplé à la XFEM. Le CZM décrit ladégradation progressive des propriétés mécaniques du matériau de la bande de localisation induite par une augmentation de la coalescence des microcavités et conduisant finalement à l’ouverture de la fissure. Un intérêt particulier est porté aux critères de transition entre endommagement diffus et localisation et à la détermination de l’orientation du plan de localisation. La localisation est traitée ici comme un phénomène résultant soit d’une instabilité plastique soit de la coalescence des microcavités. En conséquence, deux critères sont considérés : le premier est basé sur l’analyse de bifurcation, tandis que le second repose sur une porosité critique et rend compte de la compétition entre Mode I et Mode II fonction du taux de triaxialité local. Ainsi, la bifurcation ‘flat-to-slant’ du plan de rupture (l’effet ‘cup and cone’ lors de la traction d’une éprouvette axisymétrique) est correctement reproduite numériquement. En outre, des critères opérationnels d’amorçage et de propagation de la bande de localisation en trois dimensions sont proposés. Un autre aspect important de la méthodologie est le nouveau schéma utilisé pour l’intégration numérique des équations d’équilibre, à savoir l’intégration par moyennage sur les volumes (VAI). La méthodologie est implémentée en tant que routine utilisateur (UEL) dans le code de calcul commercial par éléments finis ABAQUS et ses performances sont évaluées par des simulations numériques tridimensionnelles de divers cas de chargement. Une étude paramétrique de l’effet des constantes du modèle est réalisée, montrant que la méthodepeut reproduire une grande variété de réponses dans le régime adoucissant. La méthodologie proposée se révèle être indépendante du maillage et capable de reproduire correctement les surfaces de rupture. Elle donne également des résultats prometteurs concernant les réponses globales des structures étudiées par rapport aux données expérimentales.The project in which this PhD work is carried out concerns the determination of the durability of large metal structures when they are subjected to accidental overloads (collision, explosion, bird strike, etc.). To this end has been developed a three-dimensional unified numerical methodology capable of reproducing qualitatively and quantitatively the successive stages of degradation leading to the ultimate rupture of the structures made ofductile metals and alloys (steels, aluminum and titanium alloys, etc.). These steps typically include (i) damage induced by microcavity growth, (ii) localization of deformation/damage in a narrow band, and (iii) formation and propagation of macro cracks. In the first step (i), the material is assumed to conform to Gurson’s microporous plasticity model using the standard finite element method (FEM), while in the third step (iii) the kinematic consequences of crack opening are described by the Extended Finite Element (XFEM) method. The intermediate stage (ii) of localization, which is probably the most complex to reproduce from a physical and numerical point of view, is described via a cohesive zone model (CZM) coupled to the XFEM. The CZM describes the progressive degradation of the mechanical properties of the localization band material induced by an increase in the coalescence of the microcavities and ultimately leading to the crack opening. A particular interest has been given to the criteria of transition between diffuse damage and localization and the determination of the orientation of the localization plan. Localization is treated here as a phenomenon resulting either from plastic instability or from the coalescence of microcavities. As a result, two criteria are considered: the first is based on bifurcation analysis, while the second is based on a critical porosity and reflects the competition between Mode I and Mode II as a function of the local triaxiality ratio. Thus, the flat-to-slant bifurcation of the fracture plane (the ’cup and cone effect’ when tension loading an axisymmetric specimen) is correctly reproduced numerically. In addition, operational criteria for the initiation and propagation of the three-dimensional localization band are proposed. Another important aspect of the methodology is the newscheme used for numerical integration of equilibrium equations, namely volume averaging integration (VAI). The methodology is implemented as a user subroutine (UEL) in the ABAQUS commercial Finite Element computation code and its performance is evaluated by threedimensional numerical simulations involving various loading cases. A parametric study of the effect of the model constants is performed, showing that the method can reproduce awide variety of responses in the softening regime. The proposed methodology proves to be mesh independent and capable of correctly reproducing the fracture surfaces. It also gives promising results concerning the overall responses of the structures studied compared to the experimental data