Dual virtual element method for discrete fractures networks

Discrete fracture networks is a key ingredient in the simulation of physical processes which involve fluid flow in the underground, when the surrounding rock matrix is considered impervious. In this paper we present two different models to compute the pressure field and Darcy velocity in the system. The first allows a normal flow out of a fracture at the intersections, while the second grants also a tangential flow along the intersections. For the numerical discretization, we use the mixed virtual finite element method as it is known to handle grid elements of, almost, any arbitrary shape. The flexibility of the discretization allows us to loosen the requirements on grid construction, and thus significantly simplify the flow discretization compared to traditional discrete fracture network models. A coarsening algorithm, from the algebraic multigrid literature, is also considered to further speed up the computation. The performance of the method is validated by numerical experiments

Laboratory tests in out-of-hours services in Norway. Studies with special emphasis on use and consequences of C-reactive protein test in children

Background: Children with infections and respiratory symptoms have the highest contact rate with Norwegian out-of-hours (OOH) services, especially in the youngest age group, and during the winter season. Many contacts are non-urgent in a strict medical sense. Onsite measurement of C-reactive protein (CRP) is the most frequent laboratory test in Norwegian OOH, used in 60% of all contacts with children with infections and respiratory diseases. The aim of CRP is to differentiate between bacterial infections, viral and/or non-serious infections and to keep the use of antibiotics as low as possible. Nevertheless, the use of antibiotics has increased since the test was introduced as a point of care test until 2013. Several studies have investigated the diagnostic value of laboratory tests for children with fever, but not in primary health care where the prevalence of serious bacterial infections is low. Objective: Investigate the use of laboratory tests at Norwegian out-of-hours services and which characteristics of the doctor, patient, diagnoses and geography that affects it. Compare the RGPs’ rate of CRP use at daytime and at OOH in consultations with children. Evaluate if pre-consultation CRP screening affects the choice of treatment Identify predictors for prescription of antibiotics and referral to hospital for children at OOH services. Design/method: The first and second papers are from two cross-sectional registry based studies, based on electronic compensation claims from consultations in primary care. The third study (third paper) was a randomized controlled observational study at OOH-services in Norway, including children 20 mg/L, signs on ear examination and use of paracetamol during the previous 24 hours. A high respiratory rate, low oxygen saturation and signs of auscultation were predictors for referral to hospital. In addition, parents’ assessment of the seriousness was significantly associated with prescription of antibiotics and referral to hospital. Conclusion: Paper I and II: CRP is extensively used at Norwegian OOH services and the differences in use cannot be explained by different diagnoses. Paper III: CRP screening of children with fever or respiratory symptoms will not reduce the prescription of antibiotics. Paper IV: Predictors for prescription are signs on ear examination, slightly elevated CRP values and the parents’ assessment. Disturbed respiration is the most important sign predicting hospital admission

High-accuracy phase-field models for brittle fracture based on a new family of degradation functions

Phase-field approaches to fracture based on energy minimization principles have been rapidly gaining popularity in recent years, and are particularly well-suited for simulating crack initiation and growth in complex fracture networks. In the phase-field framework, the surface energy associated with crack formation is calculated by evaluating a functional defined in terms of a scalar order parameter and its gradients, which in turn describe the fractures in a diffuse sense following a prescribed regularization length scale. Imposing stationarity of the total energy leads to a coupled system of partial differential equations, one enforcing stress equilibrium and another governing phase-field evolution. The two equations are coupled through an energy degradation function that models the loss of stiffness in the bulk material as it undergoes damage. In the present work, we introduce a new parametric family of degradation functions aimed at increasing the accuracy of phase-field models in predicting critical loads associated with crack nucleation as well as the propagation of existing fractures. An additional goal is the preservation of linear elastic response in the bulk material prior to fracture. Through the analysis of several numerical examples, we demonstrate the superiority of the proposed family of functions to the classical quadratic degradation function that is used most often in the literature.Comment: 33 pages, 30 figure

Numerical Treatment of State-Dependent Permeability in Multiphysics Problems

Constitutive laws relating fluid potentials and fluxes in a nonlinear manner are common in several porous media applications, including biological and reactive flows, poromechanics, and fracture deformation. Compared to the standard, linear Darcy's law, such enhanced flux relations increase both the degree of nonlinearity, and, in the case of multiphysics simulations, coupling strength between processes. While incorporating the nonlinearities into simulation models is thus paramount for computational efficiency, correct linearization, as is needed for incorporation in Newton's method, is challenging from a practical perspective. The standard approach is therefore to ignore nonlinearities in the permeability during linearization. For finite volume methods, which are popular in porous media applications, complete linearization is feasible only for the simplest flux discretization, namely the two-point flux approximation. We introduce an approximated linearization scheme for finite volume methods that is exact for the two-point scheme and can be applied to more advanced and accurate discretizations, exemplified herein by a multi-point flux stencil. We test the new method for both nonlinear porous media flow and several multiphysics simulations. Our results show that the new linearization consistently outperforms the standard approach. Moreover our scheme achieves asymptotic second order convergence of the Newton iterations, in contrast to the linear convergence obtained with the standard approach.publishedVersio

A fully coupled numerical model of thermo-hydro-mechanical processes and fracture contact mechanics in porous media

A range of phenomena in the subsurface is characterised by the interplay between coupled thermal, hydraulic and mechanical processes and deforming structures such as fractures. Modelling subsurface dynamics can provide valuable phenomenological understanding, but requires models which faithfully represent the dynamics involved; these models, therefore are themselves highly complex. This paper presents a mixed-dimensional thermo-hydro-mechanical model designed to capture the process-structure interplay using a discrete-fracture-matrix framework. It incorporates tightly coupled thermo-hydro-mechanical processes based on laws for momentum, mass and entropy in subdomains representing the matrix and the lower-dimensional fractures and fracture intersections. The deformation of explicitly represented fractures is modelled by contact mechanics relations and a Coulomb friction law, with particular attention on coupling of fracture dilation to the governing equations in both fractures and matrix. The model is discretised using multi-point finite volumes for the balance equations and a semismooth Newton scheme for the contact conditions and is implemented in the open source fracture simulation toolbox PorePy. Finally, simulation studies demonstrate the model's convergence, investigate process-structure coupling effects, explore different fracture dilation models and show an application of the model to a 3d geothermal pressure stimulation and long-term cooling scenario

Biased Thinking in the Face of Uncertainty: Exploring the Process of Diagnostic Decision Making Within the Field of Psychiatry

A considerable amount of medical errors are indicated to occur during the diagnostic process. Various forms of faulty cognition on the part of the physicians have been identified as sources of diagnostic errors, including the susceptibility to several cognitive biases. It is therefore of interest to increase our knowledge of how biases may occur in diagnostic decision making. Our study aims to contribute to this knowledge by investigating the occurrence of anchoring bias, confirmation bias, and unwarranted confidence when making diagnostic decisions under uncertainty. For this purpose, we conducted two experiments on samples of Norwegian medical students (N = 128). The participants were presented with two hypothetical psychiatric cases. Anchoring bias was explored by manipulating presentation of the symptoms featured in the initial vignettes, investigating whether participants locked onto whichever symptoms were presented first when making a preliminary diagnosis. Confirmation bias and corresponding developments in diagnostic confidence were investigated through the participants’ subsequent selections and interpretations of additional information. The results from each experiment did not indicate any occurrence of the three investigated phenomena. The non-findings are discussed in terms of the methodological aspects of the study, shedding light on challenges that may exist when investigating the different phenomena simultaneously. We also discuss the true prevalence and detectability of such biases. Based on our discussions, we make several proposals for how our design may be revised and expanded for future studies.En betydelig andel feil innen medisin antydes å forekomme i den diagnostiske prosessen. Diverse former for feiltenkning blant leger har blitt identifisert som kilder til diagnostiske feil, deriblant sårbarhet for ulike kognitive bias. Det er derfor hensiktsmessig å øke vår forståelse for hvordan slike bias kan forekomme i diagnostisk beslutningstaking. Vår studie tar sikte på å bidra til denne forståelsen ved å undersøke forekomst av ankerbias, bekreftelsesbias og ubegrunnet sikkerhet i diagnostisering av usikre pasienttilfeller. Med dette formålet utførte vi to eksperimenter på utvalg av norske medisinstudenter (N = 128). Deltakerne ble presentert for to skrevne psykiatriske kasuistikker. Ankerbias ble testet ved å manipulere presentasjonen av symptomene i introduksjonen av pasienten, og å undersøke hvorvidt deltakere i sine valg av preliminære diagnoser tenderte til å vektlegge de symptomene de hadde blitt presentert for først. Bekreftelsesbias og korresponderende utviklinger i diagnostisk sikkerhet ble undersøkt i deltakernes påfølgende valg og tolkninger av ytterligere informasjon. Resultatene fra hvert eksperiment indikerte ingen forekomst av noen av de tre undersøkte fenomenene. Disse nullfunnene diskuteres i lys av de metodologiske aspektene ved studien, der det blir redegjort for diverse utfordringer som kan eksistere når de ulike fenomenene undersøkes samtidig. Vi diskuterer også den reelle forekomsten, såvel som identifiserbarheten, til fenomenene. Basert på diskusjonene kommer vi med ulike forslag til hvordan vårt design kan revideres og utvides for fremtidige studier.Masteroppgave i psykologiMAPSYK36