An embedded boundary approach for efficient simulations of viscoplastic fluids in three dimensions

We present a methodology for simulating three-dimensional flow of incompressible viscoplastic fluids modelled by generalised Newtonian rheological equations. It is implemented in a highly efficient framework for massively parallelisable computations on block-structured grids. In this context, geometric features are handled by the embedded boundary approach, which requires specialised treatment only in cells intersecting or adjacent to the boundary. This constitutes the first published implementation of an embedded boundary algorithm for simulating flow of viscoplastic fluids on structured grids. The underlying algorithm employs a two-stage Runge-Kutta method for temporal discretisation, in which viscous terms are treated semi-implicitly and projection methods are utilised to enforce the incompressibility constraint. We augment the embedded boundary algorithm to deal with the variable apparent viscosity of the fluids. Since the viscosity depends strongly on the strain rate tensor, special care has been taken to approximate the components of the velocity gradients robustly near boundary cells, both for viscous wall fluxes in cut cells and for updates of apparent viscosity in cells adjacent to them. After performing convergence analysis and validating the code against standard test cases, we present the first ever fully three-dimensional simulations of creeping flow of Bingham plastics around translating objects. Our results shed new light on the flow fields around these objects

High performance simulations of yield stress fluids in a structured adaptive mesh refinement framework with embedded boundaries

Viscoplastic fluids are a class of non-Newtonian liquids characterised by their yield stress. Unless an external stress is applied which is larger than this threshold value, the fluid does not flow, but exhibits rigid body behaviour. Above the yield stress, applied forces cause viscous deformation. Such fluids play important roles in a range of fields, notably in wellbore drilling, which is the application that motivated this project. One aspect of this operation requires displacement of drilling fluid by cement in the annulus between casing and geological surroundings, and both of these fluids are viscoplastics. Ensuring that this is done properly is of utmost importance to the overall safety of the drilling operation. Often, numerical simulations are the only viable way of experimenting with the effect of drilling parameters and fluid properties on the flow configuration and resulting behaviour. Unfortunately, the presence of a yield stress leads to a singularity in the apparent viscosity at zero strain. This causes substantial computational expense for the algorithms used to simulate fluid flow numerically, even when regularisation techniques are employed to alleviate the problem. Consequently, most published results in the literature on computational viscoplasticity has been restricted to two-dimensional and steady-state flows. In an attempt to address this, we have applied state-of-the-art techniques from high-performance computational fluid dynamics to the viscoplastic flow problem. Specifically, we utilise spatio-temporal adaptive mesh refinement on structured meshes in this context for the first time. This is achieved through the software framework AMReX, which includes state-of-the-art numerical tools for solving partial differential equations with optimal parallel scaling. The ability to rapidly simulate unsteady viscoplastic flow problems in three dimensions is demonstrated by novel numerical experiments in a lid-driven cavity. In order to investigate flows in more interesting domain geometries and around objects, an embedded boundary algorithm has been developed which works alongside the viscoplastic flow solver. We show how this methodology can be utilised to simulate flow inside non-rectangular objects, and investigate fully three-dimensional viscoplastic flow past several shapes of bodies for the first time.EPSRC Centre for Doctoral Training in Computational Methods for Materials Science grant number EP/L015552/1 BP International Centre for Advanced Materials (BP-ICAM) Extra support towards living expenses through the Aker Scholarshi

Numerical Investigations on Nanobubble Stability

This thesis discusses the stability and lifetime of bulk and surface nanobubbles. Two numerical models are presented which have been developed in order to investigate aspects of the dynamic equilibrium mechanism using computational methods. By using a steady-state approach where a surface nanobubble is sustained by electrolysis, it is found that for a given nanobubble size and geometry there is only a very narrow range of possible molar fluxes into the system which balance it. This demonstrates the importance of the dynamic equilibrium mechanism in sustaining the surface nanobubbles. An effort to find a better approximation of the expected lifetimes of bulk nanobubbles has also proven fruitful. Preliminary results show lifetimes that are an order of magnitude longer than those previously presented in the literature

Highly parallelisable simulations of time-dependent viscoplastic fluid flow simulations with structured adaptive mesh refinement

We present the extension of an efficient and highly parallelisable framework for incompressible fluid flow simulations to viscoplastic fluids. The system is governed by incompressible conservation of mass, the Cauchy momentum equation and a generalised Newtonian constitutive law. In order to simulate a wide range of viscoplastic fluids, we employ the Herschel-Bulkley model for yield-stress fluids with nonlinear stress-strain dependency above the yield limit. We utilise Papanastasiou regularisation in our algorithm to deal with the singularity in apparent viscosity. The resulting system of partial differential equations is solved using the IAMR code (Incompressible Adaptive Mesh Refinement), which uses second-order Godunov methodology for the advective terms and semi-implicit diffusion in the context of an approximate projection method to solve on adaptively refined meshes. By augmenting the IAMR code with the ability to simulate regularised Herschel-Bulkley fluids, we obtain efficient numerical software for time-dependent viscoplastic flow in three dimensions, which can be used to investigate systems not considered previously due to computational expense. We validate results from simulations using this new capability against previously published data for Bingham plastics and power-law fluids in the two-dimensional lid-driven cavity. In doing so, we expand the range of Bingham and Reynolds numbers which have been considered in the benchmark tests. Moreover, extensions to time-dependent flow of Herschel-Bulkley fluids and three spatial dimensions offer new insights into the flow of viscoplastic fluids in this test case, and we provide missing benchmark results for these extensions.Funding and technical support from BP through the BP International Centre for Advanced Materials (BP-ICAM) which made this research possible

Training auscultatory skills: computer simulated heart sounds or additional bedside training? A randomized trial on third-year medical students

<p>Abstract</p> <p>Background</p> <p>The present study compares the value of additional use of computer simulated heart sounds, to conventional bedside auscultation training, on the cardiac auscultation skills of 3^rdyear medical students at Oslo University Medical School.</p> <p>Methods</p> <p>In addition to their usual curriculum courses, groups of seven students each were randomized to receive four hours of additional auscultation training either employing a computer simulator system or adding on more conventional bedside training. Cardiac auscultation skills were afterwards tested using live patients. Each student gave a written description of the auscultation findings in four selected patients, and was rewarded from 0-10 points for each patient. Differences between the two study groups were evaluated using student's t-test.</p> <p>Results</p> <p>At the auscultation test no significant difference in mean score was found between the students who had used additional computer based sound simulation compared to additional bedside training.</p> <p>Conclusions</p> <p>Students at an early stage of their cardiology training demonstrated equal performance of cardiac auscultation whether they had received an additional short auscultation course based on computer simulated training, or had had additional bedside training.</p

Mobility and associations with levels of cerebrospinal fluid amyloid β and tau in a memory clinic cohort

BackgroundMobility impairments, in terms of gait and balance, are common in persons with dementia. To explore this relationship further, we examined the associations between mobility and cerebrospinal fluid (CSF) core biomarkers for Alzheimer’s disease (AD).MethodsIn this cross-sectional study, we included 64 participants [two with subjective cognitive decline (SCD), 13 with mild cognitive impairment (MCI) and 49 with dementia] from a memory clinic. Mobility was examined using gait speed, Mini-Balance Evaluation Systems test (Mini-BESTest), Timed Up and Go (TUG), and TUG dual-task cost (TUG DTC). The CSF biomarkers included were amyloid-β 42 (Aβ42), total-tau (t-tau), and phospho tau (p-tau181). Associations between mobility and biomarkers were analyzed through correlations and multiple linear regression analyses adjusted for (1) age, sex, and comorbidity, and (2) SCD/MCI vs. dementia.ResultsAβ42 was significantly correlated with each of the mobility outcomes. In the adjusted multiple regression analyses, Aβ42 was significantly associated with Mini-BESTest and TUG in the fully adjusted model and with TUG DTC in step 1 of the adjusted model (adjusting for age, sex, and comorbidity). T-tau was only associated with TUG DTC in step 1 of the adjusted model. P-tau181 was not associated with any of the mobility outcomes in any of the analyses.ConclusionBetter performance on mobility outcomes were associated with higher levels of CSF Aβ42. The association was strongest between Aβ42 and Mini-BESTest, suggesting that dynamic balance might be closely related with AD-specific pathology

Drug Use before and after Initiating Treatment with Acetylcholinesterase Inhibitors

Background/Aims: The aim was to study the prevalence of use of different drugs prescribed for behavioral and psychological symptoms of dementia in persistent users of acetylcholinesterase inhibitors (AChEIs) before and after AChEI initiation, and to compare with the use in the general population. Methods: Use of antidepressants, antipsychotics, and analgesics in the 4 years before and 2 years after AChEI initiation was studied based on data from the Norwegian Prescription Database 2004–2016. Results: The prevalence of use of antidepressants and antipsychotics the year before AChEI initiation was twice the prevalence in the age-adjusted general population and continued to rise in the first 2 years after initiation of AChEIs. The prevalence of weak analgesics and antipsychotics increased strongly in the last year before AChEI initiation. The increase in the use of antidepressants started at least 4 years before initiation of AChEIs. Opioid use was generally lower than in the general population and was not influenced by AChEI initiation. Conclusion: Increased use of antidepressants and antipsychotics was observed both before and after initiation of AChEIs and may indicate that behavioral symptoms occur in a preclinical or early phase of Alzheimer’s disease. The prescription pattern of analgesics with a low use of opioids may indicate an undertreatment of pain in people with dementia

Comparison of organic and conventional food and food Production. Part V: Human health – pesticide residues

publishedVersio

Comparison of organic and conventional food and food Production. Part V: Human health – pesticide residues

publishedVersio