560 research outputs found
A partition of unity approach to fluid mechanics and fluid-structure interaction
For problems involving large deformations of thin structures, simulating
fluid-structure interaction (FSI) remains challenging largely due to the need
to balance computational feasibility, efficiency, and solution accuracy.
Overlapping domain techniques have been introduced as a way to combine the
fluid-solid mesh conformity, seen in moving-mesh methods, without the need for
mesh smoothing or re-meshing, which is a core characteristic of fixed mesh
approaches. In this work, we introduce a novel overlapping domain method based
on a partition of unity approach. Unified function spaces are defined as a
weighted sum of fields given on two overlapping meshes. The method is shown to
achieve optimal convergence rates and to be stable for steady-state Stokes,
Navier-Stokes, and ALE Navier-Stokes problems. Finally, we present results for
FSI in the case of a 2D mock aortic valve simulation. These initial results
point to the potential applicability of the method to a wide range of FSI
applications, enabling boundary layer refinement and large deformations without
the need for re-meshing or user-defined stabilization.Comment: 34 pages, 15 figur
Multilevel convergence analysis of multigrid-reduction-in-time
This paper presents a multilevel convergence framework for
multigrid-reduction-in-time (MGRIT) as a generalization of previous two-grid
estimates. The framework provides a priori upper bounds on the convergence of
MGRIT V- and F-cycles, with different relaxation schemes, by deriving the
respective residual and error propagation operators. The residual and error
operators are functions of the time stepping operator, analyzed directly and
bounded in norm, both numerically and analytically. We present various upper
bounds of different computational cost and varying sharpness. These upper
bounds are complemented by proposing analytic formulae for the approximate
convergence factor of V-cycle algorithms that take the number of fine grid time
points, the temporal coarsening factors, and the eigenvalues of the time
stepping operator as parameters.
The paper concludes with supporting numerical investigations of parabolic
(anisotropic diffusion) and hyperbolic (wave equation) model problems. We
assess the sharpness of the bounds and the quality of the approximate
convergence factors. Observations from these numerical investigations
demonstrate the value of the proposed multilevel convergence framework for
estimating MGRIT convergence a priori and for the design of a convergent
algorithm. We further highlight that observations in the literature are
captured by the theory, including that two-level Parareal and multilevel MGRIT
with F-relaxation do not yield scalable algorithms and the benefit of a
stronger relaxation scheme. An important observation is that with increasing
numbers of levels MGRIT convergence deteriorates for the hyperbolic model
problem, while constant convergence factors can be achieved for the diffusion
equation. The theory also indicates that L-stable Runge-Kutta schemes are more
amendable to multilevel parallel-in-time integration with MGRIT than A-stable
Runge-Kutta schemes.Comment: 26 pages; 17 pages Supplementary Material
A method for intramedullary fixation offractures and segmental grafts in the rat tibia
A new technique for intramedullary nailing ot the tibia in rats is described. The results of fracture and segmental graft fixations with the nail are presented. The modular nail consists of three parts that are threaded into and past each other, givingit a conical outer diameter which fits the tibial inedullary canal. The biomechanieal properties of this nail are similar to those ofintact tibias of 230 g male rats. which makes it suitable for experiments with rats weighing more than 200 g
Weight gain and the risk of total hip replacement a population-based prospective cohort study of 265,725 individuals
SummaryObjectiveTo study the association between change in the body mass index (BMI) at different ages and the risk of a later total hip replacement (THR) due to primary osteoarthritis (OA).DesignA total of 265,725 individuals who had two repeated measurements of weight and height were included from national health screenings. These individuals were followed prospectively. The data were matched with the Norwegian Arthroplasty Register and 4,442 of these individuals were identified as having received a THR for primary OA. Cox proportional hazard regression was used to calculate sex-specific relative risks for having a THR according to age at screening and BMI change.ResultsMen and women aged 20 years or younger at the first screening in the quartile with the greatest BMI change per year had more than twice the risk of later having a THR compared with those in the quartile with the smallest BMI change per year. For men older than 30 years at the first screening, there was no relationship between BMI gain, or weight gain, and later risk of THR. For older women, BMI gain was associated with risk of THR, but to a lesser degree than in younger women.ConclusionThere was a clear relationship between change in BMI and the risk of later THR in young men and women, whereas the association was absent in older men and weaker in older women. It is important to focus on weight control to prevent future OA, and the preventive strategy should be focused on the young population
Patterns of Nonrandom Mating Within and Across 11 Major Psychiatric Disorders
Psychiatric disorders are heritable, polygenic traits, which often share risk alleles and for which nonrandom mating has been suggested. However, despite the potential etiological implications, the scale of nonrandom mating within and across major psychiatric conditions remains unclear
Risk factors for knee replacement due to primary osteoarthritis, a population based, prospective cohort study of 315,495 individuals
Background: Osteoarthritis (OA) of the knee is a common and disabling condition. We wanted to investigate the modifiable risk factors Body Mass Index (BMI) and physical activity, using knee replacement (KR) as a marker for severely symptomatic disease, focusing on the interaction between these risk factors. Methods: 315,495 participants (mean age 43.0 years) from national health screenings were followed prospectively with respect to KR identified by linkage to the Norwegian Arthroplasty Register. Data were analysed by Cox proportional hazard regression. Results: During 12 years of follow up 1,323 individuals received KR for primary OA. There was a dose–response relationship between BMI and heavy labour, and later KR. Comparing the highest versus the lowest quarter of BMI, the relative risk was 6.2 (95% CI: 4.2-9.0) in men and 11.1 (95% CI: 7.8-15.6) in women. Men reporting intensive physical activity at work had a relative risk of 2.4 (95% CI: 1.8-3.2) versus men reporting sedentary activity at work, the corresponding figure in women being 2.3 (95% CI: 1.7-3.2). The effect of BMI and physical activity at work was additive. The heaviest men with the most strenuous work had a RR of 11.7 (95% CI: 5.9-23.1) compared to the ones with the lowest BMI and most sedentary work. For women the corresponding RR was 15.8 (95% CI: 8.2-30.3). There was no association between physical activity during leisure and KR. Conclusion: We found that a high BMI and intensive physical activity at work both contribute strongly to the risk of having a KR. As the two risk factors seem to act independently, people with strenuous physical work with a high BMI are at particularly high risk for severely disabling OA of the knee, and should be targeted with effective preventive measures.publishedVersio
A Data-Driven Computational Model for Engineered Cardiac Microtissues
Engineered heart tissues (EHTs) present a potential solution to some of the
current challenges in the treatment of heart disease; however, the development
of mature, adult-like cardiac tissues remains elusive. Mechanical stimuli have
been observed to improve whole-tissue function and cardiomyocyte (CM)
maturation, although our ability to fully utilize these mechanisms is hampered,
in part, by our incomplete understanding of the mechanobiology of EHTs. In this
work, we leverage the experimental data produced by a mechanically tunable
experimental setup to generate tissue-specific computational models of EHTs.
Using imaging and functional data, our modeling pipeline generates models with
tissue-specific ECM and myofibril structure, allowing us to estimate CM active
stress. We use this experimental and modeling pipeline to study different
mechanical environments, where we contrast the force output of the tissue with
the computed active stress of CMs. We show that the significant differences in
measured experimental forces can largely be explained by the levels of
myofibril formation achieved by the CMs in the distinct mechanical
environments, with active stress showing more muted variations across
conditions. The presented model also enables us to dissect the relative
contributions of myofibrils and extracellular matrix to tissue force output, a
task difficult to address experimentally. These results highlight the
importance of tissue-specific modeling to augment EHT experiments, providing
deeper insights into the mechanobiology driving EHT function.Comment: 19 pages, 7 figure
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