Structure-preserving integrators based on a new variational principle for constrained mechanical systems

A new variational principle for mechanical systems subject to holonomic constraints is presented. The newly proposed GGL principle is closely related to the often used Gear-Gupta-Leimkuhler (GGL) stabilization of the differential–algebraic equations governing the motion of constrained mechanical systems. The GGL variational principle relies on an extension of the Livens principle (sometimes also referred to as Hamilton–Pontryagin principle) to mechanical systems subject to holonomic constraints. In contrast to the original GGL stabilization, the new approach facilitates the design of structure-preserving integrators. In particular, new variational integrators are presented, which result from the direct discretization of the GGL variational principle. These variational integrators are symplectic and conserve momentum maps in the case of systems with symmetry. In addition to that, a new energy–momentum scheme is developed, which results from the discretization of the Euler–Lagrange equations pertaining to the GGL variational principle. The numerical properties of the newly devised schemes are investigated in representative examples of constrained mechanical systems

Port-Hamiltonian formulation and structure-preserving discretization of hyperelastic strings

Port-Hamiltonian (PH) systems provide a framework for modeling, analysis and control of complex dynamical systems, where the complexity might result from multi-physical couplings, non-trivial domains and diverse nonlinearities. A major benefit of the PH representation is the explicit formulation of power interfaces, so-called ports, which allow for a power-preserving interconnection of subsystems to compose flexible multibody systems in a modular way. In this work, we present a PH representation of geometrically exact strings with nonlinear material behaviour. Furthermore, using structure-preserving discretization techniques a corresponding finite-dimensional PH state space model is developed. Applying mixed finite elements, the semi-discrete model retains the PH structure and the ports (pairs of velocities and forces) on the discrete level. Moreover, discrete derivatives are used in order to obtain an energy-consistent time-stepping method. The numerical properties of the newly devised model are investigated in a representative example. The developed PH state space model can be used for structure-preserving simulation and model order reduction as well as feedforward and feedback control design.Comment: Submitted as a proceeding to the ECCOMAS Thematic Conference on Multibody Dynamics 202

Empirical evidence to understand the human factor for effective rapid testing against SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) rapid antigen point-of-care and home tests are available to laypeople. In four cross-sectional mixed-methods data collections conducted between December 2020 and March 2021 (n = 4,026), we showed that a majority of subjects were willing to test despite mistrust and ignorance regarding rapid tests’ validity. Experimental evidence shows that low costs and access to events could increase testing intentions. Mandatory reporting and isolation after positive results were not identified as major barriers. Instead, assuming that testing and isolation can slow down the pandemic and the possibility to protect others were related to greater willingness to get tested. While we did not find evidence for risk compensation for past tests, experimental evidence suggests that there is a tendency to show less mask wearing and physical distancing in a group of tested individuals. A short communication intervention reduced complacent behavior. The derived recommendations could make rapid testing a successful pillar of pandemic management

The four weeks before lockdown during the COVID-19 pandemic in Germany: A weekly serial cross-sectional survey on risk perceptions, knowledge, public trust and behaviour, 3 to 25 March 2020

Background: During the COVID-19 pandemic, public perceptions and behaviours have had to adapt rapidly to new risk scenarios and radical behavioural restrictions. Aim: To identify major drivers of acceptance of protective behaviours during the 4-week transition from virtually no COVID-19 cases to the nationwide lockdown in Germany (3–25 March 2020). Methods: A serial cross-sectional online survey was administered weekly to ca 1,000 unique individuals for four data collection rounds in March 2020 using non-probability quota samples, representative of the German adult population between 18 and 74 years in terms of age × sex and federal state (n = 3,910). Acceptance of restrictions was regressed on sociodemographic variables, time and psychological variables, e.g. trust, risk perceptions, self-efficacy. Extraction of homogenous clusters was based on knowledge and behaviour. Results: Acceptance of restrictive policies increased with participants’ age and employment in the healthcare sector; cognitive and particularly affective risk perceptions were further significant predictors. Acceptance increased over time, as trust in institutions became more relevant and trust in media became less relevant. The cluster analysis further indicated that having a higher education increased the gap between knowledge and behaviour. Trust in institutions was related to conversion of knowledge into action. Conclusion: Identifying relevant principles that increase acceptance will remain crucial to the development of strategies that help adjust behaviour to control the pandemic, possibly for years to come. Based on our findings, we provide operational recommendations for health authorities regarding data collection, health communication and outreach

COVID-19 Population Survey of Iran (COPSIR) study protocol: Repeated survey on knowledge, risk perception, preventive behaviors, psychological problems, essential needs, and public trust during COVID-19 epidemic

Background: The worldwide emergence and rapid expansion of COVID-19 emphasizes the need to assess the knowledge gap and to predict the disease-related behaviors and reactions during this epidemic. Methods and design: COVID19 Population Survey of Iran (COPSIR) is a repeated cross sectional survey that will be conducted in 8 waves. In each wave, 515 Iranian adults aged 18 years or older will be randomly selected and interviewed by phone. The study waves will be performed at approximately weekly intervals. The survey tool is adapted from COSMO (COVID-19 Snapshot MOnitoring) study. This study will provide information on trends of knowledge, risk perception, preventive behaviors, psychological problems, essential needs, and public trust among Iranian adults during COVID-19 epidemic. Discussion: The key findings of each wave will be immediately reported to the National Headquarters for Coronavirus Control to set better policies for disease control and prevention. Moreover, if a message is extracted from the results of this study that needs to be communicated to the public, it will be done through the mass media. © Iran University of Medical Sciences

Observation of strong final-state effects in pi+ production in pp collisions at 400 MeV

Differential cross sections of the reactions $pp \to d\pi^+$ and $pp \to pn\pi^+$ have been measured at $T_p = 400$ MeV by detecting the charged ejectiles in the angular range $4^0 \leq \Theta_{Lab} \leq 21^\circ$. The deduced total cross sections agree well with those published previously for neighbouring energies. The invariant mass spectra are observed to be strongly affected by $\Delta$ production and $NN$ final-state interaction. The data are well described by Monte Carlo simulations including both these effects. The ratio of $pp \to pn\pi^+$ and $pp \to d\pi^+$ cross sections also compares favourably to a recent theoretical prediction which suggests a dominance of $np$-production in the relative $^3S_1$-state.Comment: 17 pages, 5 figure