Assessment of a semi-probabilistic safety concept for reinforced concrete columns using non-linear finite element analyses

This thesis assesses the reliability of methods for slender concrete column design, including Non-Linear Finite Element Analysis (NLFEA). The applicability of the Eurocode in slender column design is investigated, and the current safety format is assessed. The Partial Safety Factor (PSF) method is a semi-probabilistic method where partial safety factors have been calibrated based on a linear Limit State Function (LSF). In slender structures, significant second-order effects cause geometric non-linearity. The combination of geometric non-linearity and the non-linear behavior of concrete, assessed in an NLFEA software, violate the assumption of a linear limit state. Nevertheless, the Eurocode suggests applying the PSF method to problems solved with NLFEA. The PSF method is compared with two alternative safety formats, namely the Global Resistance Factor Method (GRFM) and the method of Estimate of Coefficient Of Variation (ECOV). Since the PSF method currently is embedded in the Eurocode, a new approach for applying PSFs to slender column design is sought. A new set of PSFs is inquired through reliability analyses combined with both hand-calculation methods and NLFEA. Five stochastic variables are used in the analyses, including: The concrete compressive strength, the reinforcement yield strength, the concrete stiffness, the eccentricity and the load. Inverse reliability analyses are conducted to find the optimal combination of PSFs for the different slenderness ratios. The minimum eccentricity in the Eurocode is considered too conservative and a new approach to treat eccentricity is suggested. GRFM is a more conservative alternative to the PSF method, while ECOV might be non-conservative if the material parameters are included by the values given in Eurocode 2-1-1. It is, therefore, proposed to apply the in-situ adjusted concrete strength with the ECOV method. The results from the inverse analyses indicate that a new slenderness limit should be developed to distinguish between compression and yield failure. Two separate sets of PSFs are proposed, for columns below and above the slenderness limit

Extrapolating into no man's land enables accurate estimation of surface properties with multiparameter equations of state

Thermodynamic properties of homogeneous fluids in the metastable and unstable regions are needed to describe confined fluids, interfaces, nucleating embryos and estimate critical mass flow rates. The most accurate equations of state (EoS) called multiparameter EoS, have a second, non-physical Maxwell loop that renders predictions unreliable in these regions. We elaborate how information from the stable region can be used to reconstruct the metastable and unstable regions. For a simple interaction potential, comparison to results from molecular simulations reveals that isochoric expansion of the pressure from stable states reproduces simulation results in the metastable regions. By constructing a dome that extends above the critical point, we obtain an extrapolated pressure from multiparameter EoS that is free of second Maxwell loops. A reconstructed EoS is developed next, by integrating the extrapolated pressure from a stable state to obtain the Helmholtz energy. The consistency of the reconstructed EoS is gauged by computing phase equilibrium densities, pressures, and enthalpies of evaporation, which are in reasonable agreement with experimental values. Combined with density gradient theory, the reconstructed EoS yields surface tensions of water, carbon dioxide, ammonia, hydrogen and propane that deviate, on average, 4.4%, 1.6%, 6.0%, 0.7% and 5.4% from experimental values respectively. The results reveal a potential to develop more accurate extrapolation protocols, which can be leveraged to obtain prediction of metastable properties, surface properties or used as constraints in fitting multiparmeter EoS.Extrapolating into no man's land enables accurate estimation of surface properties with multiparameter equations of statepublishedVersio

Influenza A virus H10N7 detected in dead harbor seals (Phoca vitulina) at several locations in Denmark 2014.

Influenza A virus (IAV) affects a wide range of species, though waterfowl is regarded the natural host for most IAV subtypes. Avian influenza (AI) viruses replicate in the intestinal tract of birds and are mainly transmitted by the fecal-oral route. Pinnipeds share the same shoreline habitats as many waterfowl species and are therefore potentially exposed to AIV. Outbreaks of AI in seals have been described in North America and Asia but prior to 2014 never in Europe. In 2014 massive deaths of harbor seals (Phoca vitulina) were reported in Northern Europe. In Denmark, harbor seals were initially found dead on the Danish island Anholt in Kattegat, which is the sea surrounded by Denmark, Norway and Sweden. Between June and August, 152 harbor seals were found dead. Four seals were submitted to the National Veterinary Institute in Dennmark and diagnosed with severe pneumonia. Influenza A virus of the subtype H10N7 was detected in two out of four seals. Subsequently IAV was detected in dead harbor seals at several locations in Denmark. The IAV outbreak appeared to move with time to the west through the Limfjord to the North Sea and further down south along the west coast of Jutland to the Wadden Sea. Outbreaks were subsequently reported from Germany and The Netherlands. The aim of this study was to characterize the viruses detected at the several locations by molecular and phylogenetic analysis. All viruses were subtyped as H10N7 with genes of avian origin. The HA and NA genes of the viruses were highly similar to H10N7 IAV detected in harbor seals in Sweden in the spring of 2014 and in Germany in the autumn of 2014, suggesting that the same strain of virus had spread from Sweden to Denmark and further on to Germany

Equation of State for Solid Argon Valid for Temperatures up to 300 K and Pressures up to 16 GPa

A new equation of state (EoS) is presented for solid argon. The EoS is based on the quasi-harmonic approximation and formulated in terms of the Helmholtz energy, with temperature and molar volume as independent variables. To ensure high accuracy over a wide range of pressures, the static energy is represented semi-analytically by a Buckingham potential with three-body corrections. The vibrational modes are represented by Debye and Einstein contributions, and the EoS includes an anharmonic correction. A comprehensive collection of available experimental data has been used in the parameter optimization, including pressure and volume measurements along the co-existence curves, heat capacities, thermal expansivities and isothermal compressibilites. The EoS reproduces the molar volumes along the sublimation coexistence curve within an estimated uncertainty of 0.03%. For the heat capacity, the uncertainty is estimated to 1% in the range 20–50 K, 2% at higher temperatures, and 6% at lower temperatures. The isentropic and isothermal compressibilities have estimated uncertainties of 4% and 3%. For the thermal expansivity, the EoS has an estimated uncertainty of 2% above, and 5% below 30 K. For the pressure along the phase coexistence curves, the EoS has an estimated uncertainty of 0.4% for melting and 5% for sublimation. For the calculation of pressure as function of temperature and molar volume, the average relative deviation with respect to all available data is 5%. The EoS is valid up to pressures of 16 GPa and temperatures of 300 K, yet extrapolates well at temperatures beyond this range. The EoS represents the coexistence of solid argon in argon–hydrogen and argon–helium fluid mixtures nearly within the experimental uncertainty, provided that the EoS used to represent the fluid phase is sufficiently accurate.acceptedVersio

Choice of reference, influence of non-additivity and present challenges in thermodynamic perturbation theory for mixtures

This work revisits the fundamentals of thermodynamic perturbation theory for fluid mixtures. The choice of reference and governing assumptions can profoundly influence the accuracy of the perturbation theory. The statistical associating fluid theory for variable range interactions of the generic Mie form equation of state is used as a basis to evaluate three choices of hard-sphere reference fluids: single component, additive mixture, and non-additive mixture. Binary mixtures of Lennard-Jones fluids are investigated, where the ratios of σ (the distance where the potential is zero) and the ratios of ϵ (the well depth) are varied. By comparing with Monte Carlo simulations and results from the literature, we gauge the accuracy of different theories. A perturbation theory with a single-component reference gives inaccurate predictions when the σ-ratio differs significantly from unity but is otherwise applicable. Non-additivity becomes relevant in phase-equilibrium calculations for fluids with high ϵ-ratios or when the mixing rule of σ incorporates non-additivity through an adjustable parameter. This can be handled in three ways: by using a non-additive hard-sphere reference, by incorporating an extra term in the additive hard-sphere reference, or with a single-component reference when the σ-ratio is close to unity. For σ- and ϵ-ratios that differ significantly from unity, the perturbation theories overpredict the phase-equilibrium pressures regardless of reference. This is particularly pronounced in the vicinity of the critical region for mixtures with high ϵ-ratios. By comparing with Monte Carlo simulations where we compute the terms in the perturbation theory directly, we find that the shortcomings of the perturbation theory stem from an inaccurate representation of the second- and third-order perturbation terms, a2 and a3. As mixtures with molecules that differ significantly in size and depths of their interaction potentials are often encountered in industrial and natural applications, further development of the perturbation theory based on these results is an important future work.acceptedVersionThis is the authors’ accepted and refereed manuscript to the article. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in (citation of published article) and may be found at http://dx.doi.org/10.1063/1.514277

Estimating metastable thermodynamic properties by isochoric extrapolation from stable states

The description of metastable fluids, those in local but not global equilibrium, remains an important problem of thermodynamics, and it is crucial for many industrial applications and all first order phase transitions. One way to estimate their properties is by extrapolation from nearby stable states. This is often done isothermally, in terms of a virial expansion for gases or a Taylor expansion in density for liquids. This work presents evidence that an isochoric expansion of pressure at a given temperature is superior to an isothermal density expansion. Two different isochoric extrapolation strategies are evaluated, one best suited for vapors and one for liquids. Both are exact for important model systems, including the van der Waals equation of state. Moreover, we present a simple method to evaluate all the coefficients of the isochoric expansion directly from a simulation in the canonical ensemble. Using only the properties of stable states, the isochoric extrapolation methods reproduce simulation results with Lennard-Jones potentials, mostly within their uncertainties. The isochoric extrapolation methods are able to predict deeply metastable pressures accurately even from temperatures well above the critical. Isochoric extrapolation also predicts a mechanical stability limit, i.e., the thermodynamic spinodal. For water, the liquid spinodal pressure is predicted to be monotonically decreasing with decreasing temperature, in contrast to the re-entrant behavior predicted by the direct extension of the reference equation of state. © 2024 Author(s).Estimating metastable thermodynamic properties by isochoric extrapolation from stable statesacceptedVersio

A flashing flow model for the rapid depressurization of CO2 in a pipe accounting for bubble nucleation and growth

Flashing flow is encountered in many industrial systems involving nozzles, valves and decompression of vessels and pipes. In the context of CO2 capture and storage (CCS), the design of safe and efficient CO2 transportation systems requires accurate flashing models, e.g., for safety analysis of pipe fractures and to predict the mass flow through relief valves. We propose a homogeneous flashing model (HFM) for flashing flow accounting for the underlying physical phenomena of the phase change: bubble nucleation, coalescence, break-up and growth. Homogeneous nucleation is modeled using classical nucleation theory and heterogeneous nucleation is approximated with constant rates of bubble creation and mass transfer from liquid to vapor. The flashing flow model is fitted for CO2 pipe depressurization data at various initial conditions. We find that the same, constant model parameters can be applied for the whole set of depressurization cases considered, as opposed to the conventional homogeneous relaxation model which typically is tuned on a case-by-case basis. For depressurization paths where the fluid state passes close to the critical point, we demonstrate that an accurate description of the flashing process along the length of the pipe can only be achieved when both homogeneous and heterogeneous nucleation are accounted for.publishedVersio

Free energy of critical droplets—from the binodal to the spinodal

Arguably, the main challenge of nucleation theory is to accurately evaluate the work of formation of a critical embryo in the new phase, which governs the nucleation rate. In Classical Nucleation Theory (CNT), this work of formation is estimated using the capillarity approximation, which relies on the value of the planar surface tension. This approximation has been blamed for the large discrepancies between predictions from CNT and experiments. In this work, we present a study of the free energy of formation of critical clusters of the Lennard-Jones fluid truncated and shifted at 2.5σ using Monte Carlo simulations, density gradient theory, and density functional theory. We find that density gradient theory and density functional theory accurately reproduce molecular simulation results for critical droplet sizes and their free energies. The capillarity approximation grossly overestimates the free energy of small droplets. The incorporation of curvature corrections up to the second order with the Helfrich expansion greatly remedies this and performs very well for most of the experimentally accessible regions. However, it is imprecise for the smallest droplets and largest metastabilities since it does not account for a vanishing nucleation barrier at the spinodal. To remedy this, we propose a scaling function that uses all relevant ingredients without adding fitting parameters. The scaling function reproduces accurately the free energy of the formation of critical droplets for the entire metastability range and all temperatures examined and deviates from density gradient theory by less than one kB