On the effect of contraction ratio in viscoelastic flow through abrupt contractions

A numerical study of the creeping flow of a PTT fluid through planar sudden contractions was carried out to quantify the effect of contraction ratio upon the flow characteristics (streamlines and size and intensity of recirculation vortices). The relevant governing equations were solved with a finite volume method embodying a new high-resolution scheme (Alves et al. [Int. J. Numer. Meth. Fluids 41 (2003) 47]) for the discretisation of convection terms, which is here explained and shown to yield improved accuracy and robustness. The results of the simulations, in terms of streamline patterns, give further evidence for a lip-vortex enhancement mechanism and are in remarkable agreement with flow visualization photographs from the literature. In addition, the results show that the variation of flow features in the vicinity of the re-entrant corner, such as lip vortex size and streamlines, are dominated by downstream quantities and scale with the common definition for the Deborah number in this flow, while flow characteristics in the salient corner region scale with that Deborah number divided by the contraction ratio

Viscoelastic flow in a 3D square/square contraction: Visualizations and simulations.

The inertialess three-dimensional (3D) flow of viscoelastic shear-thinning fluids in a 4:1 sudden square-square contraction was investigated experimentally and numerically and compared with the flow of inelastic fluids. Whereas for a Newtonian fluid the vortex length remains unchanged at low Reynolds numbers, with the non-Newtonian fluid there is a large increase in vortex length with fluid elasticity leading to unstable periodic flow at higher flow rates. In the steady flow regime the vortices are 3D and fluid particles enter the vortex at the middle plane, rotate towards its eye, drift sideways to the corner-plane vortex, rotate to its periphery, and exit to the downstream duct. Such dynamic process is reverse of that observed and predicted with Newtonian fluids. Numerical predictions using a multimode Phan-Thien–Tanner viscoelastic model are found to match the visualizations accurately and in particular are able to replicate the observed flow reversal. The effect of fluid rheology on flow reversal, vortex enhancement, and entry pressure drop is investigated in detail

Modelação estatística dos índices de sinistralidade, aplicados ao setor da construção

Face ao quadro problemático das estatísticas de sinistralidade e considerando a Segurança na construção uma missão vital e de primordial significado para a sociedade, pretendeu-se com o presente estudo desenvolver modelos matemáticos, com capacidade explicativa do comportamento dos índices de sinistralidade em função de variáveis setoriais e globais como a taxa de desemprego e a taxa de crescimento do PIB de Portugal. Foram utilizados dados oficiais relativos a uma amostra de conveniência composta por 58 empresas do setor da construção, referente ao período de 6 anos. Foi efetuado o tratamento estatístico dos dados e desenhados modelos com base em regressões lineares. No final, dois dos modelos desenvolvidos apresentaram capacidade explicativa. Sendo que nestes aparecem como variáveis relevantes, a taxa de desemprego e os custos de formação por trabalhador. Apesar da capacidade explicativa dos modelos e da possibilidade da sua utilização pelas empresas do setor, como ferramenta potenciadora da Prevenção dos acidentes ocupacionais, é ainda questionada a eventual falta de consistência dos dados e a qualidade da formação ministrada nas empresas

Purely Elastic Flow Asymmetries

Using a numerical technique we demonstrate that the flow of the simplest differential viscoelastic fluid model (i.e., the upper-convected Maxwell model) goes through a bifurcation to a steady asymmetric state when flowing in a perfectly symmetric ‘‘cross-slot’’ geometry. We show that this asymmetry is purely elastic in nature and that the effect of inertia is a stabilizing one. Our results are in qualitative agreement with very recent experimental visualizations of a similar flow in the microfluidic apparatus of Arratia et al. [Phys. Rev. Lett. 96, 144502 (2006)]

Uniform flow of viscoelastic fluids past a confined falling cylinder

Uniform steady flow of viscoelastic fluids past a cylinder placed between two moving parallel plates is investigated numerically with a finite-volume method. This configuration is equivalent to the steady settling of a cylinder in a viscoelastic fluid, and here, a 50% blockage ratio is considered. Five constitutive models are employed (UCM, Oldroyd-B, FENE-CR, PTT and Giesekus) to assess the effect of rheological properties on the flow kinematics and wake patterns. Simulations were carried out under creeping flow conditions, using very fine meshes, especially in the wake of the cylinder where large normal stresses are observed at high Deborah numbers. Some of the results are compared with numerical data from the literature, mainly in terms of a drag coefficient, and significant discrepancies are found, especially for the constant-viscosity constitutive models. Accurate solutions could be obtained up to maximum Deborah numbers clearly in excess of those reported in the literature, especially with the PTT and FENECR models. The existence or not of a negative wake is identified for each set of model parameters

The effect of expansion ratio for creeping expansion flows of UCM fluids

A systematic numerical investigation on creeping flows in planar sudden expansions of viscoelastic fluids obeying the upper-convected Maxwell model is carried out to assess the combined effects of viscoelasticity, through the Deborah number, and expansion ratio (ER), which was varied between 1.25 and 32. At large expansion ratios (ER≥4) the flow becomes dominated by the downstream duct size and appropriately normalized quantities tend to be independent of ER. The recirculation size and strength become decreasing functions of De, whereas the Couette correction (the normalized entry pressure drop due to the presence of the expansion) increases. At small ER (ER≤3), however, no simple scaling laws are found and there is a complex interaction between De and ER leading to non-monotonic variations, with an initial decrease in the recirculation length at low Deborah numbers, followed by an enhancement as De increases

Plane sudden expansion flows of viscoelastic liquids

We report a systematic numerical investigation of the creeping flow of three different viscoelastic models, the UCM, Oldroyd-B and the linear form of the PTT model, through a 1:3 planar sudden expansion. Although the effect of elasticity is to reduce both the length and intensity of the recirculation region downstream of the expansion, we show that this reduction is much lower than previous studies have suggested and that, at high Deborah number, a significant region of recirculation still exists for all of the models studied

Modelling anaerobic digestion of sewage sludge: mechanistic models vs machine learning

Anaerobic digestion processes are one of the technologies most used by wastewater treatment plants (WWTPs) to stabilize and decrease the organic content of sludge. This process decreases the costs of disposal while increasing the energetic efficiency of WWTPs. In order to optimize this process, three model approaches were implemented. First, we calibrated and validated the anaerobic digestion model no.1 (ADM1) using data from an anaerobic lab digester treating sewage sludge (Phases I, II, III), and further receiving glycerol pulses (Phases IV, V). Then, to optimize the calibration and parameter estimation, an iterative procedure was applied by minimizing the root mean square error (RMSE). The second approach consisted of applying a machine learning (ML) model to the biogas and methane produced. The results showed that the ADM1 model adjusted well to the experimental results, especially to biogas, methane and pH. The optimization routine was useful to identify the most sensitive parameters, improving model calibration. Overall, the ML approach was more reliable to predict anaerobic reactors performance but did not respond so well to process perturbations (glycerol pulses).This work was supported by the Portuguese Foundation for Science and Technology (FCT) under the scope of the strategic funding of UIDB/04469/2020 and UIDB/00319/2020 units and the PAMWater Project (DSAIPA/Al/0099/2019).info:eu-repo/semantics/publishedVersio

Burnout and dropout intention in medical students: the protective role of academic engagement

Introduction: The infuence of burnout, academic engagement, and their interaction in dropout intention among medical students should be further studied. Current research shows its consequences are relevant, however, there is little understanding on burnout and academic engagement moderation in dropout intention. The current study tested a model that relates the efects of coping strategies, social support satisfaction, general distress on academic engagement, burnout, and dropout intention, on medical students. Methods: Through an online survey a non-probabilistic sample of one Medical Faculty’s 1st- and 2nd-year students was recruited. Cross-sectional data were collected using psychometric instruments (Maslach Burnout Inventory – Student Survey, Social Support Satisfaction Scale for College Students, Brief COPE Scale for College Students, University Student Engagement Inventory, and Depression, Anxiety and Stress Scale), sociodemographic and academic variables, and analyzed using structural equation modeling. Results: 532 students (76% response rate) enrolled in the study. Latent variables structural model presented a satisfactory ft to the data and confrmed the expected negative path between burnout and dropout intention (βDI<-SB=0.430; p<.001) and the latent moderation burnout x engagement (βDI<-SB*SE=-0.218; p<.001). Conclusion: Academic engagement attenuates the impact of burnout on dropout intention, working as a protective factor. Social support satisfaction and adaptive coping are associated with increased levels of academic engagement, and general distress and maladaptive coping are associated with burnout. Medical Schools should develop interventions to prevent dropout intention, tackle students’ stress and academic challenges, and develop their academic engagement levels.Fundação para a Ciência e Tecnologia - FCTinfo:eu-repo/semantics/publishedVersio