Moment-based boundary conditions for lattice Boltzmann simulations of natural convection in cavities

We study a multiple relaxation time lattice Boltzmann model for natural convection with moment-based boundary conditions. The unknown primary variables of the algorithm at a boundary are found by imposing conditions directly upon hydrodynamic moments, which are then translated into conditions for the discrete velocity distribution functions. The method is formulated so that it is consistent with the second order implementation of the discrete velocity Boltzmann equations for fluid flow and temperature. Natural convection in square cavities is studied for Rayleigh numbers ranging from 103 to 108. An excellent agreement with benchmark data is observed and the flow fields are shown to converge with second order accuracy

Mixed-Methods in Information Systems Research: Status Quo, Core Concepts, and Future Research Implications

Mixed-methods studies are increasing in information systems research, as they deliver robust and insightful inferences combining qualitative and quantitative research. However, there is considerable divergence in conducting such studies and reporting their findings. Therefore, we aim (1) to evaluate how mixed-methods studies have developed in information systems research under the existence of heavily used guidelines and (2) to reflect on those observations in terms of potential for future research. During our review, we identified 52 mixed-methods papers and quantitatively elaborated on the adherence to the three core concepts of mixed-methods in terms of purpose, meta-inferences, and validation. Findings discover that only eight adhere to all three of them. We discuss the significance of our results for current and upcoming mixed-methods research and derive specific suggestions for authors. With our study, we contribute to mixed-methods research by showing how to leverage the insights from existing guidelines to strengthen future research and by contributing to the discussion of the legislation associated with research guidelines, in general, presenting the status quo in current literature

The lattice Boltzmann method for complex flows

This thesis presents the extension of the lattice Boltzmann equation (LBE) to several well-known flows. First, the flow over a cylinder is studied using the LBE and the numerical predictions are shown to compare well with those obtained using a stylised finite volume method. A clear and formal perturbation analysis of the generalised LBE is also presented. A LBE for axisymmetric flows is developed, the precise form of which is derived through a Chapman-Enskog analysis so that the additional axisymmetric contributions to the Navier-Stokes equation are furnished when written in the cylindrical polar coordinate system. Stokes' flow over a sphere is studied and excellent agreement is found between the numerical and analytical predictions. A lattice Boltzmann model for immiscible binary fluids with variable viscosities and density ratio is developed. In the macroscopic limit this model is shown to recover the Navier-Stokes equations for two phase flow. A theoretical expression for surface tension is determined. The validity of this analysis is confirmed by comparing numerical and theoretical predictions of surface tension as a function of density. A number of numerical simulations are presented and shown to be in good agreement with analytical results. Finally, an axisymmetric multiphase lattice Boltzmann model has been proposed. This model is easy to implement and some test cases have been performed to demonstrate its capabilities. A review of the extension of the lattice Boltzmann equation to viscoelasticity is also presented

Lattice Boltzmann simulations of pressure-driven flows in microchannels using Navier-Maxwell slip boundary conditions

We present lattice Boltzmann simulations of rarefied flows driven by pressure drops along two-dimensional microchannels. Rarefied effects lead to non-zero cross-channel velocities, and nonlinear variations in the pressure along the channel. Both effects are absent in flows driven by uniform body forces. We obtain second-order accuracy for the two components of velocity and the pressure relative to asymptotic solutions of the compressible Navier–Stokes equations with slip boundary conditions. Since the common lattice Boltzmann formulations cannot capture Knudsen boundary layers, we replace the usual discrete analogs of the specular and diffuse reflection conditions from continuous kinetic theory with a moment-based implementation of the first-order Navier–Maxwell slip boundary conditions that relate the tangential velocity to the strain rate at the boundary. We use these conditions to solve for the unknown distribution functions that propagate into the domain across the boundary. We achieve second-order accuracy by reformulating these conditions for the second set of distribution functions that arise in the derivation of the lattice Boltzmann method by an integration along characteristics. Our moment formalism is also valuable for analysing the existing boundary conditions. It reveals the origin of numerical slip in the bounce-back and other common boundary conditions that impose conditions on the higher moments, not on the local tangential velocity itself

The “social” aspect of social-ecological systems : a critique of analytical frameworks and findings from a multisite study of coastal sustainability

The work described here was partly funded by the European Commission’s FP6 contract 036992.We evaluate whether society can adequately be conceptualized as a component of social-ecological systems, given social theory and the current outputs of systems-based research. A mounting critique from the social sciences posits that resilience theory has undertheorized social entities with the concept of social-ecological systems. We trace the way that use of the term has evolved, relating to social science theory. Scientometic and network analysis provide a wide range of empirical data about the origin, growth, and use of this term in academic literature. A content analysis of papers in Ecology and Society demonstrates a marked emphasis in research on institutions, economic incentives, land use, population, social networks, and social learning. These findings are supported by a review of systems science in 18 coastal assessments. This reveals that a systems-based conceptualization tends to limit the kinds of social science research favoring quantitative couplings of social and ecological components and downplaying interpretive traditions of social research. However, the concept of social-ecological systems remains relevant because of the central insights concerning the dynamic coupling between humans and the environment, and its salient critique about the need for multidisciplinary approaches to solve real world problems, drawing on heuristic devices. The findings of this study should lead to more circumspection about whether a systems approach warrants such claims to comprehensiveness. Further methodological advances are required for interdisciplinarity. Yet there is evidence that systems approaches remain highly productive and useful for considering certain social components such as land use and hybrid ecological networks. We clarify advantages and restrictions of utilizing such a concept, and propose a reformulation that supports engagement with wider traditions of research in the social sciences.Publisher PDFPeer reviewe

Reaction-diffusion models of decontamination

A contaminant, which also contains a polymer is in the form of droplets on a solid surface. It is to be removed by the action of a decontaminant, which is applied in aqueous solution. The contaminant is only sparingly soluble in water, so the reaction mechanism is that it slowly dissolves in the aqueous solution and then is oxidized by the decontaminant. The polymer is insoluble in water, and so builds up near the interface, where its presence can impede the transport of contaminant. In these circumstances, Dstl wish to have mathematical models that give an understanding of the process, and can be used to choose the parameters to give adequate removal of the contaminant. Mathematical models of this have been developed and analysed, and show results in broad agreement with the effects seen in experiments

Use of outpatient medical care by headache patients in Germany: a population-based cross-sectional study

Background: Headache sufferers in need of professional health care often do not utilize the care available, and factors influencing headache-specific physician consultation are not yet understood. Objectives of this study are (1) to assess self-reported headache-specific physician consultations and (2) to identify headache-related and sociodemographic predictors. Methods: Data of a random sample of the general population in Germany aged ≥14 years were analyzed (N = 2461). A multivariate binary logistic regression was conducted to identify a parsimonious model to predict physician consultation. Results: 50.7% of the participants with headache reported at least one headache-specific physician consultation during lifetime. Of these, 53.6% had seen one, 26.1% two, and 20.3% more than two physicians because of their headaches. The odds of physician consultation increased with the number of headache days per month (HDM) (reference HDM 1) HDM 1–3 (OR = 2.29), HDM 4–14 (OR = 2.41), and HDM ≥15 (OR = 4.83) and increasing Headache Impact Test score (HIT-6) (reference “no or little impact”) moderate impact (OR = 1.74), substantial impact (OR = 3.01), and severe impact (OR = 5.08). Middle-aged participants were more likely to have consulted than younger and older ones (reference 14–34 years) 35–54 years (OR = 1.90), 55–74 years (OR = 1.96), ≥75 years (OR = 1.02). The odds of physician consultation among self-employed subjects were lower than among employed manual workers (OR = 0.48). The living environment (rural versus urban) did not have an influence on the consultation frequency. Conclusion: The results indicate that apart from burden-related factors (headache frequency; headache impact), health care utilization patterns are also influenced by patients’ occupational status and age. Further research is needed to analyze whether the lower consultation rate means that the self-employed have a higher risk of chronification or that they have more effective self-management strategies regarding headache

The ratio of soluble fms–like tyrosine kinase 1 to placental growth factor predicts time to delivery and mode of birth in patients with suspected preeclampsia- a secondary analysis of the INSPIRE trial

Background: The ratio of soluble fms–like tyrosine kinase 1 to placental growth factor (sFLT1/PLGF) is a useful biomarker for preeclampsia. Since it is a measure of placental dysfunction, it could also be a predictor of clinical deterioration and fetal tolerance to intrapartum stress. Objectives: We tested the hypothesis that sFLT1/PLGF ratio predicts time to delivery. Secondary objectives were to examine associations between the sFLT1/PLGF ratio and mode of birth, fetal distress, need for labor induction and birthweight z-score. Study design: Secondary analysis of the INSPIRE trial, a randomized interventional study on prediction of preeclampsia/eclampsia in which women with suspected preeclampsia were recruited and their blood sFLT1/PLGF ratio was assessed. We stratified participants into three groups according to the ratio result: category 1 (sFLT1/PLGF≤38); category 2 (sFLT1/PLGF>38 and <85); and category 3 (sFLT1/PLGF≥85). We modelled time from sFLT1/PLGF determination to delivery using Kaplan-Meier curves and compared the three ratio categories adjusting for gestational age at sFLT1/PLGF determination and trial arm with Cox Regression. The association between ratio category and mode of delivery, induction of labour and fetal distress was assessed using a multivariable logistic regression adjusting for gestational age at sampling and trial arm. The association between birthweight z-score and sFLT1/PLGF ratio was evaluated using multiple linear regression. Subgroup analysis was conducted in women with no preeclampsia and spontaneous onset of labor; women with preeclampsia; and participants in the non-reveal arm. Results: Higher ratio categories were associated with a shorter latency from sFLT1/PLGF determination to delivery (37 vs 13 vs 10 days for ratios categories 1-3 respectively), hazards ratio for category 3 ratio of 5.64 (95%CI 4.06-7.84, p<0.001). A sFLT/PlGF ratio≥85 had specificity of 92.7%(95%CI 89.0-95.1%) and sensitivity of 54.72% (95% CI, 41.3-69.5) for prediction of preeclampsia indicated delivery within 2 weeks. A ratio category 3 was also associated with decreased odds of spontaneous vaginal delivery (OR 0.47, 95%CI 0.25-0.89); an almost six fold increased risk of emergency cesarean section (OR 5.89, 95%CI 3.05-11.21); and a three-fold increased risk for intrapartum fetal distress requiring operative delivery or cesarean section (OR 3.04, 95%CI 1.53-6.05) when compared to patients with ratios≤38. Higher ratio categories were also associated with higher odds of induction of labor when compared to ratios category 1 (category 2, OR 2.20, 95%CI 1.02-4.76; category 3, OR 6.0, 95%CI 2.01-17.93); and lower median birthweight z-score. Within subgroups of women a)without preeclampsia and with spontaneous onset of labor and b)women with preeclampsia, the log ratio was significantly higher in patients requiring intervention for fetal distress or failure to progress compared to those who delivered vaginaly without intervention. In the subset of women with no preeclampsia and spontaneous onset of labour, those who required intervention for fetal distress or failure to progress had a significantly higher log ratio than those who delivered vaginaly without needing intervention. Conclusion: The sFLT1/PLGF ratio might be helpful in risk-stratification of patients who present with suspected preeclampsia regarding clinical deterioration, intrapartum fetal distress and mode of birth (including the need for intervention in labour)

The virtual source approach to non-linear potential flow simulations

In this paper, we develop the Virtual Source Method for simulation of incompressible and irrotational fluid flows. The method is based upon the integral equations derived by using Green’s identity with Laplace’s equation for the velocity potential. The velocity potential within the fluid domain is completely determined by the potential on a virtual boundary located above the fluid. This avoids the need to evaluate singular integrals. Furthermore, the solution method developed here is meshless in space in that discretisation is in terms of the spectral components of the solution along this virtual boundary. These are determined by specifying non-linear boundary conditions on the velocity potential on the air/water surface using Bernoulli’s equation. A fourth-order Runge-Kutta procedure is used to update the spectral components in time. The method is used to model high-amplitude standing waves and sloshing. Results are compared with theory where applicable and some interesting physical phenomena are identified