Simulation of the Scalar Transport above and within the Amazon Forest Canopy

The parallelized large-eddy simulation model (PALM) was used to understand better the turbulent exchanges of a passive scalar above and within a forested region located in the central Amazon. Weak (2 ms−1) and strong (6 ms−1) wind conditions were simulated. A passive scalar source was introduced to the forest floor for both simulations. The simulations reproduced the main characteristics of the turbulent flow and of the passive scalar transport between the forest and the atmosphere. Noteworthily, strong and weak wind conditions presented different turbulence structures that drove different patterns of scalar exchange both within and above the forest. These results show how passive scalar concentration is influenced by the wind speed at the canopy top. Additionally, higher wind speeds are related to stronger sweep and ejection regimes, generating more intense plumes that are able to reduce the passive scalar concentration inside the forest canopy. This work was the first that used PALM to investigate scalar transport between the Amazon rainforest and the atmosphere.

P–048 Effects of bisphenol S and bisphenol F on human spermatozoa: an in vitro study

Study question Are plasticizers bisphenol S (BPS) and bisphenol F (BPF) safer alternatives to bisphenol A (BPA) for human sperm function? Summary answer Unlike BPA, the analogues BPS and BPF do not significantly affect human sperm mitochondrial functions, motility and viability What is known already The widespread distribution of BPA, along with its reputation to be an endocrine disruptor has generated concerns about possible adverse effects for human health, thus prompting the European Food Safety Authority and the Food and Drug Administration to ban the use of this chemical in many plastic products. Following such restrictions, several substitutes have been developed, with BPS and BPF representing the main replacements to BPA. While it has been demonstrated that BPA promotes oxidative damages in spermatozoa from different species, including human, the possible effects exerted by BPS and BPF on human sperm, have not yet been investigated. Study design, size, duration We explored the effect of 4 h in vitro exposure to scalar concentrations of BPS and BPF (from 10 to 400 μM), and to 400 μM BPA on sperm motility, viability, mitochondrial membrane potential (Δm) and mitochondrial generation of reactive oxygen species (ROS). In a set of experiments, the effect of a combination of both BPF (400 μM) and BPS (400 μM) on ΔΨm and mitochondrial ROS generation was also assessed. Participants/materials, setting, methods Sperm Δm was analyzed by flow cytometry with the fluorescent lipophilic cationic dye JC–1. Flow cytometric assessment of mitochondrial generation of ROS was carried out using the lipid soluble cation MitoSOX red (MSR). Sperm motility and viability were evaluated by Computer-Aided Semen Analysis (CASA) and eosin assay, respectively. Main results and the role of chance The exposure to scalar concentration of BPS did not significantly affect sperm motility and viability with respect to untreated controls. A lower, albeit not significant, sperm motility was registered in samples exposed to the highest concentrations of BPF (300 μM and 400 μM). As expected, 400 μM BPA produced a complete sperm immobilization along with a dramatically loss in sperm viability. No significant differences were observed in sperm Δm and ROS generation after exposure to scalar concentration of BPS compared to untreated controls and the trend towards lower Δm and higher mitochondrial ROS generation at the highest concentrations of BPF did not reach statistical significance. On the contrary, after 4 h exposure to 400 μM BPA a significant lower Δm and higher mitochondrial ROS generation were observed. Finally, the exposure to a combination of BPF and BPS at high concentrations (400 μM) did not significantly affect sperm Δm, or mitochondrial ROS generation, when compared to 400 μM BPA, used as positive control. Limitations, reasons for caution: The present study only evaluated BPS and BPF effects, but in daily-life people are exposed to several plasticizers containing different bisphenols at different concentrations. Therefore, adverse effects of synergic exposure to BPA analogues other than BPS and BPF, alone or in combination with BPA, cannot be ruled out. Wider implications of the findings: The analogues BPS and BPF, alone or in combination, appeared to be safer alternatives to BPA on sperm biology as they exert a neutral effect on sperm motility, viability, and mitochondrial functions even at high concentrations. These results could be useful to identify more secure plasticizer components. Trial registration number Not applicable

Wall-Modelled Large Eddy Simulations of Axial Turbine Rim Sealing

This paper presents WMLES simulations of a chute type turbine rim seal. Configurations with an axisymmetric annulus flow and with nozzle guide vanes fitted (but without rotor blades) are considered. The passive scalar concentration solution and WMLES are validated against available data in the literature for uniform convection and a rotor-stator cavity flow. The WMLES approach is shown to be effective, giving significant improvements over an eddy viscosity turbulence model, in prediction of rim seal effectiveness compared to research rig measurements. WMLES requires considerably less computational time than wall-resolved LES, and has the potential for extension to engine conditions. All WMLES solutions show rotating inertial waves in the chute seal. Good agreement between WMLES and measurements for sealing effectiveness in the configuration without vanes is found. For cases with vanes fitted the WMLES simulation shows less ingestion than the measurements, and possible reasons are discussed.

Wall-Modelled Large Eddy Simulations of Axial Turbine Rim Sealing

This paper presents WMLES simulations of a chute type turbine rim seal. Configurations with an axisymmetric annulus flow and with nozzle guide vanes fitted (but without rotor blades) are considered. The passive scalar concentration solution and WMLES are validated against available data in the literature for uniform convection and a rotor-stator cavity flow. The WMLES approach is shown to be effective, giving significant improvements over an eddy viscosity turbulence model, in prediction of rim seal effectiveness compared to research rig measurements. WMLES requires considerably less computational time than wall-resolved LES, and has the potential for extension to engine conditions. All WMLES solutions show rotating inertial waves in the chute seal. Good agreement between WMLES and measurements for sealing effectiveness in the configuration without vanes is found. For cases with vanes fitted the WMLES simulation shows less ingestion than the measurements, and possible reasons are discussed.

A Comprehensive Presentation of the Turbulent Plane Jet Theory with Passive Scalar

We present a unified vision of the existing theoretical models for the turbulent plane jet, leading to new analytical profiles for scalar concentration and turbulent quantities, including a complete turbulent kinetic energy budget. Integrals of the budget terms are also computed. The present model is split into two variants. Both compare fairly well with referenced experimental data.

Dispersion in the large-deviation regime. Part 2. Cellular flow at large Péclet number

A standard model for the study of scalar dispersion through the combined effect of advection and molecular diffusion is a two-dimensional periodic flow with closed streamlines inside periodic cells. Over long time scales, the dispersion of a scalar released in this flow can be characterized by an effective diffusivity that is a factor$\mathit{Pe}^{1/2}$larger than molecular diffusivity when the Péclet number$\mathit{Pe}$is large. Here we provide a more complete description of dispersion in this regime by applying the large-deviation theory developed in Part 1 of this paper. Specifically, we derive approximations to the rate function governing the scalar concentration at large time$t$by carrying out an asymptotic analysis of the relevant family of eigenvalue problems. We identify two asymptotic regimes and, for each, make predictions for the rate function and spatial structure of the scalar. Regime I applies to distances$|\boldsymbol {x}|$from the scalar release point that satisfy$|\boldsymbol {x}|= O(\mathit{Pe}^{1/4} t)$. The concentration in this regime is isotropic at large scales, is uniform along streamlines within each cell, and varies rapidly in boundary layers surrounding the separatrices between adjacent cells. The results of homogenization theory, yielding the$O(\mathit{Pe}^{1/2})$effective diffusivity, are recovered from our analysis in the limit$|\boldsymbol {x}|\ll \mathit{Pe}^{1/4} t$. Regime II applies when$|\boldsymbol {x}|=O(\mathit{Pe}\, t/{\rm log}\, \mathit{Pe})$and is characterized by an anisotropic concentration distribution that is localized around the separatrices. A novel feature of this regime is the crucial role played by the dynamics near the hyperbolic stagnation points. A consequence is that in part of the regime the dispersion can be interpreted as resulting from a random walk on the lattice of stagnation points. The two regimes overlap so that our asymptotic results describe the scalar concentration over a large range of distances$|\boldsymbol {x}|$. They are verified against numerical solutions of the family of eigenvalue problems yielding the rate function.

Dispersion in the large-deviation regime. Part 1: shear flows and periodic flows

The dispersion of a passive scalar in a fluid through the combined action of advection and molecular diffusion is often described as a diffusive process, with an effective diffusivity that is enhanced compared with the molecular value. However, this description fails to capture the tails of the scalar concentration distribution in initial-value problems. To remedy this, we develop a large-deviation theory of scalar dispersion that provides an approximation to the scalar concentration valid at much larger distances away from the centre of mass, specifically distances that are$O(t)$rather than$O(t^{1/2})$, where$t \gg 1$is the time from the scalar release. The theory centres on the calculation of a rate function characterizing the large-time form of the scalar concentration. This function is deduced from the solution of a one-parameter family of eigenvalue problems which we derive using two alternative approaches, one asymptotic, the other probabilistic. We emphasize the connection between the large-deviation theory and the homogenization theory that is often used to compute effective diffusivities: a perturbative solution of the eigenvalue problems in the appropriate limit reduces at leading order to the cell problem of homogenization theory. We consider two classes of flows in some detail: shear flows and periodic flows with closed streamlines (cellular flows). In both cases, large deviation generalizes classical results on effective diffusivity and captures new phenomena relevant to the tails of the scalar distribution. These include approximately finite dispersion speeds arising at large Péclet number$\mathit{Pe}$(corresponding to small molecular diffusivity) and, for two-dimensional cellular flows, anisotropic dispersion. Explicit asymptotic results are obtained for shear flows in the limit of large$\mathit{Pe}$. (A companion paper, Part 2, is devoted to the large-$\mathit{Pe}$asymptotic treatment of cellular flows.) The predictions of large-deviation theory are compared with Monte Carlo simulations that estimate the tails of concentration accurately using importance sampling.