Conditional statistics in a turbulent premixed flame derived from direct numerical simulation

The objective of this paper is to briefly introduce conditional moment closure (CMC) methods for premixed systems and to derive the transport equation for the conditional species mass fraction conditioned on the progress variable based on the enthalpy. Our statistical analysis will be based on the 3-D DNS database of Trouve and Poinsot available at the Center for Turbulence Research. The initial conditions and characteristics (turbulence, thermo-diffusive properties) as well as the numerical method utilized in the DNS of Trouve and Poinsot are presented, and some details concerning our statistical analysis are also given. From the analysis of DNS results, the effects of the position in the flame brush, of the Damkoehler and Lewis numbers on the conditional mean scalar dissipation, and conditional mean velocity are presented and discussed. Information concerning unconditional turbulent fluxes are also presented. The anomaly found in previous studies of counter-gradient diffusion for the turbulent flux of the progress variable is investigated

Community-powered urban stream restoration: A vision for sustainable and resilient urban ecosystems

Urban streams can provide amenities to people living in cities, but those benefits are reduced when streams become degraded, potentially even causing harm (disease, toxic compounds, etc.). Governments and institutions invest resources to improve the values and services provided by urban streams; however, the conception, development, and implementation of such projects may not include meaningful involvement of community members and other stakeholders. Consequently, project objectives may be misaligned with community desires and needs, and projects may fail to achieve their goals. In February 2020, the 5(th) Symposium on Urbanization and Stream Ecology, an interdisciplinary meeting held every 3 to 5 y, met in Austin, Texas, USA, to explore new approaches to urban stream projects, including ways to maximize the full range of potential benefits by better integrating community members into project identification and decision making. The symposium included in-depth discussion about 4 nearby field case studies, participation of multidisciplinary urban stream experts from 5 continents, and input from the Austin community. Institutional barriers to community inclusion were identified and analyzed using real-world examples, both from the case studies and from the literature, which clarified disparities in power, equity, and values. Outcomes of the symposium have been aggregated into a vision that challenges the present institutional approach to urban stream management and a set of strategies to systematically address these barriers to improve restoration solutions. Integrating community members and other stakeholders throughout the urban restoration process, and a transparent decision-making process to resolve divergent objectives, can help identify appropriate goals for realizing both the ecological and social benefits of stream restoration

Community-powered urban stream restoration: A vision for sustainable and resilient urban ecosystems

Este artículo contiene 16 páginas, 2 tablas, 3 figuras.Urban streams can provide amenities to people living in cities, but those benefits are reduced when streams become degraded, potentially even causing harm (disease, toxic compounds, etc.). Governments and institutions invest resources to improve the values and services provided by urban streams; however, the conception, development, and implementation of such projects may not include meaningful involvement of community members and other stakeholders. Consequently, project objectives may be misaligned with community desires and needs, and projects may fail to achieve their goals. In February 2020, the 5th Symposium on Urbanization and Stream Ecology, an interdisciplinary meeting held every 3 to 5 y, met in Austin, Texas, USA, to explore new approaches to urban stream projects, including ways to maximize the full range of potential benefits by better integrating community members into project identification and decision making. The symposium included in-depth discussion about 4 nearby field case studies, participation of multidisciplinary urban stream experts from 5 continents, and input from the Austin community. Institutional barriers to community inclusion were identified and analyzed using real-world examples, both from the case studies and from the literature, which clarified disparities in power, equity, and values. Outcomes of the symposium have been aggregated into a vision that challenges the present institutional approach to urban stream management and a set of strategies to systematically address these barriers to improve restoration solutions. Integrating community members and other stakeholders throughout the urban restoration process, and a transparent decision-making process to resolve divergent objectives, can help identify appropriate goals for realizing both the ecological and social benefits of stream restoration.Publication costs were covered by an award from the Society of Freshwater Science’s Endowed Publication Fund (https:// freshwater-science.org/publications/endowed-publication-fund).Peer reviewe

RNAi-mediated suppression of isoprene emission in poplar transiently impacts phenolic metabolism under high temperature and high light intensities: a transcriptomic and metabolomic analysis

In plants, isoprene plays a dual role: (a) as thermo-protective agent proposed to prevent degradation of enzymes/membrane structures involved in photosynthesis, and (b) as reactive molecule reducing abiotic oxidative stress. The present work addresses the question whether suppression of isoprene emission interferes with genome wide transcription rates and metabolite fluxes in grey poplar (Populusxcanescens) throughout the growing season. Gene expression and metabolite profiles of isoprene emitting wild type plants and RNAi-mediated non-isoprene emitting poplars were compared by using poplar Affymetrix microarrays and non-targeted FT-ICR-MS (Fourier transform ion cyclotron resonance mass spectrometry). We observed a transcriptional down-regulation of genes encoding enzymes of phenylpropanoid regulatory and biosynthetic pathways, as well as distinct metabolic down-regulation of condensed tannins and anthocyanins, in non-isoprene emitting genotypes during July, when high temperature and light intensities possibly caused transient drought stress, as indicated by stomatal closure. Under these conditions leaves of non-isoprene emitting plants accumulated hydrogen peroxide (H2O2), a signaling molecule in stress response and negative regulator of anthocyanin biosynthesis. The absence of isoprene emission under high temperature and light stress resulted transiently in a new chemo(pheno)type with suppressed production of phenolic compounds. This may compromise inducible defenses and may render non-isoprene emitting poplars more susceptible to environmental stress

Experiments on a reacting plume—2. Conditional concentration statistics

A detailed study is made of reactive scalar statistics conditional on the value of a conserved scalar in a non-buoyant reactive plume of NO in a turbulent grid flow doped with O3 using the reaction NO + O3 → NO2 + O2 + 200 kJ mol−1 (in the absence of UV radiation). Use of such conditional statistics makes it possible to separate the effects of turbulent mixing and chemical reaction. Simultaneous measurements of two reactive scalars and two orthogonal turbulent velocities are made using high-resolution instruments. Radial profiles of scalar statistics are taken through the plume. It is found that the reactive scalar means conditional on a conserved scalar lack significant dependence on radial position and that conditional variance around the conditional mean is relatively small. This is in agreement with similar findings in completely different types of turbulent reacting flows. Detailed experiments have been conducted at a single location where the ratio of initial reactant concentrations has been varied by a factor of 30 and the Damko¨hler number (the ratio of the flow timescale to that of the chemical reaction) by a factor of 6, but without measurable effect on the flow field. The conditional reactive scalar results are presented and their usefulness for investigating the reactive behaviour of the plume is demonstrated. Use is made of the stoichiometiic distance, the location where the unconditional mean conserved scalar is equal to the initial concentration of ambient reactant. Preliminary results of the conditional moment closure (CMC) model of Klimenko (1990) and Bilger (1993) are compared with conditional experimental data. The CMC model leads to significant simplification of the equations governing variation of reactive scalars in the flow. The equations are solved numerically with low computational requirements. Weighting of the conditional values with the conserved scalar probability density function yields the conventional reactive scalar means. CMC modelling results are shown to be in good agreement with the experimental data. Limiting cases (equilibrium and reaction dominated) for the reactive scalar statistics are derived from the conserved scalar statistics and form bounds on both the experimental and CMC predictions as expected from theoretical considerations. Application of the CMC model to atmospheric flows is discussed. A good estimate of the covariance is also made from the conditional reactive scalar means

Experiments on a reacting plume—1. Conventional concentration statistics

Experiments have been carried out under laboratory conditions to study turbulent mixing with chemical reaction. Such flows are common in the environment but there are few high-resolution measurements of them under controlled conditions. The reaction NO + O3 → NO2 + O2 + 200 kJ (mol)−1 (in the absence of UV radiation) has been used by introducing a non-buoyant reactive plume of NO into a turbulent grid flow doped with O3. The experiments have been conducted over a wide range of conditions by varying the Damko¨hler number (the ratio of the flow timescale to that of the chemical reaction) by a factor of 6 and varying the ratio of initial reactant concentrations by a factor of 30. The data are presented in two park. Part 1 (present paper) presents conventional concentration statistics. Part 2 presents concentration statistics conditionally averaged by a mixing parameter and discusses their usefulness for investigating; the reactive behaviour of the plume. Specially developed chemiluminescent analysers are used to measure NO and O3 concentrations with high-frequency response and a spatial resolution of four Kolmogorov scales (in this flow). The mixing field of the plume is studied using conserved scalar theory and is compared. to that obtained in other non-reactive flows. Results for the means, variances and joint statistics of the reactive species are presented and compared with frozen and equilibrium limits obtained from the conserved scalar theory for reacting flows. Statistics for NO show no significant trend with Damko¨hler number while those for O3 and NO2 have increased reaction at higher Damko¨hler number. The reactive scalar means are bounded by the frozen and equilibrium limits as they should be according to conserved scalar theory. Other reactive scalar statistics are not necessarily bounded by the limits. A simple relation between moments of O3 and N02 is found in the experimental results and is confirmed by conserved scalar theory. Contribution to the mean reaction rate from the reactant covariance and product of means is compared and it is found that both are significant under the present conditions. Intensity of segregation varies over a wide range and is sensitive to initial reactant concentrations

Modeling of nonreacting and reacting turbulent spray jets using a fully stochastic separated flow approach

Numerical modeling of several turbulent nonreacting and reacting spray jets is carried out using a fully stochastic separated flow (FSSF) approach. As is widely used, the carrier-phase is considered in an Eulerian framework, while the dispersed phase is tracked in a Lagrangian framework following the stochastic separated flow (SSF) model. Various interactions between the two phases are taken into account by means of two-way coupling. Spray evaporation is described using a thermal model with an infinite conductivity in the liquid phase. The gas-phase turbulence terms are closed using the k-epsilon model. A novel mixture fraction based approach is used to stochastically model the fluctuating temperature and composition in the gas phase and these are then used to refine the estimates of the heat and mass transfer rates between the droplets and the surrounding gas-phase. In classical SSF (CSSF) methods, stochastic fluctuations of only the gas-phase velocity are modeled. Successful implementation of the FSSF approach to turbulent nonreacting and reacting spray jets is demonstrated. Results are compared against experimental measurements as well as with predictions using the CSSF approach for both nonreacting and reacting spray jets. The FSSF approach shows little difference from the CSSF predictions for nonreacting spray jets but differences are significant for reacting spray jets. In general, the FSSF approach gives good predictions of the flame length and structure but further improvements in modeling may be needed to improve the accuracy of some details of the Predictions. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved