Letteratura scientifica selezionata sul tema "Disturbance energy budgets"

AbstractThis paper presents an analysis of the energy transported by disturbances in gaseous combustion. It extends the previous work of Myers (J. Fluid Mech., vol. 226, 1991, 383–400) and so includes non-zero mean-flow quantities, large-amplitude disturbances, varying specific heats and chemical non-equilibrium. This extended form of Myers’ ‘disturbance energy’ then enables complete identification of the conditions under which the famous Rayleigh source term can be derived from the equations governing combusting gas motion. These are: small disturbances in an irrotational, homentropic, non-diffusive (in terms of species, momentum and energy) and stationary mean flow at chemical equilibrium. Under these assumptions, the Rayleigh source term becomes the sole source term in a conservation equation for the classical acoustic energy. It is also argued that the exact disturbance energy flux should become an acoustic energy flux in the far-field surrounding a (reacting or non-reacting) jet. In this case, the volume integral of the disturbance energy source terms are then directly related to the area-averaged far-field sound produced by the jet. This is demonstrated by closing the disturbance energy budget over a set of aeroacoustic, direct numerical simulations of a forced, low-Mach-number, laminar, premixed flame. These budgets show that several source terms are significant, including those involving the mean-flow and entropy fields. This demonstrates that the energetics of sound generation cannot be examined by considering the Rayleigh source term alone.

This paper presents a comparative analysis of the budgets of acoustic energy and Myers' second-order ‘disturbance energy’ in a simple inhomogeneous flow with heat communication. The flow considered is non-diffusive and one-dimensional, with excitation by downstream-travelling acoustic and entropic disturbances. Two forms of heat communication are examined: a case with only steady heat communication and another in which unsteady heat addition cancels the generation of entropy disturbances throughout the inhomogeneous region.It is shown that significant entropic disturbances are usually generated at low frequency when a flow with steady heat communication is excited either acoustically or entropically. However, for acoustic excitation and regardless of the form of heat communication, entropic disturbances are not created at high frequency, inferring that all source terms create mainly sound in this limit. A general method is therefore proposed for determining an approximate frequency beyond which the generation of entropy disturbances can be ignored, and the disturbance energy flux then approximates the acoustic energy flux. This frequency is shown to depend strongly on the problem under investigation, which is expected to have practical significance when studying sound generation and propagation in combusting flows in particular. Further, sound is shown to be generated by fluid motion experiencing only steady heat communication, which is consistent with the known mechanism of sound generation by the acceleration of density disturbances.

This paper explores the application of an ecosystem ecology framework to greenroof systems. It investigates how aspects of greenroof design or structure relate to functions such as rates of nutrient and energy cycling. Three main sections include energy budgets, cycling of nutrients and water, and ecosystem response to disturbance. Comparisons between greenroofs and other systems indicated that, functionally, greenroofs may be very different from ecosystem analogs. A further assessment of the greenroof energy budget called into question how food webs are supported. An evaluation of factors predicting system response to disturbance identified ways in which greenroofs may be less resilient to disturbance. One challenge with the ecosystem approach is a lack of sufficient data for fully holistic models, especially with respect to management practices. Ecosystem ecology is nevertheless shown to be a valuable framework for integrating existing greenroof research as well as targeting areas for future research and model development.

We examined whether the fall activity budgets of greater snow geese (Chen caerulescens atlantica) vary between years and sites by comparing time-budget data gathered from 1982 to 1984 at the Montmagny and Cap-Saint-Ignace sanctuaries in Quebec. Feeding and resting accounted for more than 70% of the activities during the daylight period. At Cap-Saint-Ignace, geese spent less time feeding and more time resting during the 2nd year of the study, whereas no such annual variation was found at Montmagny. Snow geese also spent more time feeding and less time resting at Montmagny than at Cap-Saint-Ignace. These differences are discussed in relation to energy requirements, quality of feeding sites, temperature, and disturbance associated with hunting.

Anthropogenic underwater noise may negatively affect marine animals. Yet, while fishes are highly sensitive to sounds, effects of acoustic disturbances on fishes have not been extensively studied at the population level. In this study, we use a size-structured model based on energy budgets to analyse potential population-level effects of anthropogenic noise on Atlantic cod ( Gadus morhua ). Using the model framework, we assess the impact of four possible effect pathways of disturbance on the cod population growth rate. Through increased stress, changes in foraging and movement behaviour, and effects on the auditory system, anthropogenic noise can lead to (i) increased energy expenditure, (ii) reduced food intake, (iii) increased mortality, and (iv) reduced reproductive output. Our results show that population growth rates are particularly sensitive to changes in energy expenditure and food intake because they indirectly affect the age of maturation, survival and fecundity. Sub-lethal effects of sound exposure may thus affect populations of cod and fishes with similar life histories more than lethal effects of sound exposure. Moreover, anthropogenic noise may negatively affect populations when causing persistent increases of energy expenditure or decreases of food intake. Effects of specific acoustic pollutants on energy acquisition and expenditure should therefore be further investigated.

Abstract Time-activity budgets are fundamental to behavioural studies, allowing examination of how individuals allocate their time, and potentially energy, and how these patterns vary spatially and temporally and in relation to habitat, individual identity, sex, social status and levels of anthropogenic disturbance. Direct observations of animal behaviour, especially in the wild, are often limited to daylight hours; therefore, many activity budgets relate to diurnal activity only, or assumptions are made about nocturnal activity. Activity budgets have been a key component of many behavioural and energetics studies of breeding grey seals (Halichoerus grypus, Fabricius, 1791), and yet very little is known about nocturnal activity of grey seals, and a general, implicit assumption of no significant change from day to night seems to pervade the literature. Here we use a combination of high resolution digital video and thermal imaging video camera to follow known individual grey seal mothers from day into night to examine activity patterns during lactation. We show distinct differences in nocturnal activity budgets relative to diurnal activity budgets. Mothers spent significantly more time resting with a reduction of time spent in the alert and comfort move behavioural categories during nocturnal periods. It is clear that diurnal time-activity patterns of breeding female grey seals cannot be extrapolated to represent activity across a 24-hour cycle. These considerations are particularly critical in studies that aim to use time-activity budgets as proxies for energy budgets.

Anthropogenic disturbance of farmed animals can be detrimental by adversely affecting behaviours and metabolic rate, potentially reducing their commercial value. However, relatively little is known about the normal behavioural time budgets and associated metabolism of many such species, particularly for example pectinid bivalves, which use anaerobic metabolism during periods of short-burst activity. In the present study, we used the accelerometry technique to measure scallop overall dynamic body acceleration in combination with respirometry in order to obtain and compare the behavioural time budgets and associated metabolism of 10 scallops, Pecten maximus , in an aquaculture hatchery and 10 in the wild. Scallops in the wild typically spent only 0.1 per cent of the time moving (less than 2 min d −1 ), yet, on average, the estimated metabolism of such movement represented 16.8 per cent of daily energy expenditure. Furthermore, owing to their reliance on anaerobic pathways during such activity, movement resulted in the wild scallops having a raised metabolic rate for, on average, an estimated 7.8 per cent of the time, during which oxygen debts accumulated during movement were paid off. Hatchery scallops also typically spent only 0.1 per cent of the time moving but estimated metabolism of such movement represented 41.8 per cent of daily energy expenditure. Estimated mean daily metabolism of scallops in the hatchery was significantly higher than scallops in the wild (169.1 versus 120.7 mg O 2 d −1 ) because anthropogenic disturbance in the hatchery caused energetically costly non-feeding behaviours. Consequently, hatchery scallops also spent a far greater amount of time with a raised metabolic rate (an estimated 26.6% of the time) than wild scallops. While short-term bursts of movement in pectinid bivalves may appear innocuous, they result in large expenditures of energy and an oxygen debt that is paid off over long periods of time that together limit further movement. These findings have implications for the farming industry; mitigating anthropogenic disturbances to farmed colonies may minimize non-feeding behaviours and hence maximize growth rates by reducing the costs of such movements and increasing the opportunity to feed.

La conception des futurs statoréacteurs bénéficierait d'une meilleure compréhension des instabilités de combustion apparaissant dans les écoulements rapides présentant des recirculations : une problématique étudiée à l'ONERA avec le banc MICAEDI au LAboratoire des Ecoulements Réactifs et de leurs Techniques d'Etude (LAERTE) de l'ONERA. Dans ce dispositif, une flamme de prémélange méthane-air s’établit au voisinage d’une marche descendante, dans un environnement présentant des similitudes avec les foyers de statoréacteurs. Pour certains points de fonctionnement, d’importantes oscillations de pression accompagnées de remontées de flammes (flashbacks) périodiques y apparaissent sous l'effet de l'excitation de modes acoustiques longitudinaux. Dans le cadre cette thèse, le code de calcul CEDRE est utilisé afin de restituer ces instabilités, les analyser et en améliorer la compréhension. Pour cela, une méthodologie de post-traitement basée sur les bilans d’énergie des perturbations (BEP) a été appliquée tout d'abord dans le cas d'une flamme de prémélange unidimensionnelle forcée acoustiquement. Grâce à ce formalisme, la dynamique de cette flamme peut alors être décrite au travers d'une loi d'échelle au moyen de deux nombres adimensionnels : (i) un nombre de Strouhal associé aux mouvements de la flamme qui compare son amplitude de battement à l’épaisseur de flamme laminaire et (ii) l’amplitude de la perturbation en vitesse adimensionnée par la vitesse de propagation de flamme laminaire. Sont ensuite réalisées des simulations numériques aux grandes échelles de flammes s'établissant au voisinage d'une marche descendante. En deux dimensions (2D), une telle approche permet de restituer, à moindre coût de calcul, une dynamique de combustion similaire à celle observée sur le banc MICAEDI. Une étude de sensibilité conduite sur les principaux paramètres opératoires met en évidence les processus associés au développement d'instabilités de combustion et à l'apparition de flashbacks. Il apparaît que la rétroactions entre des modes acoustiques longitudinaux et les oscillations du front de flamme sont favorisées si la symétrie des plissements de la flamme est rompue par l’action de la bulle de recirculation s'établissant au pied de la marche descendante. Ce mécanisme est favorisé si le mode acoustique présente un nœud de vitesse au niveau de la marche descendante et si sa fréquence est basse et/ou proche de la fréquence caractéristique de l'instabilité de Kelvin-Helmholtz. Des oscillations peuvent également apparaître à de plus basses fréquences lors du battement du point de raccordement de la flamme à la paroi supérieure. Ces oscillations peuvent provoquer des flashbacks suivant deux mécanismes : (i) synchronisation entre les oscillations des modes acoustiques et les détachements périodiques de la bulle de recirculation s'élevant au-dessus de la marche, bloquant la flamme et la faisant temporairement remonter en amont, avant d'être convectée plus en aval et (ii) action de la flamme qui, lors de sa propagation dans la couche limite à de basses fréquences, y cause son décollement et favorise ainsi l'apparition de nouveaux flashbacks à de plus hautes fréquences. Si les détachements de la bulle provoquent des flashbacks pour des niveaux importants d'oscillations de vitesse (la direction de l'écoulement est proche de s'inverser, voire s'inverse périodiquement), les remontées de flamme par la couche limite sont susceptibles d'apparaître pour des niveaux d'oscillation plus modérés. Pour les simulations en trois dimensions (3D), l'utilisation d'une géométrie représentative du foyer MICAEDI permet une restitution fine des instabilités observées. En effet, le cycle limite obtenu numériquement présente de nombreuses similitudes avec les données expérimentales. Les principaux mécanismes analysés sur les simulations 2D sont également retrouvés sur ce cas 3D assurant une modélisation plus fidèle de la turbulence et de l'acoustique, validant ainsi l'ensemble
The design of future ramjets would benefit from a better understanding of the combustion instabilities that occur in flows featuring recirculation zones : a problem studied at ONERA by the means of the MICAEDI combustor from the LAERTE test facility. In this experimental setup, a methane-air premixed flame stabilizes at the vicinity of a backward-facing step, in an environment comparable to ramjet combustors. At certain operating points, significant pressure oscillations accompanied by periodic flame flashbacks appear due to the triggering of longitudinal acoustic modes. In this thesis work, the CEDRE CFD solver is used in order to restitute those instabilities, to study them and to improve their understanding. A post-processing methodology based on disturbance energy budgets (DEB) is elaborated. Its first application to the case of a one-dimensional acousticallyforced premixed flame puts into evidence two dimensionless numbers that can be used to describe its dynamicsby means of scaling laws : (i) a Strouhal number associated with the the flames motions that compares its flapping magnitude to the laminar flame thickness, and (ii) the magnitude of the velocity perturbation normalized using the laminar flame propagation speed. Large-eddy simulations of flames stablized in the vicinity of a backward-facing step are then carried out. In two dimensions (2D), this approach makes it possible to reproduce combustion dynamics similar to that observed in the MICAEDI experiment at a moderate computational cost. A sensitivity study is conducted on the operating parameters to clarify the phenomenology associated with the development of combustion instabilities and the occurence of flame flashbacks. It appears that the feedback loop between longitudinal acoustic modes and flame front oscillations is favoured if the symmetry of the flame wrinkles is broken by the action of the recirculation bubble establishing itself at the foot of the step. This mechanism is favoured if the acoustic mode displays a velocity node at the vicinity of the step and if its frequency is low and/or close to the characteristic frequency of the Kelvin-Helmholtz instability. Oscillations may also occur at lower frequencies as a result of flapping of the point where the flame re-attaches to the combustor upper wall.These oscillations can cause flashbacks by the mean of two mechanisms : (i) synchronization between the oscillations of the acoustic modes and the periodic detachments of the recirculation bubble which then rises above the step before being convected downstream, a process during which the flame is transported upstream and (ii)action of the flame which, during its propagation in the boundary layer at low frequencies, causes its detachment and thus favours the birth of new flashbacks at higher frequencies. While bubble detachment causes flashbacks at high levels of velocity oscillation (the direction of the flow is close to reversing, or even reverses periodically), boundary layer flashbacks are likely to occur at more moderate levels. In three dimensions (3D), the use of a geometry representative of the MICAEDI combustor allows a detailed reproduction of the observed instabilities. Indeed, the limit cycle obtained numerically shows many similarities with the experimental data. The main mechanisms analysed on the 2D simulations are also observed on this 3D case, ensuring a more accurate modelling of the turbulence and acoustics, thus validating the whole approach followed in this manuscript

Wildfires can profoundly impact many aspects of matter flows and energy budgets in ecosystems. Exacerbated by projected shifts in climate, land use, and forest management, changes in fire regimes can lead to decreased ecosystem resilience, regime shifts, and ecosystem collapse. Thorough assessments of ecosystem resilience to wildfires are thus critical to bridge gaps between science, policy, and management. To that end, approaches based on ecosystem functioning offer an integrative view of ecosystem responses to wildfire-induced changes and provide quicker, quantifiable responses to disturbances that are more directly connected to ecosystem services. In that regard, satellite remote sensing can be employed to easily and frequently monitor multiple dimensions of ecosystem functioning over large areas and across time, and to evaluate ecosystem functioning resilience to wildfires. This study describes an approach for identifying potential regime shifts based on satellite-based surrogates of four key dimensions of ecosystem functioning: primary production, water content, albedo, and sensible heat. To that end, we classified the trajectories after wildfires in 2005, in NW Iberian Peninsula, for the 2000–2018 period, into five main types, using two metrics of medium-to-long term post-fire recovery. Then, we derived a synthetic indicator to analyse the overall “strength-of-evidence” of potential regime shifts across dimensions. Potential regime shifts were identified for each dimension of ecosystem functioning considered, with the main effects associated with the sudden removal of vegetation. For primary production, regime shifts may be linked to changes in land cover and use, as well as management. Changes in the concentrations of impervious and radiation-absorbing materials following wildfires may be responsible for regime shifts in water content and albedo, with loss of canopy moisture due to fire-related damage leading to vegetation mortality during post-fire recovery. On the other hand, regime shifts in sensible heat were less frequent, since wildfires tend to have transient effects on this dimension of ecosystem functioning. Overall, our results show that our approach successfully captured different patterns of post-fire recovery and resilience across multiple dimensions of ecosystem functioning. We argue that our approach can provide an enhanced characterization of ecosystem resilience to wildfires, and support the identification of potential regime shifts after such disturbances, ultimately upholding promising implications for post-fire ecosystem management.

Production equipment such as machines have crucial impact on the overall performance of production operations in manufacturing industries, since there is a strong correlation between the machines and working conditions and performance on the shop floor. Well designed production equipment has the potential to achieve economic gain by reducing the disturbances during the operational phase, to fulfill environmental commitment by reducing emissions and resources consumption and utility, and to increase employee satisfaction ensuring safety and good ergonomics. Therefore, when acquiring production equipment it is important to consider different sustainability aspects relevant to its usage during the operational phase. This study aims at exploring the critical features of production equipment to facilitate different practices in the context of sustainable production operational system, and how manufacturing companies are considering sustainability aspects when acquiring production equipment. The data has been collected based on a literature study, interviews conducted in different manufacturing companies located in Sweden, attending group discussion sessions, and reviewing machines’ technical regulation guidelines. Some of the critical features identified are error proofing, setup time, one-piece flow, automatic generation of required data, reduction of energy and resource consumption, together with worker’s health and safety, etc. The data indicates that companies specify different features of machines based on the requirements of operational performance and these features are aligned with different lean techniques, green practice, and safety issues. However, during acquisition process of production equipment the environmental issues are still not prioritized yet compared to lean and safety aspects. Budget constraint, insufficient information of the whole life cycle costing and lack of innovation from the equipment suppliersÂť side are exampled of major barriers for acquiring more environment-friendly production equipment.

The noise generated by the passage of acoustic and entropy perturbations through subsonic and choked nozzle flows is investigated numerically using an energetic approach. Low-order models are used to reproduce the experimental results of the Hot Acoustic Test rig (HAT) of DLR and energy budgets are performed to characterize the reflection, transmission and dissipation of the fluctuations. Because acoustic and entropy perturbations are present in the flow in the general case, classical acoustic energy budgets cannot be used and the disturbances energy budgets proposed by Myers (J. Fluid Mech. 226 (1991) 383–400) are used instead. Numerical results are in very good agreement with the experiments in terms of acoustic transmission and reflection coefficients. The normal shock present in the diffuser for choked regimes is shown to attenuate the scattered acoustic fluctuations, either by pure dissipation effect or by converting a part of the acoustic energy into entropy fluctuations.

Electrohydrodynamic jet (E-jet) printing is a recent technique for high resolution additive micromanufacturing. With high resolution comes sensitivity to small disturbances, which has kept this technique from reaching its industrial potential. Closed loop control of E-jet printing can overcome these disturbances, but it requires an improved understanding of ink droplet spreading on the substrate and a physical model to predict printed feature locations and geometries from process inputs and disturbances. This manuscript examines a model of ink droplet spreading that uses assumptions that are important to the e-jet process. Our model leverages previous energy balance models that were derived for larger length scale droplets. At the smaller length scale, we find that viscous losses are a significant portion of the energy budget and must be accounted for; this is in contrast to models at length scales two orders of magnitude larger. Our model predicts the droplet height, base radius and contact angle in time from an initial volume and E-jet printing control parameters. The model is validated with published droplet spreading data and new measurements of E-jet printed droplets of diameter 8 μm. The viscous friction calculated in the new model is found to be significant compared to surface energy.

Vegetation phenology is the study of periodically recurring patterns of growth and development of plants, which affect terrestrial ecosystem carbon, energy budget balance, fire disturbance, and climate– biosphere interactions. The increases in surface temperature had already altered the extent of vegetation phenology. Vegetation phenology can make some responses to climate factors, and the current climate change has attracted more research for the trend of vegetation phenology and its causes. The purpose of this paper is to investigate the spatial and temporal trend of forest phenology at mid and high latitude in the Northern Hemisphere (50°N-90°N, 180°W-180°E) over the period 2001–2017 using Collection 6 MODIS Land Cover Dynamics (MCD12Q2) datasets. The results indicated that SOS has a significant advanced trend, EOS has a significant delayed trend and LOS showed a significant extended trend on the whole. The significant advancement of SOS and extension of LOS mainly occurred in central Russia, the north and southwest of North America. Meanwhile, EOS showed a delayed trend in the south of Russia, the north and southwest of Canada and Alaska.

Abstract Due to the narrow operating range with the high efficiency of Wells turbines, the performance of Oscillating Water Column (OWC) systems is extremely limited by the stall issue. In this study, a detailed investigation on the flow physics at three typical flow rates corresponding to the design, stall inception, and deep stall conditions is performed, aiming to explore the mechanisms governing the evolution of the leading-edge vortex in terms of the circulation budget analysis. For this purpose, 3D unsteady Reynolds-averaged Navier-Stokes equations with the shear-stress-transport turbulence model are solved for a monoplane Wells turbine with high solidity under the steady-state inflow conditions. The numerical results show encouraging agreement with the available experimental data. The transient flow behaviors of stall inception and deep stall mode are captured by the Proper Orthogonal Decomposition (POD) method. Furthermore, the contributions of the spanwise convection of the vorticity gradient, vorticity tilting term, and the shear layer flux to the circulation of the leading-edge vortex are quantitatively determined within the spanwise control surface. The results indicate that, for stall inception condition, the high turbulent kinetic energy regions are attached to the leading edge of the suction surface with a disturbance of 0.33 times of the rotor frequency. The leading-edge vortex appears after the turbine stall, and its stable development time is less than 1/4 of the rotor period. Although leading-edge vortex occurs periodically after the first shedding with a significant decrease of circulation, it is almost not reattached to the blade surface, which speeds up the shedding and vorticity dissipation. The first shedding of the leading-edge vortex is related to the increase of radial vorticity gradient, but not directly related to the radial velocity. Furthermore, the spatiotemporal distribution of the circulation is similar to the spanwise convection, but there is a phase difference. In addition, the tilting term changes greatly in the evolution of the leading-edge vortex, indicating that the tangential and axial vorticity are tilted, which is verified by the spatial streamlines.

Numerical investigation of the compressible flow in the Turbine Center Frame (TCF) duct was carried out using a Reynolds-averaged Navier-Stokes (RANS) method, and a Hybrid RANS/Large Eddy Simulation (HLES) method, i.e. Stress-Blended Eddy Simulation (SBES). The reference Reynolds number based on the TCF inlet condition is 530,000, and the inlet Mach number is 0.41. It is found that the boundary layer flow behavior is very sensitive to the incoming turbulence characteristics, so the upstream grid used to generate turbulence in the experiment is also included in the computational domain. Results have been validated carefully against experimental data, in terms of static pressure distribution on hub and casing walls, total pressure and Mach number profiles on the TCF measurement planes, as well as over-all pressure loss coefficient. Further, various fundamental mechanisms dictating the intricate flow phenomena, including concave and convex curvature effects, interactions between inlet turbulent structures and boundary layer, and turbulent kinetic energy budget, have been studied systematically. The current study is to evaluate the performance of HLES method for TCF flows and develop a further understanding of unsteady flow physics in the TCF duct. The results obtained in this work provide physical insight into the mechanisms relevant to the turbine intercase or TCF duct flows subjected to complex inlet disturbances.