Turbulent flows are characterized by the non-linear cascades of energy and other inviscid invariants across a huge range of scales, from where they are injected to where they are dissipated. ...Recently, new experimental, numerical and theoretical works have revealed that many turbulent configurations deviate from the ideal three and two dimensional homogeneous and isotropic cases characterized by the presence of a strictly direct and inverse energy cascade, respectively. New phenomena appear that alter the global and local transfer properties. In this review, we provide a critical summary of historical and recent works from a unified point of view and we present a classification of all known transfer mechanisms. Beside the classical cases of direct and inverse energy cascades, the different scenarios include: split cascades for which an invariant flows both to small and large scales simultaneously, multiple/dual cascades of different quantities, bi-directional cascades where direct and inverse transfers of the same invariant coexist in the same scale-range and finally equilibrium states where no cascades are present, including the case when a large scale condensate is formed. We classify all possible transitions from one scenario to another as the control parameters are changed and we analyse when and why different configurations are observed. Our discussion is based on a set of paradigmatic applications: helical turbulence, rotating and/or stratified flows, magnetohydrodynamics (MHD) turbulence, and passive/active scalars where the transfer properties are altered as one changes the embedding dimensions, the thickness of the domain or other relevant control parameters, as, e.g., the Reynolds, Rossby, Froude, Péclet, or Alfvén numbers. We briefly discuss the presence of anomalous scaling laws in 3D hydrodynamics and in other configurations, in connection with the intermittent nature of the energy dissipation in configuration space. A quick overview is also provided concerning the importance of cascades in other applications such as bounded flows, quantum fluids, relativistic and compressible turbulence, and active matter, together with a discussion of the implications for turbulent modelling. Finally, we present a series of open problems and challenges that future work needs to address.
Recently, the use of multi-rotor (MR) unmanned aerial vehicles (UAVs) has emerged as a promising solution for establishing flexible free-space optical communication links. We address, in this paper, ...the accurate channel modeling to assess the benefits of MR UAV-based deployment for such links. In particular, in the absence of active tracking subsystems, we derive statistical models for ground-to-UAV, UAV-to-UAV, and UAV-to-ground links over both Gamma-Gamma and log-normal atmospheric turbulence models. Unlike previous works on this topic, our proposed model considers the joint effect of atmospheric turbulence along with position and angle-of-arrival fluctuations. The high accuracy of the proposed analytical models is verified by comparing numerically solved and Monte Carlo simulation results in terms of link outage probability. We further discuss the impact of different transmitter/receiver parameters and their optimization in view of maximizing the link availability.
This paper investigates pollutant removal at pedestrian level in urban canopy layer (UCL) models of medium packing density (λp = λf = 0.25) using computational fluid dynamics (CFD) simulations. Urban ...size, building height variations, wind direction and uniform wall heating are investigated. The standard and RNG k−ε turbulence models, validated against wind tunnel data, are used. The contribution of mean flows and turbulent diffusion in removing pollutants at pedestrian level is quantified by three indicators: the net escape velocity (NEV), the pollutant transport rate (PTR) across UCL boundaries and their contribution ratios (CR).
Results show that under parallel approaching wind, after a wind-adjustment region, a fully-developed region develops. Longer urban models attain smaller NEV due to pollutant accumulation. Specifically, for street-scale models (∼100 m), most pollutants are removed out across leeward street openings and the dilution by horizontal mean flows contributes mostly to NEV. For neighbourhood-scale models (∼1 km), both horizontal mean flows and turbulent diffusion contribute more to NEV than vertical mean flows which instead produce significant pollutant re-entry across street roofs. In contrast to uniform height, building height variations increase the contribution of vertical mean flows, but only slightly influence NEV. Finally, flow conditions with parallel wind and uniform wall heating attain larger NEV than oblique wind and isothermal condition.
The paper proves that by analysing the values of the three indicators it is possible to form maps of urban breathability according to prevailing wind conditions and known urban morphology that can be of easy use for planning purposes.
•Pollutant removal in UCL models of medium building density is studied.•Pollutant transport rate (PTR) and net escape velocity (NEV) are used as ventilation indicators.•NEV is smaller in UCLs with longer streets due to pollutant accumulation effect.•Parallel wind and wall heating get bigger NEV than oblique wind and isothermal case.•Building height variations enhance pollutant removal across street roofs.
En las toberas supersónicas, ocurren diferentes patrones de flujo y su comportamiento está influenciado por las geometrías de las superficies internas de las paredes. En el presente trabajo se simula ...en 2D el campo de flujo sobrexpandido en la tobera cónica experimental ULA-1B fuera de diseño, para dos casos de longitudes de garganta: L.sub.g = 15 mm y L.sub.g = 1 nun; con el fin de analizar el campo de número de Mach, presión y temperatura. Se empleó el código ANSYS-Fluent y se aplicó el modelo RANS; las ecuaciones gobernantes: conservación de la masa, cantidad de movimiento, energía, y estado; así como el modelo de turbulencia de Menter y la ecuación de Sutherland para la viscosidad en función de la temperatura. En la sección de la garganta de mayor longitud, los resultados mostraron choques oblicuos, fluctuaciones de velocidad, presión y temperatura; para la garganta de menor longitud no hubo fluctuaciones; para ambos casos, el flujo en la divergente presentó picos de velocidad en el rango de 2,5 - 3 Mach. Se concluye que, para la menor longitud de garganta el flujo se acelera sin perturbaciones en dicha sección; en la divergente se presenta un chorro supersónico y una separación de flujo.
We present Multi Unit Spectroscopic Explorer (MUSE) observations of ESO137−001, a spiral galaxy infalling towards the centre of the massive Norma cluster at z ∼ 0.0162. During the high-velocity ...encounter of ESO137−001 with the intracluster medium, a dramatic ram-pressure stripping event gives rise to an extended gaseous tail, traced by our MUSE observations to >30 kpc from the galaxy centre. By studying the Hα surface brightness and kinematics in tandem with the stellar velocity field, we conclude that ram pressure has completely removed the interstellar medium from the outer disc, while the primary tail is still fed by gas from the inner regions. Gravitational interactions do not appear to be a primary mechanism for gas removal. The stripped gas retains the imprint of the disc rotational velocity to ∼20 kpc downstream, without a significant gradient along the tail, which suggests that ESO137−001 is fast moving along a radial orbit in the plane of the sky. Conversely, beyond ∼20 kpc, a greater degree of turbulence is seen, with velocity dispersion up to ≳100 km s−1. For a model-dependent infall velocity of v
inf ∼ 3000 km s−1, we conclude that the transition from laminar to turbulent flow in the tail occurs on time-scales ≥6.5 Myr. Our work demonstrates the terrific potential of MUSE for detailed studies of how ram-pressure stripping operates on small scales, providing a deep understanding of how galaxies interact with the dense plasma of the cluster environment.
Supply risk or the likelihood of supply disruptions is emerging as a key challenge to supply chain management. The ability to identify which supplier has greater potential of a disruption is a ...critical first step in managing the frequency and impact of these disruptions that often significantly impact on the supply chain. This paper presents preliminary research concepts regarding a new approach to the identification and prediction of supply risk. This approach to the assessment and classification of suppliers is based on supplier's attributes, performances and supply chain characteristics, while it is also modified by factors in the supplier's specific environment. The challenges posed to supply chains due to a turbulent environment (both from within the industry and external influences) are examined. A new method for the assessment and classification of suppliers based on their characteristics, performances and the environment of the industry in which they operate is presented. The findings are explained within the contingency theory.
Context. We examine the dynamics of low-frequency waves in differentially rotating stellar radiation zones, the angular velocity being taken as generally as possible depending both on radius and on ...latitude in stellar interiors. The associated induced transport of angular momentum, which plays a key role in the evolution of rotating stars, is derived. Aims. We focus on the wave-induced transport of angular momentum, taking into account the Coriolis acceleration in the case of strong radial and latitudinal differential rotation. We thus go beyond the “weak differential rotation” approximation, where rotation is almost a solid-body one plus a residual radial differential rotation. As has been shown in previous works, the Coriolis acceleration modifies such transport. Methods. We built analytically a complete formalism that allows the study of rotational transport in stellar radiation zones taking into account the wave action modified by a general strong differential rotation. Results. The different approximations possible for low-frequency waves in a differentially rotating stably stratified radiative region, namely the traditional and the JWKB approximations, are examined and discussed. The complete bidimensional structure of regular elliptic gravito-inertial waves, which verify these approximations, is derived and compared to those in the “weak differential rotation” case. Next, associated transport of energy and of angular momentum in the vertical and in the horizontal directions and the dynamical equations, respectively for the mean radial differential rotation and the latitudinal one, are obtained. Conclusions. The complete formalism, which takes into account low-frequency wave action, is derived and can be used for the study of secular hydrodynamics of radiative regions and of the associated mixing. The modification of waves due to general strong differential rotation and their feed-back on the angular momentum transport are treated rigourously. In a forthcoming paper (Paper II), this formalism will be applied to the case of solar differential rotation. However, the case of hyperbolic gravito-inertial waves should be carefully studied.
Desde los primeros desarrollos en el secado por aspersión en 1870, se han realizado numerosas contribuciones para obtener parámetros de diseño. Sin embargo, se requieren estudios experimentales para ...diseñar, optimizar y construir equipos de secado por aspersión para aplicaciones específicas. A partir de la información disponible en libros, catálogos y patentes, se han seleccionado las dimensiones y condiciones de operación para realizar el modelado matemático de un secador por aspersión en co- corriente, comenzando con un modelo relativamente simple de flujo monofásico incompresible y luego utilizando un modelo más complejo que contempla el flujo multifásico. Los objetivos de este trabajo fueron: (i) evaluar tres diseños diferentes de un equipo de secado por aspersión mediante Dinámica de Fluidos Computacional (CFD); y (ii) seleccionar el diseño con las mejores características de flujo y transferencia de calor. Se evaluaron los perfiles de velocidad, las líneas de flujo, los mapas de presión y los perfiles de temperatura en cada diseño. A partir de los resultados, se construyó un nuevo prototipo con la modificación en la tapa del equipo y se realizaron pruebas experimentales para demostrar la efectividad del nuevo diseño.
Since the first developments in spray drying in 1870, attempts have been made to obtain design parameters. However, experimental studies are required to design, optimize, and build spray drying equipment for specific applications. From the information reported in books, catalogs and patents, the dimensions and operating conditions have been selected to develop the mathematical modeling of a co-current spray dryer, starting with a relatively simple model of incompressible single-phase flow, and then using a more complex method with multi- hase flow. The objectives of this study were to: (i) evaluate three different designs of spray-drying equipment; and (ii) select the design that provided the best flow and heat transfer characteristics. The velocity profiles, flow lines, pressure maps and temperature profiles of each design were evaluated. Based on the results obtained, a new prototype with the modification on the equipment lid was built and experimental tests were conducted to demonstrate the effectiveness of the new design.
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