Particle-laden two-dimensional elastic turbulence

The aggregation properties of heavy inertial particles in the elastic turbulence regime of an Oldroyd-B fluid with periodic Kolmogorov mean flow are investigated by means of extensive numerical simulations in two dimensions. Both the small and large scale features of the resulting inhomogeneous particle distribution are examined, focusing on their connection with the properties of the advecting viscoelastic flow. We find that particles preferentially accumulate on thin highly elastic propagating waves and that this effect is largest for intermediate values of particle inertia. We provide a quantitative characterization of this phenomenon that allows to relate it to the accumulation of particles in filamentary highly strained flow regions producing clusters of correlation dimension close to 1. At larger scales, particles are found to undergo turbophoretic-like segregation. Indeed, our results indicate a close relationship between the profiles of particle density and fluid velocity fluctuations. The large-scale inhomogeneity of the particle distribution is interpreted in the framework of a model derived in the limit of small, but finite, particle inertia. The qualitative characteristics of different observables are, to a good extent, independent of the flow elasticity. When increased, the latter is found, however, to slightly reduce the globally averaged degree of turbophoretic unmixing.Comment: 12 pages, 9 figures. Submitted to EPJ

Invasions in heterogeneous habitats in the presence of advection

We investigate invasions from a biological reservoir to an initially empty, heterogeneous habitat in the presence of advection. The habitat consists of a periodic alternation of favorable and unfavorable patches. In the latter the population dies at fixed rate. In the former it grows either with the logistic or with an Allee effect type dynamics, where the population has to overcome a threshold to grow. We study the conditions for successful invasions and the speed of the invasion process, which is numerically and analytically investigated in several limits. Generically advection enhances the downstream invasion speed but decreases the population size of the invading species, and can even inhibit the invasion process. Remarkably, however, the rate of population increase, which quantifies the invasion efficiency, is maximized by an optimal advection velocity. In models with Allee effect, differently from the logistic case, above a critical unfavorable patch size the population localizes in a favorable patch, being unable to invade the habitat. However, we show that advection, when intense enough, may activate the invasion process.Comment: 16 pages, 11 figures J. Theor. Biol. to appea

Bile leak after elective laparoscopic cholecystectomy: Role of MR imaging

Increasing hepatobiliary laparoscopic surgeries have lead to a rise in injury to the biliary tree and other complications like bile leak. Ultrasonography (US) and computed tomography (CT) cannot reliably distinguish bile from other postoperative fluid collections. Magnetic resonance (MR) imaging with hepatobiliary agents and MR cholangiopancreatography provide anatomic and functional information that allows for prompt diagnosis and excludes any other concomitant complications. We report a case of post-cholecystectomy bile leak in a 42-year-old female who presented with persistent dull abdominal pain after the intervention; we emphasize the role of MR imaging in achieving the correct diagnosis

Kako smo poboljšanim mjerama praćenja izloženosti i zaštite na radu snizili razine formaldehida u zraku u sveučilišnoj bolnici u Italiji: sažetak 20 godina iskustava

The last two decades have been crucial for the assessment of airborne formaldehyde (FA) exposure in healthcare environments due to changes in limits and reference values, definition of carcinogenicity, and new monitoring methods. The aim of this study was to analyse twenty years (1999–2019) of experience in automatic, continuous airborne FA monitoring in the Pathology Laboratory and operating rooms at the Careggi University Hospital, Florence, Italy. These 20 years saw gradual improvements in FA monitoring of exposed employees considered at maximum risk, including improvements in analytical methods of detection and sampling strategies, which came with changes in procedures and workflow operations. In 2019, after the adoption of safe practices, including a closed-circuit system using pre-loaded containers and a vacuum sealing, 94 % of the total measurements (FA concentrations) were lower than 16 μg/m3, and only 6 % ranged from 21 to 75 μg/m3. In the studied work units, the ratio between area and personal readings ranged from 0.9 to 1.0, both for long and short-term sampling. Personal sampling was simplified with a new workstation, which integrated different monitoring systems into an innovative ergonomic armchair equipped with personal sampling devices. Area monitoring was also improved with a real-time, continuous photoacoustic instrument. Over these 20 years, FA exposure significantly dropped, which coincided with optimised histology workflow and implementation of safety practices. For high-throughput screening and cost savings we propose an innovative ergonomic armchair station which allows remote continuous monitoring.Posljednja dva desetljeća bila su iznimno važna za procjenu izloženosti formaldehidu (FA) u zraku u zdravstvenim ustanovama zahvaljujući promjenama u preporučenim maksimalnim i referentnim vrijednostima, definiciji njegove kancerogenosti i novim metodama mjerenja/praćenja. Cilj je ovog istraživanja bio analizirati dvadeset godina (1999. – 2000.) iskustva u automatskom, kontinuiranom mjerenju razina FA-a u laboratoriju za patologiju i operacijskim dvoranama talijanske sveučilišne bolnice Careggi u Firenzi. Tijekom tih dvadeset godina bolnica je postupno poboljšavala metode praćenja razina FA-a i osoblja izloženoga najvećem riziku, analitičke metode detekcije i strategije uzorkovanja koje su bile popraćene promjenama u odgovarajućim postupcima i organizaciji rada. Nakon usvajanja novih postupaka zaštite na radu 2019., uključujući i zatvoreni sustav rukovanja spremnicima i sustav vakuumskoga zatvaranja, razine FA-a u 94 % izmjera bile su niže od 16 μg/m3, a samo 6 % izmjera kretalo se u rasponu od 21 do 75 μg/m3. Omjer izmjerenih razina prostornih i osobnih skupljača uzoraka u ispitanim scenarijima kretao se od 0,9 do 1,0, bez obzira na to je li posrijedi kratkoročno ili dugoročno mjerenje. Mjerenje osobnim mjeračima dodatno je pojednostavljeno novom radnom stanicom u obliku ergonomskoga radnog stolca, koji u sebi objedinjuje različite sustave praćenja i osobne mjerače. Nisu zaostala ni poboljšanja u prostornom mjerenju, budući da je uveden novi fotoakustični uređaj za kontinuirano mjerenje u stvarnom vremenu. U tih 20 godina izloženost FA-u drastično se smanjila, što je popraćeno poboljšanom organizacijom obrade histoloških uzoraka i zaštite na radu. Radnu stanicu u obliku ergonomskoga stolca svakako preporučujemo zbog velikog kapaciteta protoka za probirno mjerenje, značajnih ušteda i mogućnosti daljinskog upravljanja

Paralinearization and extended lifespan for solutions of the α \alpha -SQG sharp front equation

In this paper we paralinearize the contour dynamics equation for sharp-fronts of $\alpha$-SQG, for any $\alpha \in (0,1) \cup (1,2)$, close to a circular vortex. This turns out to be a quasi-linear Hamiltonian PDE. After deriving the asymptotic expansion of the linear frequencies of oscillations at the vortex disk and verifying the absence of three wave interactions, we prove that, in the most singular cases $\alpha \in (1,2)$, any initial vortex patch which is $\varepsilon$-close to the disk exists for a time interval of size at least $\sim \varepsilon^{-2}$. This quadratic lifespan result relies on a paradifferential Birkhoff normal form reduction and exploits cancellations arising from the Hamiltonian nature of the equation. This is the first normal form long time existence result of sharp fronts.Comment: 58 page

The random walk of a low-Reynolds-number swimmer

Swimming at a micrometer scale demands particular strategies. Indeed when inertia is negligible as compared to viscous forces (i.e. Reynolds number $Re$ is lower than unity), hydrodynamics equations are reversible in time. To achieve propulsion at low Reynolds number, swimmers must then deform in a way that is not invariant under time reversal. Here, we investigate dispersal properties of self propelled organisms by means of microscopy and cell tracking. Our system of interest is the micro-alga \textit{Chlamydomonas Reinhardtii}, a motile single celled green alga about 10 micrometers in diameter that swims with to two front flagella. In the case of dilute suspensions, we show that tracked trajectories are well modeled by a correlated random walk. This process is based on short time correlations in the direction of movement called persistence. At longer times, correlations are lost and a standard random walk characterizes the trajectories. Moreover, high speed imaging enables us to show how the back-and-forth motion of flagella at very short times affects the statistical description of the dynamics. Finally we show how drag forces modify the characteristics of this particular random walk

Surface light modulation by sea ice and phytoplankton survival in a convective flow model

Plankton dynamics depend in a complex manner on a variety of physical phenomena, according to both experimental and numerical data. In particular, experimental field studies have highlighted the relation between phytoplankton survival and turbulent upwelling and downwelling from thermal convection. Recent numerical works have also shown the importance of accounting for advective transport by persistent structures in simulation models. In nutrient-rich polar marine environments phytoplankton blooms are critically limited by light availability under ice-covered waters. Such heterogeneity of the light intensity distribution, in association with a large-scale coherent fluid flow, can give rise to nontrivial growth dynamics. In this work we extend a previous advection-reaction-diffusion model of phytoplankton light-limited vertical dynamics in the presence of convective transport. Specifically, we consider horizontally heterogeneous light conditions through the use of two regions with different production regimes, modelling the absence (presence) of light under (in between) obstacles. Such a model is intended as an idealized representation of nonuniformly ice-covered polar waters. By means of numerical simulations, we find that the main role of advective transport is to hinder phytoplankton growth, but also that such effect depends on the positions of the obstacles with respect to the upwelling and downwelling flow regions. Furthermore, we show that the sinking speed due to the density difference between phytoplankton organisms and water, while small, plays an important role, which depends on how it adds to the flow. These results indicate that advective transport can have a crucial impact on the survival conditions of sinking phytoplankton species in polar environments.Comment: 10 pages, 5 figure

Silviculture: forest products, certification and wood chain in Italy

This paper outlines the potential and critical issues of forest production in Italy, according to recent forest statistics (Italian National Institute of Statistics and Italian National Forest Inventory) and to the findings from the Third Italian National Congress of Silviculture. There is currently a growing interest towards forest products which meet sustainability goals through reduced impact on the forest ecosystems, improvement of human well being and a strong link to the territory; this concerns notably the demand for: wood as an optimal material for sustainable building; non-wood products; wood biomass for energy use. Indeed, sustainable forest management is not in conflict with a productive use of forestland, without which abandonment and land neglection could further occur. To meet these new societal demands through a viable and sustainable use of forest resources is a true challenge; forest owners must be supported in this regard, to supply a variety of forest goods and services while maintaining economic viability. To this end operative strategies are outlined