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1. Turbulence, entrainment and low-order description of a transitional variable-density jet

   https://doi.org/10.1017/jfm.2017.822  

   Viggiano, B.; Dib, T.; Ali, N.; Mastin, L. G.; Cal, R. B.; Solovitz, S. A. (February 2018, Journal of Fluid Mechanics)

   Geophysical flows occur over a large range of scales, with Reynolds numbers and Richardson numbers varying over several orders of magnitude. For this study, jets of different densities were ejected vertically into a large ambient region, considering conditions relevant to some geophysical phenomena. Using particle image velocimetry, the velocity fields were measured for three different gases exhausting into air – specifically helium, air and argon. Measurements focused on both the jet core and the entrained ambient. Experiments considered relatively low Reynolds numbers from approximately 1500 to 10 000 with Richardson numbers near 0.001 in magnitude. These included a variety of flow responses, notably a nearly laminar jet, turbulent jets and a transitioning jet in between. Several features were studied, including the jet development, the local entrainment ratio, the turbulent Reynolds stresses and the eddy strength. Compared to a fully turbulent jet, the transitioning jet showed up to 50 % higher local entrainment and more significant turbulent fluctuations. For this condition, the eddies were non-axisymmetric and larger than the exit radius. For turbulent jets, the eddies were initially smaller and axisymmetric while growing with the shear layer. At lower turbulent Reynolds number, the turbulent stresses were more than 50 % higher than at higher turbulent Reynolds number. In either case, the low-density jet developed faster than a comparable non-buoyant jet. Quadrant analysis and proper orthogonal decomposition were also utilized for insight into the entrainment of the jet, as well as to assess the energy distribution with respect to the number of eigenmodes. Reynolds shear stresses were dominant in Q1 and Q3 and exhibited negligible contributions from the remaining two quadrants. Both analysis techniques showed that the development of stresses downstream was dependent on the Reynolds number while the spanwise location of the stresses depended on the Richardson number. 

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2. Estimating pathogen‐spillover risk using host–ectoparasite interactions

   https://doi.org/10.1002/ece3.11509  

   Brennan, Reilly_N; Paulson, Sally_L; Escobar, Luis_E (June 2024, Ecology and Evolution)

   Abstract Pathogen spillover corresponds to the transmission of a pathogen or parasite from an original host species to a novel host species, preluding disease emergence. Understanding the interacting factors that lead to pathogen transmission in a zoonotic cycle could help identify novel hosts of pathogens and the patterns that lead to disease emergence. We hypothesize that ecological and biogeographic factors drive host encounters, infection susceptibility, and cross‐species spillover transmission. Using a rodent–ectoparasite system in the Neotropics, with shared ectoparasite associations as a proxy for ecological interaction between rodent species, we assessed relationships between rodents using geographic range, phylogenetic relatedness, and ectoparasite associations to determine the roles of generalist and specialist hosts in the transmission cycle of hantavirus. A total of 50 rodent species were ranked on their centrality in a network model based on ectoparasites sharing. Geographic proximity and phylogenetic relatedness were predictors for rodents to share ectoparasite species and were associated with shorter network path distance between rodents through shared ectoparasites. The rodent–ectoparasite network model successfully predicted independent data of seven known hantavirus hosts. The model predicted five novel rodent species as potential, unrecognized hantavirus hosts in South America. Findings suggest that ectoparasite data, geographic range, and phylogenetic relatedness of wildlife species could help predict novel hosts susceptible to infection and possible transmission of zoonotic pathogens. Hantavirus is a high‐consequence zoonotic pathogen with documented animal‐to‐animal, animal‐to‐human, and human‐to‐human transmission. Predictions of new rodent hosts can guide active epidemiological surveillance in specific areas and wildlife species to mitigate hantavirus spillover transmission risk from rodents to humans. This study supports the idea that ectoparasite relationships among rodents are a proxy of host species interactions and can inform transmission cycles of diverse pathogens circulating in wildlife disease systems, including wildlife viruses with epidemic potential, such as hantavirus. 

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3. Simplified models of aerosol collision and deposition for disease transmission

   https://doi.org/10.1038/s41598-023-48053-0  

   Jung, Sunghwan Sunny (December 2023, Scientific Reports)

   Abstract Fluid-mechanics research has focused primarily on droplets/aerosols being expelled from infected individuals and transmission of well-mixed aerosols indoors. However, aerosol collisions with susceptible hosts earlier in the spread, as well as aerosol deposition in the nasal cavity, have been relatively overlooked. In this paper, two simple fluid models are presented to gain a better understanding of the collision and deposition between a human and aerosols. The first model is based on the impact of turbulent diffusion coefficients and air flow in a room on the collisions between aerosols and humans. Infection rates can be determined based on factors such as air circulation and geometry as an infection zone expands from an infected host. The second model clarifies how aerosols of different sizes adhere to different parts of the respiratory tract. Based on the inhalation rate and the nasal cavity shape, the critical particle size and the deposition location can be determined. Our study offers simple fluid models to understand the effects of geometric factors and air flows on the aerosol transmission and deposition. 

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4. Liquid Coolant Jet Breakup with Application to Grinding

   https://doi.org/10.2514/6.2024-85879  

   Sakib, Sheikh Ahmad; Povitsky, Alex (January 2024, American Institute of Aeronautics and Astronautics)

   Liquid jets in surrounding air face capillary and shear forces which eventually disintegrate the jet into droplets or spray. The instabilities developed in the flow inevitably break down an initial laminar (coherent) jet into a turbulent one. In the manufacturing process called grinding, one of the oldest approaches of shaping metals and other materials, liquid coolant jets are frequently used. A non-coherent or turbulent jet has a reduced flow rate due to cavitation, air entrapment and atomization of the fluid particles. The jet spread does not allow the coolant jet to effectively breach the high-speed rotating air layer, created by entrainment of air along the surface of rapidly rotating grinding wheel. The coherent, nearly columnal jet should be sufficiently long to maintain its initial velocity to penetrate the layer of air rotating with the grinding wheel. Thus, in many critical grinding applications, it is advised to use a coherent jet instead of a spray to eradicate defects of ground surface. In this study, we present simulations of liquid jet flows to see how the jet develops and breaks due to surface tension and shear forces. Creating an accurate model to predict liquid jet characteristics, especially for high-speed applications such as grinding wheel cooling would require wellresolving numerical grids and turbulence model selection. The problem being multi-phased with a density ratio of coolant-to-air being order of 1000 adds to the computational complexity. The presented numerical model and results are different compared to the previous simulations of liquid jets as the characteristics of jet disintegration are explored under conditions that closely resemble a grinding cooling application. Finite volume discretization of the flow domain and calculation of flow field characteristics were done by commercial software ANSYS Mesh and ANSYS Fluent modules, respectively. The numerical calculation and visualization of disintegration of free jet and the jet impinging into grinding wheel will be presented. 

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5. What determines the structure of short gamma-ray burst jets?

   https://doi.org/10.1093/mnras/stab723  

   Urrutia, Gerardo; De Colle, Fabio; Murguia-Berthier, Ariadna; Ramirez-Ruiz, Enrico (April 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT The discovery of GRB 170817A, the first unambiguous off-axis short gamma-ray burst (sGRB) arising from a neutron star merger, has challenged our understanding of the angular structure of relativistic jets. Studies of the jet propagation usually assume that the jet is ejected from the central engine with a top-hat structure and its final structure, which determines the observed light curve and spectra, is primarily regulated by the interaction with the nearby environment. However, jets are expected to be produced with a structure that is more complex than a simple top-hat, as shown by global accretion simulations. We present numerical simulations of sGRBs launched with a wide range of initial structures, durations, and luminosities. We follow the jet interaction with the merger remnant wind and compute its final structure at distances ≳1011 cm from the central engine. We show that the final jet structure, as well as the resulting afterglow emission, depends strongly on the initial structure of the jet, its luminosity, and duration. While the initial structure of the jet is preserved for long-lasting sGRBs, it is strongly modified for jets barely making their way through the wind. This illustrates the importance of combining the results of global simulations with propagation studies in order to better predict the expected afterglow signatures from neutron star mergers. Structured jets provide a reasonable description of the GRB 170817A afterglow emission with an off-axis angle θobs ≈ 22.5°. 

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