Eruption Style, Emplacement Dynamics and Geometry of Peralkaline Ignimbrites: Insights From the Lajes-Angra Ignimbrite Formation, Terceira Island, Azores

Ignimbrites are relatively uncommon on ocean island volcanoes and yet they constitute a significant portion of the stratigraphy of Terceira Island (Azores). The Lajes-Angra Ignimbrite Formation (ca. 25 cal ka BP) contains the youngest ignimbrites on Terceira and records two ignimbrite-forming eruptions of Pico Alto volcano that occurred closely spaced in time. Here, we present the first detailed lithofacies analysis and architecture of the Angra and Lajes ignimbrites, complemented by petrographic, mineral chemical, whole rock and groundmass glass geochemical data. The two ignimbrites have the same comenditic trachyte composition, but show considerable variability in trace element and groundmass glass compositions, revealing complex petrogenetic processes in the Pico Alto magma reservoir prior to eruption. The Angra Ignimbrite has a high-aspect ratio and is massive throughout its thickness. It was formed by a small-volume but sustained pyroclastic density current (PDC) fed by a short-lived, low pyroclastic fountain. Overall, the PDC had high particle concentration, granular fluid-based flow conditions and was mostly channelled into a valley on the south part of Terceira. By contrast, the Lajes Ignimbrite has a low-aspect ratio and shows vertical and lateral lithofacies variations. It was formed by a sustained quasi-steady PDC generated from vigorous and prolonged pyroclastic fountaining. The ignimbrite architecture reveals that depositional conditions of the parent PDC evolved as the eruption waxed. The dilute front of the current rapidly changed to a high particle concentration, granular fluid-based PDC that extended to the north and south coasts, with limited capacity to surmount topographic highs. Contrary to what is commonly assumed, the low-aspect ratio of the Lajes Ignimbrite is interpreted to result from deposition of a relatively low velocity PDC over a generally flat topography. This work highlights that the geometry (aspect ratio) of ignimbrites does not necessarily reflect the kinetic energy of PDCs and thus should not be used as a proxy for PDC emplacement dynamics. Although the probability of an ignimbrite-forming eruption on Terceira is relatively low, such a scenario should not be underestimated, as a future event would have devastating consequences for the island’s 55,000 inhabitants.

Sustainable Ambient Environment to Prevent Future Outbreaks: How Ambient Environment Relates to COVID-19 Local Transmission in Lima, Peru

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), universally recognized as COVID-19, is currently is a global issue. Our study uses multivariate regression for determining the relationship between the ambient environment and COVID-19 cases in Lima. We also forecast the pattern trajectory of COVID-19 cases with variables using an Auto-Regressive Integrated Moving Average Model (ARIMA). There is a significant association between ambient temperature and PM10 and COVID-19 cases, while no significant correlation has been seen for PM2.5. All variables in the multivariate regression model have R2 = 0.788, which describes a significant exposure to COVID-19 cases in Lima. ARIMA (1,1,1), during observation time of PM2.5, PM10, and average temperature, is found to be suitable for forecasting COVID-19 cases in Lima. This result indicates that the expected high particle concentration and low ambient temperature in the coming season will further facilitate the transmission of the coronavirus if there is no other policy intervention. A suggested sustainable policy related to ambient environment and the lessons learned from different countries to prevent future outbreaks are also discussed in this study.

Friction and Lubrication Behavior of a Selective Laser Melted Hydraulic Spool Valve Under Contaminated Conditions

Selective laser melting (SLM) technology has a great potential to reduce size and weight of hydraulic valves. However, the tribological performance of an SLMed valve has not been studied which is crucial for the performance and reliability of the valve, especially under contaminated conditions. In this study, the friction and lubrication behavior between an SLMed valve body and a traditional spool were studied using a scaled reciprocating test rig under various contaminated conditions (frequency at 5 Hz and 25 Hz; particle concentration at 0.4 mg/ml and 4 mg/ml; particle size at 1.6 µm and 15 µm). Three types of SLMed samples were fabricated using different exposure times: one has many large surface pores (pores area > 1000 µm2 accounts for 7.167% of the sample surface); one has a few small surface pores (pores area between 100 µm2 and 1000 µm2 accounts for 0.574% of the sample surface); and one only has micropores (pores area < 300 µm2 accounts for 0.168% of the sample surface). The density, hardness, microstructures, and pore characterization of the SLMed samples were investigated. The results indicated that the frequency greatly influenced friction and lubrication behaviors by determining lubrication regimes. The influence of surface pores on the lubrication and friction depends on contact conditions: pores which served as particle containers to reduce friction are prominent under 5 Hz frequency and high particle concentration; extra lubrication by the surface pores is observed under 25 Hz frequency and low particle concentration.

Deposition of Colloidal Particles during the Evaporation of Sessile Drops: Dilute Colloidal Dispersions

The deposition of colloidal silica particles during the evaporation of sessile drops on a smooth substrate has been modeled by the simultaneous solution of the Navier–Stokes equations, the convective-diffusive equation for particles, and the diffusion equation for evaporated vapor in the gas phase. Isothermal conditions were assumed. A mapping was created to show the conditions for various deposition patterns for very dilute suspensions. Based on values of the Peclet (Pe) number and Damkholer numbers (Da and Da−1), the effects of adsorption and desorption were discussed according to the map. Simulations were also done for suspensions with a high particle concentration to form a solid phase during the evaporation by using a packing criterion. The simulations predicted the height and width of the ring deposit near the contact line, and the results compared favorably to experimental particle deposition patterns.

Optimization of Temporary Plugging Parameters Under Rough Fractures

Temporary-plugging-and-diverting (TPD) fracturing technology is widely used in the development of the unconventional reservoir. The operational procedure of temporary plugging and the size and combination of diverters are very much concerned by field engineers. This study compares different pumping procedure of diverters and optimizes the combination and pumping rate of diverters under the different width of the fracture. The experimental method is based on a simulated fracture apparatus, which is manufactured by the 3D printing technology. The surface morphology of the fracture is obtained through a 3D scanning of a fracture. The experimental procedure is pumping the carrier liquid and diverter mixtures into the fracture while recording pumping pressure and the outlet volume of carrier fluid. The fracture plugging efficiency was evaluated through the recorded parameters. The diverter concentration and composition were optimized at a wide range of fracture width (1 mm to 4 mm). Low the diverter concentration could help to reduce the operational risk of the diversion. Under the low concentration of the diverters, the plugging mechanism is that the large particle diverters bridge in the fracture due to the fracture tortuosity and roughness; the smaller particle and fiber diverters then fill the voids of the large particles and form a strong and low permeable diverter pack. The results indicate that pumping the mixture particle and fiber diverters are more beneficial to plug the fracture than pumping them separately and sequentially. High particle concentration has a two-sided effect, which leads to the existence of an optimal fiber-to-particle ratio. The concentration of diverters could decrease when the size of diverters is increased. At a constant fracture width, higher pumping rates can help to temporarily plug the fracture more efficiently.

IMEX_SfloW2D 1.0: a depth-averaged numerical flow model for pyroclastic avalanches

Pyroclastic avalanches are a type of granular flow generated at active volcanoes by different mechanisms, including the collapse of steep pyroclastic deposits (e.g., scoria and ash cones), fountaining during moderately explosive eruptions, and crumbling and gravitational collapse of lava domes. They represent end-members of gravity-driven pyroclastic flows characterized by relatively small volumes (less than about 1 Mm3) and relatively thin (1–10 m) layers at high particle concentration (10–50 vol %), manifesting strong topographic control. The simulation of their dynamics and mapping of their hazards pose several different problems to researchers and practitioners, mostly due to the complex and still poorly understood rheology of the polydisperse granular mixture and to the interaction with the complex natural three-dimensional topography, which often causes rapid rheological changes. In this paper, we present IMEX_SfloW2D, a depth-averaged flow model describing the granular mixture as a single-phase granular fluid. The model is formulated in absolute Cartesian coordinates (whereby the fluid flow equations are integrated along the direction of gravity) and can be solved over a topography described by a digital elevation model. The numerical discretization and solution algorithms are formulated to allow for a robust description of wet–dry conditions (thus allowing us to accurately track the front propagation) and an implicit solution to the nonlinear friction terms. Owing to these features, the model is able to reproduce steady solutions, such as the triggering and stopping phases of the flow, without the need for empirical conditions. Benchmark cases are discussed to verify the numerical code implementation and to demonstrate the main features of the new model. A preliminary application to the simulation of the 11 February pyroclastic avalanche at the Etna volcano (Italy) is finally presented. In the present formulation, a simple semi-empirical friction model (Voellmy–Salm rheology) is implemented. However, the modular structure of the code facilitates the implementation of more specific and calibrated rheological models for pyroclastic avalanches.

IMEX_SfloW2D 1.0. A depth-averaged numerical flow model for pyroclastic avalanches

Pyroclastic avalanches are a type of granular flow generated at active volcanoes by different mechanisms, including the collapse of steep pyroclastic deposits (e.g., scoria and ash cones) and fountaining during moderately explosive eruptions. They represent end-members of gravity-driven pyroclastic flows, characterized by relatively small volumes (less than about 1 Mm3) and relatively thin (1–10 m) layers at high particle concentration (1–50 vol.%), manifesting strong topographic control. The simulation of their dynamics and mapping of their hazards pose several different problems to researchers and practitioners, mostly due to the complex and still poorly understood rheology of the polydisperse granular mixture, and to the interaction with the complex natural three-dimensional topography, which often causes rapid rheological changes. In this paper, we present IMEX_SfloW2D, a depth-averaged flow model describing the granular mixture as a single-phase granular fluid. The model is formulated in absolute Cartesian coordinates (where the fluid flow equations are integrated along the direction of gravity) and can be solved over a topography described by a Digital Elevation Model. The numerical discretization and solution algorithms are formulated to allow a robust description of wet-dry conditions (thus allowing to accurately track the front propagation) and to implicitly solve the non-linear friction terms. Owing to these features, the model is able to reproduce steady solutions, such as the triggering and stopping phases of the flow, without the need of empirical conditions. Benchmark cases are discussed to verify the numerical code implementation and to demonstrate the main features of the new model. A preliminary application to the simulation of the February 11th pyroclastic avalanche at Etna volcano (Italy) is finally presented. In the present formulation, a simple semi-empirical friction model (Voellmy-Salm rheology) is implemented. However, the modular structure of the code facilitates the implementation of more specific and calibrated rheological models for pyroclastic avalanches.

Particles Transport in Railway Braking Systems: An Experimental and Numerical Investigation

There has been a growing public health issue concerning the regulation of indoor air quality (IAQ) and the human exposure to particulate matter (PM). Today, this exposure is a major worldwide concern because ambient PM concentrations in many cities exceeded the limits set by the European air quality directive. Underground airborne particles are mainly generated by the mechanical abrasion of rail tracks, wheels and brake pads produced by urban railways transportation. For that reason, understanding the transport mechanism of particles with various size distribution is essential and crucial for understanding and accurately predicting the behavior of the main high particle concentration areas. In this framework, a simple case of particles emission inside a viscous flow in a channel has been investigated both experimentally and numerically. The suspended particles used experimentally are molybdenum solid particles with a broad size distribution (in diameter) from 1 to 80 μm (size similar to cases such as in braking systems). The experimental tests are conducted for a flow in a channel at a horizontal steady inflow velocity of uf = 0.15m/s. The solid particles are injected transversely to the horizontal bottom wall with an injection steady velocity of ui = 0.95m/s. Measurements and analysis are carried out using shadowscopy technique to determine the particles concentration fields. Finally, experimental results are compared to numerical ones predicted by a continuum computational fluid dynamics (CFD) approach using the SBM (Suspension Balance Model) implemented in “OPENFOAM” (via the Finite Volume Method).

Particle-Cluster Formation and Vorticity in Gas-Solid Flows

Fluidized beds are widely used in industry for combustion, gasification, catalytic cracking and several other purposes. Pneumatic conveying of air is popularly used in industry to transport materials such as pulverized coal through pipelines. A common observation in gas-solid flow dynamics in both of the above systems is the formation of high concentration regions of particles; known as clusters in fluidized beds and rope like structures in pipe bends and ducts. Both the clustering and roping phenomenon were clearly observed in some experiments and in simulations of both fluidized beds and gas-solid flows in pipe bends. It has been found from these simulations that there is a strong correlation between vorticity and concentration. The high particle concentration regions are bounded by vortices of clockwise and counter clockwise direction of roughly the same order of magnitude and there is very low vorticity at the high concentration regions. The goal of this study is to find the cause and effect relation between the gas vorticity and the high particle concentration regions; in particular whether the gas vorticity causes particle agglomeration into clusters or vice-versa. Numerical study has been performed on a vertical pipe by creating a vortex field. In this regard, very large eddy simulations with Lagrangian Discrete Phase model have been performed using Ansys FLUENT and MFIX software packages. The influence of particles on the vorticity has been studied. Influence of several factors such as particle size, injection velocity etc. have also been studied. Correlations among turbulent kinetic energy, vorticity, and particle clustering and/or roping are illustrated.