PDF model based on Langevin equation for polydispersed two-phase flows applied to a bluff-body gas-solid flow,

The aim of the paper is to discuss the main characteristics of a complete theoretical and numerical model for turbulent polydispersed two-phase flows, pointing out some specific issues. The theoretical details of the model have already been presented [Minier and Peirano, Physics Reports, Vol. 352/1-3, 2001 ]. Consequently, the present work is mainly focused on complementary aspects, that are often overlooked and that require particular attention. In particular, the following points are analysed : the necessity to add an extra term in the equation for the velocity of the fluid seen in the case of twoway coupling, the theoretical and numerical evaluations of particle averages and the fulfilment of the particle mass-continuity constraint. The theoretical model is developed within the PDF formalism. The important-physical choice of the state vector variables is first discussed and the model is then expressed as a stochastic differential equation (SDE) written in continuous time (Langevin equations) for the velocity of the fluid seen. The interests and limitations of Langevin equations, compared to the single-phase case, are reviewed. From the numerical point of view, the model corresponds to an hybrid Eulerian/Lagrangian approach where the fluid and particle phases are simulated by different methods. Important aspects of the Monte Carlo particle/mesh numerical method are emphasised. Finally, the complete model is validated and its performance is assessed by simulating a bluff-body case with an important recirculation zone and in which two-way coupling is noticeable.Comment: 23 pages, 10 figure

An experimental approach to analyze aerosol and splatter formations due to a dental procedure

Throughout 2020 and beyond, the entire world has observed a continuous increase in the infectious spread of the novel coronavirus (SARS-CoV-2) otherwise known as COVID-19. The high transmission of this airborne virus has raised countless concerns regarding safety measures employed in the working conditions for medical professionals. Specifically, those who perform treatment procedures on patients which intrinsically create mists of fine airborne droplets, i.e., perfect vectors for this and other viruses to spread. The present study focuses on understanding the splatter produced due to a common dentistry technique to remove plaque buildup on teeth. This technique uses a high-speed dentistry instrument, e.g., a Cavitron ultrasonic scaler, to scrape along the surface of a patient’s teeth. This detailed understanding of the velocity and the trajectory of the droplets generated by the splatter will aid in the development of hygiene mechanisms to guarantee the safety of those performing these procedures and people in clinics or hospitals. Optical flow tracking velocimetry (OFTV) method was employed to obtain droplet velocity and trajectory in a two-dimensional plane. Multiple data collection planes were taken in different orientations around a model of adult mandibular teeth. This technique provided pseudo-three-dimensional velocity information for the droplets within the splatter developed from this high-speed dental instrument. These results indicated that within the three-dimensional splatter produced there were high velocities (1–2 m/s) observed directly below the intersection point between the front teeth and the scaler. The splatter formed a cone-shape structure that propagated 10–15 mm away from the location of the scaler tip. From the droplet trajectories, it was observed that high velocity isolated droplets propagate away from the bulk of the splatter. It is these droplets which are concerning for health safety to those performing the medical procedures. Using a shadowgraphy technique, we further characterize the individual droplets’ size and their individual velocity. We then compare these results to previously published distributions. The obtained data can be used as a first step to further examine flow and transport of droplets in clinics/dental offices. [Image: see text

Interaction of Nearly-Inviscid, Multi-mode Faraday Waves and Mean Flows

Faraday waves [1] are gravity-capillary waves that are excited on the surface of a fluid when its container is vibrated vertically and the vertical acceleration exceeds a threshold value. These waves have received much attention in the literature both as a basic fluid dynamical problem and as a paradigm of a pattern-forming system [2-4]. Unfortunately, in the low viscosity limit, there are several basic issues that remain unresolved, particularly in connection with the generation of mean flows in the bulk. The viscous part of these flows (also called streaming flow or acoustic streaming) is driven by the oscillatory boundary layers attached to the solid walls and the free surface by well-known mechanisms first uncovered by Schlichting [5] and Longuet-Higgins [6]. This mean flow has been shown recently to affect the dynamics of the primary waves at leading order in a related, laterally vibrated system [7]. This is somewhat similar to the effect of an internal circulation on surface wave dynamics in drops [8]

Finite element simulation of three-dimensional free-surface flow problems

An adaptive finite element algorithm is described for the stable solution of three-dimensional free-surface-flow problems based primarily on the use of node movement. The algorithm also includes a discrete remeshing procedure which enhances its accuracy and robustness. The spatial discretisation allows an isoparametric piecewise-quadratic approximation of the domain geometry for accurate resolution of the curved free surface. The technique is illustrated through an implementation for surface-tension-dominated viscous flows modelled in terms of the Stokes equations with suitable boundary conditions on the deforming free surface. Two three-dimensional test problems are used to demonstrate the performance of the method: a liquid bridge problem and the formation of a fluid droplet

On LEM/LES methodology for two-phase flows

A two-phase subgrid combustion model developed earlier has been evaluated for applicability in large-eddy simulations (LES). Direct Numerical Simulations (DNS) of two-phase isotropic turbulence in the presence of passive, momentum-coupled and vaporizing droplets has been extensively studied to form a base-line database. Current DNS results agree with earlier studies and show that the presence of droplets increase the kinetic energy and dissipation at the small scales. LES for these same cases were also carried out to investigate what modifications are needed to incorporate the small-scale turbulence modifications seen in DNS of two-phase flows. LES subgrid modeling for two-phase mixing within the context of the new subgrid combustion model is also addressed

​​Observing Antarctic Bottom Water in the Southern Ocean​

Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW’s key role in regulating Earth’s climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system