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A Chebyshev multidomain staggered-grid method (CMSM) presented by Kopriva (Kopriva, D. A., 'A Staggered-Grid Multidomain Spectral Method for the Compressible Navier-Stokes Equations,' Journal of Computational Physics, Vol. 143, No. 1, 1998, pp. 125-158) for numerical simulation of three-dimensional compressible turbulent flows, is validated. Computations with the CMSM of an isotropic turbulence and a fully developed turbulent channel flow are compared with previously published results. Turbulence initiation of the isotropic turbulence on the Chebyshev grid is discussed. Low-resolution simulations ensure transition of an initial laminar channel flow to a turbulent channel flow. The turbulent kinetic energy spectrum has sufficient dropoff (indicating a resolved solution) at a minimum of 3 to 6 points per wave number for a polynomial approximation of 13 to 10, respectively.

A stochastic model is presented for the prediction of the gravity effects on velocity and temperature fluctuations in turbulent flows laden with solid particles. Both temporal and spatial correlations as well as the anisotropy of turbulence are included in the model. It is found that the inclusion of the spatial correlation is imperative for capturing the gravity effects. The performance of the stochastic model is assessed by comparisons of the results with direct numerical simulation data for isotropic and homogeneous shear turbulent flows. The comparisons are also used to investigate the effect of the empirical constant that appears in the expression used for the spatial correlation. [DOI: 10.1115/1.1778714]

A stochastic model is proposed for the prediction of velocity and temperature fluctuations in turbulent flows laden with nonevaporating droplets. The model is based on a first-order time series analysis, addresses the anisotropy of turbulence, and adequately takes into account the temporal correlations. The gravity effect is not considered in this study. The performance of the model is assessed by conducting simulations of droplet-laden homogeneous shear flows. The predictions of the stochastic model are compared with the results from a model derived using a kinetic approach and the data from a direct numerical simulation study Excellent agreements are observed for various velocity and temperature statistics.

The physical situation of two-phase nonisothermal turbulent fluid flows laden with nonevaporating spherical particles is considered. A closed kinetic equation for the transport of particles and their temperature is derived by solving the involved closure problem using the functional method. The equation is exact when the fluctuations of fluid flow variables along the particle path are distributed as Gaussian. For the case of homogeneous turbulent flows, macroscopic equations describing the time evolution of statistical properties related to particle velocity and temperature are obtained by taking various moments of the closed kinetic equation. These equations are computed for the case of homogeneous shear turbulent flow with constant temperature gradient, and the results are compared with the data of our direct numerical simulation study.

For the cross-stream and spanwise normal stress transport equations (Graph $i = 2$ and Graph $j = 2$; Graph $i = 3$ and Graph $j = 3$), the WilcoxRSM model is able to approximately predict the magnitudes of pressure-strain correlation (see figure 14 I b i , I c i ), which acts as the major source term. To decompose a flow variable, a filtering length scale, Graph $\varDelta$, is required and is defined as 5.1 Graph \begin{equation} \varDelta = (\varDelta {x}\varDelta {y}\varDelta {z})^{1/3}.\end{equation} Here, Graph $\varDelta {x}$, Graph $\varDelta {y}$ and Graph $\varDelta {z}$ respectively represent the grid sizes in the streamwise, cross-stream and spanwise directions. The closure coefficients Graph $\beta ^*$, Graph $\widehat {C 1}$ and Graph $\hat {\alpha }$ are respectively set as Graph $9/100$, Graph $9/5$ and Graph $181/220$, while Graph $\hat {\beta }$, Graph $\hat {\gamma }$ and Graph $C \mu$ are given as Graph $139/220$, Graph $59/55$ and Graph $9/100$, respectively (Wilcox 2006). As can be seen, the maximum Graph $(\sqrt {\langle T'^{2} \rangle }/\langle T \rangle)/(\Delta U)$ is approximately Graph $0.008$ for Graph $M c = 0.3$, and increases to Graph $0.011$ as Graph $M c$ increases to 0.7. In the other shear stress transport equations (Graph $i = 1$ and Graph $j = 3$; Graph $i = 2$ and Graph $j = 3$), which involve the shear in the homogeneous direction (spanwise direction), the production and pressure-strain correlation terms appear insignificant. [Extracted from the article]

The persisting outbreak of SARS-CoV-2 has posed an enormous threat to global health. The sustained human-to-human transmission of SARS-CoV-2 via respiratory droplets makes the medical procedures around the perioral area vulnerable to the spread of the disease. Such procedures include the ultrasonic dental cleaning method, which occurs within the oral cavity and involves cavitation-induced sprays, thus increasing the risk of pathogen transmission via advection. To understand the associated health and safety risks for patients and clinicians, it is critical to understand the flow pattern of the spray cloud around the operating region, the size and velocity distribution of the emitted droplets, and the extent of fluid dispersion until ultimate deposit on surfaces or escape through air vents. In this work, the droplet size and velocity distributions of the spray emerging from the tip of a free-standing common ultrasonic dental cleaning device were characterized via high-speed imaging. Deionized water and 1.5% and 3% aqueous hydrogen peroxide (H2O2) solutions were used as working fluids, with the H2O2—an established oxidizing agent—intended to curb the survival of virus released in aerosols generated from dental procedures. The measurements reveal that the presence of H2O2 in the working fluid increases the mean droplet size and ejection velocity. Detailed computational fluid dynamic simulations with multiphase flow models reveal benefits of adding small amounts of H2O2 in the feed stream of the ultrasonic cleaner; this practice causes larger droplets with shorter residence times inside the clinic before settling down or escaping through air vents. The results suggest optimal benefits (in terms of fluid spread) of adding 1.5% H2O2 in the feed stream during dental procedures involving ultrasonic tools. The present findings are not specific to the COVID-19 pandemic but should also apply to future outbreaks caused by airborne droplet transmission. [ABSTRACT FROM AUTHOR]

High-viscosity organic oils may be considered as an alternative to the ordinary diesel fuel. These organic oils and the diesel fuel are all Newtonian liquids; however, viscosity values of the organic oils are more than 20 times higher than that of the diesel fuel. In the present work, the electrostatic atomization of corn oil jets is studied and compared to the electrostatic atomization of diesel fuel jets. The experimental data revealed that in addition to the varicose breakup of straight jets, bending modes set in and grow in conjunction with the varicose undulations. Bending instability, kindred to the aerodynamically-driven bending instability of high-speed liquid jets moving in air, and to the electrically-driven bending instability of polymer jets in electrospinning, is significantly more pronounced in the case of the highly-viscous corn oil jets than in diesel jets. The experimental results are interpreted using the theory of bending instability developed previously for electrospinning. [ABSTRACT FROM AUTHOR]

A simplistic numerical kinetic model to predict the deposited film morphology in dusty plasma chemical vapor deposition reactors is developed. The morphological accretion of a circular object is studied in a two-dimensional geometry and the most important deposition phenomena are taken into account, i.e., surface diffusion and surface ion bombardment. Both isotropic and anisotropic plasmas are considered. It is shown that when the particle is located in an isotropic plasma, the deposited film maintains the original particle sphericity. Whereas, if the particle is assumed to levitate in the (pre) sheath forming around the bottom electrode of the reactor and ions contribute considerably to the deposition, the film develops in a nonuniform manner similar to experimental observations. [ABSTRACT FROM AUTHOR]

In this paper, the effect of collisions on the charging and shielding of a single dust particle immersed in an infinite plasma is studied. A Monte-Carlo collision (MCC) algorithm is implemented in the particle-in-cell DEMOCRITUS code to account for the collisional phenomena which are typical of dusty plasmas in plasma processing, namely, electron-neutral elastic scattering, ion-neutral elastic scattering, and ion-neutral charge exchange. Both small and large dust particle radii, as compared to the characteristic Debye lengths, are considered. The trends of the steady-state dust particle potential at increasing collisionality are presented and discussed. The ions and electron energy distributions at various locations and at increasing collisionality in the case of large particle radius are shown and compared to their local Maxwellians. The ion-neutral charge-exchange collision is found to be by far the most important collisional phenomenon. For small particle radius, collisional effects are found to be important also at low level of collisionality, as more ions are collected by the dust particle due to the destruction of trapped ion orbits. For large particle radius, the major collisional effect is observed to take place in proximity of the presheath. Finally, the species energy distribution functions are found to approach their local Maxwellians at increasing collisionality. [ABSTRACT FROM AUTHOR]

In this paper, the structure of a rf plasma glow discharge in a parallel-plate geometry is revisited through a numerical solution of the well-known local field approximation model equations. The dynamics of a dust particle injected into the plasma is elaborated in a Lagrangian framework by solving the particle equations for its motion and charge. Different ion drag expressions are considered. For particles of three different sizes, magnitudes of various forces such as gravity, electricity, and ion drag acting on a stationary particle are compared to each other. Particle potential energy, together with its possible wells, is demonstrated for each case. Taking into account the neutral drag force, damping oscillations and final locations of the particles, depending on their initial injection position (top or bottom), are captured. The transient characteristic of the charging process of smaller particles with respect to their motion time scale is discussed. The effect of inclusion of ion thermal energy in the calculation of drag force on the motion of the particle is illustrated. [ABSTRACT FROM AUTHOR]

A detailed chemical kinetics scheme of the reactions occurring in a CH4/H2 plasma, namely, electron-neutral, ion-neutral, and neutral-neutral reactions, is implemented for the prediction of the species fluxes toward the surface of a submicron particle in a low-pressure environment. Surface reactions at the particle surface are also accounted for. Kinetic theory is applied in the collisionless region within a distance of one mean free path away from the particle, while continuum theory is implemented to solve for species transport in the outer region where reactive-diffusive phenomena occur. These regions are bounded by appropriate boundary conditions. The self-consistent electric field is obtained by solving the Poisson’s equation in the continuum region. The charged and neutral species distributions are calculated and the growth rate of the amorphous carbon layer at the particle surface, as well as particle charging, are predicted. The predicted growth rate is within the range of experimental data from literature for similar conditions. This shows that the model reflects rather accurately the complicated physicochemical phenomena occurring in real systems. [ABSTRACT FROM AUTHOR]

A statistical closure scheme is used to obtain an approximate equation for probability density function to predict the statistical properties of interest of collisionless evaporating droplets suspended in isothermal isotropic turbulent flows. The resulting Fokker-Planck equation has nonlinear, time-dependent drift and diffusion coefficients that depend on the statistical properties of the droplet slip velocity. Approximate analytical expressions for these properties are derived, and the equation is solved numerically after implementing the path-integral approach. The time evolution of various statistical properties related to the droplet diameter are then calculated and compared with the data available from the stochastic (Mashayek, F., 'Stochastic Simulations of Particle-Laden Isotropic Turbulent Flow,' International Journal of Multiphase Flow, Vol. 25, No. 8, 1999, pp. 1575-1599) and direct numerical (Mashayek, F., Jaberi, F. A., Miller, R. S., and Givi, P., 'Dispersion and Polydispersity of Droplets in Stationary Isotropic Turbulence,' International Journal of Multiphase Flow, Vol. 23, No. 2, 1997, pp. 337-355) simulations.