Farzad Mashayek

Temporal analysis of capillary jet breakup

The temporal instability of a cylindrical capillary jet is analysed numerically for different liquid Reynolds numbers Re , disturbance wavenumbers k , and amplitudes e 0 . The breakup mechanism of viscous liquid jets and the formation of satellite drops are described. The results show that the satellite size decreases with decreasing Re , and increasing k and e 0 . Marginal Reynolds numbers below which no satellite drops are formed are obtained for a large range of wavenumbers. The growth rates of the disturbances are calculated and compared with those from the linear theory. These results match for low- Re jets, however as Re is increased the results from the linear theory slightly overpredict those from the nonlinear analysis. (At the wavenumber of k = 0.9, the linear theory underpredicts the nonlinear results.) The breakup time is shown to decrease exponentially with increasing the amplitude of the disturbance. The cut-off wavenumber is shown to be strongly dependent on the amplitude of the initial disturbance for amplitudes larger than approximately

Analytical description of particle/droplet-laden turbulent flows

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Droplet–turbulence interactions in low-Mach-number homogeneous shear two-phase flows

of the initial jet radius. The stable oscillations of liquid jets are also investigated. The results indicate that liquid jets with Re ∼ O (1) do not oscillate, and the disturbances are overdamped. However, liquid jets with higher Re oscillate with a period which depends on Re and e 0 . The period of the oscillation decreases with increasing Re at small e 0 ; however, it increases with increasing Re at large e 0 . Marginal Reynolds numbers below which the disturbances are overdamped are obtained for a wide range of wavenumbers and e 0 = 0.05.

Atomic-scale observation of lithiation reaction front in nanoscale SnO2 materials.

Abstract Various existing analytical descriptions for predicting turbulent flows laden with solid particles or liquid droplets are reviewed here. The main focus is on a collisionless dispersed phase, however, the two-way coupling effects are considered and discussed. The review of various methods is conducted by dividing them into two main categories. The first category includes direct numerical simulation (DNS), large-eddy simulation, and stochastic modelling, which are collectively called the ‘Lagrangian description’. The second category, under the ‘Eulerian description’, includes Reynolds averaged Navier–Stokes (RANS) and probability density function (pdf) modelling. The emphasis is placed on application of these approaches for both understanding and prediction of turbulent dispersed phase. The discussion is focused on merits and limitations of these approaches and the nature of predictions offered by them. The mathematical aspects of RANS and pdf modelling are presented in greater detail. The important role of DNS generated data in the development and assessment of other approaches is discussed with the aid of some representative examples in particle-laden homogeneous turbulent flows.

Coalescence collision of liquid drops

Several important issues pertaining to dispersion and polydispersity of droplets in turbulent flows are investigated via direct numerical simulation (DNS). The carrier phase is considered in the Eulerian context, the dispersed phase is tracked in the Lagrangian frame and the interactions between the phases are taken into account in a realistic two-way (coupled) formulation. The resulting scheme is applied for extensive DNS of low-Mach-number, homogeneous shear turbulent flows laden with droplets. Several cases with one- and two-way couplings are considered for both non-evaporating and evaporating droplets. The effects of the mass loading ratio, the droplet time constant, and thermodynamic parameters, such as the droplet specific heat, the droplet latent heat of evaporation, and the boiling temperature, on the turbulence and the droplets are investigated. The effects of the initial droplet temperature and the initial vapour mass fraction in the carrier phase are also studied. The gravity effects are not considered as the numerical methodology is only applicable in the absence of gravity. The evolution of the turbulence kinetic energy and the mean internal energy of both phases is studied by analysing various terms in their transport equations

Modeling subgrid-scale effects on particles by approximate deconvolution

In the present work, taking advantage of aberration-corrected scanning transmission electron microscopy, we show that the dynamic lithiation process of anode materials can be revealed in an unprecedented resolution. Atomically resolved imaging of the lithiation process in SnO2 nanowires illustrated that the movement, reaction, and generation of b = [1[overline]1[overline]1] mixed dislocations leading the lithiated stripes effectively facilitated lithium-ion insertion into the crystalline interior. The geometric phase analysis and density functional theory simulations indicated that lithium ions initial preference to diffuse along the [001] direction in the {200} planes of SnO2 nanowires introduced the lattice expansion and such dislocation behaviors. At the later stages of lithiation, the Li-induced amorphization of rutile SnO2 and the formation of crystalline Sn and LixSn particles in the Li2O matrix were observed.

Direct numerical simulations of evaporating droplet dispersion in forced low Mach number turbulence

The coalescence collision of two liquid drops is studied using a Galerkin finite element method in conjunction with the spine-flux method for the free surface tracking. The effects of Reynolds number, impact velocity, drop size ratio, and internal circulation on the coalescence process is investigated. The long time oscillations of the coalesced drop are also studied and curves for the variations of the period and decay factor are provided as a function of the number of oscillations. In the study of non-equal-size drop collision, traces of different fluid particles are calculated to illustrate the liquid mixing during the collision. 2002 Elsevier Science Ltd. All rights reserved.

A stochastic model for particle motion in large-eddy simulation

The approximate deconvolution is implemented to reconstruct the instantaneous velocities from the filtered velocities before using them in the momentum equations of particles in the large-eddy simulation (LES) of particle-laden turbulent flows. It is shown that the various statistics of particles obtained through deconvolution are in good agreement with those obtained by the direct numerical simulation (DNS) by conducting a priori and a posteriori tests in a particle-laden homogeneous shear turbulent flow. On the other hand, the neglect of the effects of subgrid scales on the particles results in discrepancies between DNS and LES results.

Twin boundary-assisted lithium ion transport.

Abstract Dispersion of evaporating droplets in forced low Mach number isotropic turbulence is studied using direct numerical simulation (DNS). The carrier phase is treated in the Eulerian frame, the droplets are tracked in the Lagrangian frame, and a (realistic) two-way coupling is considered. The results of the simulations are used to investigate the effects of the initial droplet time constant, the initial mass loading ratio, the initial droplet temperature, the latent heat of evaporation, the boiling temperature, and the initial vapor mass fraction on the droplet size, the temperature fields, and the vapor mass fraction. The DNS results indicate that the evaporation rate is nonlinear during the early times. The pdfs of the droplet diameter are skewed towards smaller droplets, however, they may be approximated as Gaussian for small mass loading ratios. An examination of the mean vapor mass fraction indicates that the mixture becomes nearly saturated at long times. The evolution of the fluctuating vapor mass fraction is investigated by considering the transport equation for the variance of this quantity.

Numerical investigation of reacting droplets in homogeneous shear turbulence

Most of the studies on large-eddy simulation (LES) of particle-laden flows assume that the effect of subgrid scales on the particle motion is negligible. This assumption may break down, particularly when particles have a small time constant and/or the filtered energy is significant. In this work, a stochastic model is proposed for the particle motion in LES while considering the effect of subgrid fluctuations. The model assumes that the fluid particle seen by the heavy particle evolves based on a diffusion stochastic process. For model assessment, both a priori and a posteriori tests are conducted for a particle-laden decaying isotropic turbulence. In the a priori test the filtered velocity field, obtained via filtering the DNS velocities, and in the a posteriori test the LES velocity field, obtained via the dynamic Smagorinsky model, are applied to particles through the stochastic model. The small-particle statistics obtained through the stochastic model very well match those obtained through DNS in the a priori test, once the effects of the initial conditions have decayed. There is also a good agreement with DNS results in the a posteriori test. It is shown that the neglect of subgrid fluctuations is not acceptable for these cases. For large particle time constants, the model needs to be adjusted.