Turbulent breakage of ductile aggregates

In this paper we study breakage rate statistics of small colloidal aggregates in non-homogeneous anisotropic turbulence. We use pseudo-spectral direct numerical simulation of turbulent channel flow and Lagrangian tracking to follow the motion of the aggregates, modelled as sub-Kolmogorov massless particles. We focus specifically on the effects produced by ductile rupture: This rupture is initially activated when fluctuating hydrodynamic stresses exceed a critical value, $\sigma>\sigma_{cr}$, and is brought to completion when the energy absorbed by the aggregate meets the critical breakage value. We show that ductile rupture breakage rates are significantly reduced with respect to the case of instantaneous brittle rupture (i.e. breakage occurs as soon as $\sigma>\sigma_{cr}$). These discrepancies are due to the different energy values at play as well as to the statistical features of energy distribution in the anisotropic turbulence case examined.Comment: Accepted for publication in Phys. Rev. E (April 2015

Large-eddy simulation of turbulent dispersed flows: a review of modelling approaches

In large-eddy simulation (LES) of turbulent dispersed flows, modelling and numerical inaccuracies are incurred because LES provides only an approximation of the filtered velocity. Interpolation errors can also occur (on coarse-grained domains, for instance). These inaccuracies affect the estimation of the forces acting on particles, obtained when the filtered fluid velocity is supplied to the Lagrangian equation of particle motion, and accumulate in time. As a result, particle trajectories in LES fields progressively diverge from particle trajectories in DNS fields, which can be considered as the exact numerical reference: the flow fields seen by the particles become less and less correlated, and the forces acting on particles are evaluated at increasingly different locations. In this paper, we review models and strategies that have been proposed in the Eulerian\u2013Lagrangian framework to correct the above-mentioned sources of inaccuracy on particle dynamics and to improve the prediction of particle dispersion in turbulent dispersed flows

Time persistency of floating particle clusters in free-surface turbulence

We study the dispersion of light particles floating on a flat shear-free surface of an open channel in which the flow is turbulent. This configuration mimics the motion of buoyant matter (e.g. phytoplankton, pollutants or nutrients) in water bodies when surface waves and ripples are smooth or absent. We perform direct numerical simulation of turbulence coupled with Lagrangian particle tracking, considering different values of the shear Reynolds number (Re{\tau} = 171 and 509) and of the Stokes number (0.06 < St < 1 in viscous units). Results show that particle buoyancy induces clusters that evolve towards a long-term fractal distribution in a time much longer than the Lagrangian integral fluid time scale, indicating that such clusters over-live the surface turbulent structures which produced them. We quantify cluster dynamics, crucial when modeling dispersion in free-surface flow turbulence, via the time evolution of the cluster correlation dimension

Appraisal of energy recovering sub-grid scale models for large-eddy simulation of turbulent dispersed flows

Current capabilities of Large-Eddy Simulation (LES) in Eulerian-Lagrangian studies of dispersed flows are limited by the modeling of the Sub-Grid Scale (SGS) turbulence effects on particle dynamics. These effects should be taken into account in order to reproduce accurately the physics of particle dispersion since the LES cut-off filter removes both energy and flow structures from the turbulent flow field. In this paper, we examine the possibility of including explicitly SGS effects by incorporating ad hoc closure models in the Lagrangian equations of particle motion. Specifically, we consider candidate models based on fractal interpolation and approximate deconvolution techniques. Results show that, even when closure models are able to recover the fraction of SGS turbulent kinetic energy for the fluid velocity field (not resolved in LES), prediction of local segregation and, in turn, of near-wall accumulation may still be inaccurate. This failure indicates that reconstructing the correct amount of fluid and particle velocity fluctuations is not enough to reproduce the effect of SGS turbulence on particle near-wall accumulatio

Particle interaction with binary-fluid interfaces in the presence of wetting effects

In this paper, we present an Eulerian-Lagrangian methodology to simulate the interaction between a fluid-fluid interface and a solid particle in the presence of wetting effects. The target physical problem is represented by ternary phase systems in which a solid phase and a drop phase interact inside an incompressible Newtonian carrier fluid. The methodology is based on an Eulerian-Lagrangian approach that allows for the numerical solution of the Continuity and Navier-Stokes equations by using a pseudo-spectral method, whereas the drop phase is modelled by the Phase Field Method, in which a smooth transition layer represented by an hyperbolic function is considered both across the solid-fluid interface and across the drop-fluid interface. Finally, the solid phase is described in the form of a virtual force using the Direct Forcing Immersed Boundary approach. The properties of the immersed solid phase (including wetting effects), the deformability of the drops and the characteristics of the carrier fluid flow are the main controlling parameters. To simulate a ternary phase system, the solid phase is coupled to the binary-fluid phase by introducing a single well potential in the free-energy density functional, which can also control the solid surface wetting property. The capabilities of the methodology are proven by examining first 2D and 3D validation cases in which a solid particle is settling in a quiescent fluid. Then, the interaction of solid particles with a binary-fluid interface and the effects of surface wetting on the submergence of a quasi-buoyant body are discussed. Finally, the equilibrium configuration for a solid particle interacting with an equally-sized drop at different contact angles and the relative rotation of two solid particles bridged by a drop are examined in the case the interaction is induced by shear fluid flow deformations on the drop interface

Experimental Study on the Detection of Frozen Diffused Ammonia Blockage in the Inactive Section of a Variable Conductance Heat Pipe

Variable Conductance Heat Pipes (VCHP) are mainly employed to cool down electronic systems in spacecraft applications, as they can handle high temperature fluctuations in their cold source, preventing thus the systems from damaging. These fluctuations, as well as ultra-low temperatures, are always present in outer space, and one of the key steps in a VCHP design is therefore to make sure that they endure these conditions. However, not much has been written about their resilience during and after a long exposition to subfreezing conditions, i.e. temperatures lower than the freezing point of the working fluid. In this paper we implement and validate a computational routine based on a modified Flat-Front Approach to predict the VCHP temperature profile and to determine the location of the gas-vapor front. Then we continuously expose an ammonia/stainless-steel VCHP to temperatures below the ammonia freezing point for 211 hours, to later examine the formation and subsequent dynamics of a thin block of frozen ammonia which is diffused into the inactive part of the heat pipe condenser. We describe as well how a strong correlation between the adiabatic section and the reservoir temperatures is maintained (or broken) upon the occurrence (or absence) of the blockage of frozen ammonia.Comment: Under review for possible publication in Applied Thermal Engineerin

Experimental assessment of mixing layer scaling laws in Rayleigh-Taylor instability

We assess experimentally the scaling laws that characterize the mixing region produced by the Rayleigh-Taylor instability in a confined porous medium. In particular, we wish to assess experimentally the existence of a superlinear scaling for the growth of the mixing region, which was observed in recent two-dimensional simulations. To this purpose, we use a Hele-Shaw cell. The flow configuration consists of a heavy fluid layer overlying a lighter fluid layer, initially separated by a horizontal, flat interface. When small perturbations of concentration and velocity fields occur at the interface, convective mixing is eventually produced: Perturbations grow and evolve into large finger-like convective structures that control the transition from the initial diffusion-dominated phase of the flow to the subsequent convection-dominated phase. As the flow evolves, diffusion acts to reduce local concentration gradients across the interface of the fingers. When the gradients become sufficiently small, the system attains a stably-stratified state and diffusion is again the dominant mixing mechanisms. We employ an optical method to obtain high-resolution measurements of the density fields and we perform experiments for values of the Rayleigh-Darcy number (i.e., the ratio between convection and diffusion) sufficiently large to exhibit all the flow phases just described, which we characterize via the mixing length, a measure of the extension of the mixing region. We are able to confirm that the growth of the mixing length during the convection-dominated phase follows the superlinear scaling predicted by previous simulations

Appraisal of energy recovering sub-grid scale models for large-eddy simulation of turbulent dispersed flows

Current capabilities of Large-Eddy Simulation (LES) in Eulerian-Lagrangian studies of dispersed flows are limited by the modeling of the Sub-Grid Scale (SGS) turbulence effects on particle dynamics. These effects should be taken into account in order to reproduce accurately the physics of particle dispersion since the LES cut-off filter removes both energy and flow structures from the turbulent flow field. In this paper, we examine the possibility of including explicitly SGS effects by incorporating ad hoc closure models in the Lagrangian equations of particle motion. Specifically, we consider candidate models based on fractal interpolation and approximate deconvolution techniques. Results show that, even when closure models are able to recover the fraction of SGS turbulent kinetic energy for the fluid velocity field ( not resolved in LES), prediction of local segregation and, in turn, of near-wall accumulation may still be inaccurate. This failure indicates that reconstructing the correct amount of fluid and particle velocity fluctuations is not enough to reproduce the effect of SGS turbulence on particle near-wall accumulation

Lagrangian filtered density function for LES-based stochastic modelling of turbulent dispersed flows

The Eulerian-Lagrangian approach based on Large-Eddy Simulation (LES) is one of the most promising and viable numerical tools to study turbulent dispersed flows when the computational cost of Direct Numerical Simulation (DNS) becomes too expensive. The applicability of this approach is however limited if the effects of the Sub-Grid Scales (SGS) of the flow on particle dynamics are neglected. In this paper, we propose to take these effects into account by means of a Lagrangian stochastic SGS model for the equations of particle motion. The model extends to particle-laden flows the velocity-filtered density function method originally developed for reactive flows. The underlying filtered density function is simulated through a Lagrangian Monte Carlo procedure that solves for a set of Stochastic Differential Equations (SDEs) along individual particle trajectories. The resulting model is tested for the reference case of turbulent channel flow, using a hybrid algorithm in which the fluid velocity field is provided by LES and then used to advance the SDEs in time. The model consistency is assessed in the limit of particles with zero inertia, when "duplicate fields" are available from both the Eulerian LES and the Lagrangian tracking. Tests with inertial particles were performed to examine the capability of the model to capture particle preferential concentration and near-wall segregation. Upon comparison with DNS-based statistics, our results show improved accuracy and considerably reduced errors with respect to the case in which no SGS model is used in the equations of particle motion