46 research outputs found
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
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
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
Numerical simulations of aggregate breakup in bounded and unbounded turbulent flows
Breakup of small aggregates in fully developed turbulence is studied by means
of direct numerical simulations in a series of typical bounded and unbounded
flow configurations, such as a turbulent channel flow, a developing boundary
layer and homogeneous isotropic turbulence. The simplest criterion for breakup
is adopted, whereas aggregate breakup occurs when the local hydrodynamic stress
, with being the energy dissipation
at the position of the aggregate, overcomes a given threshold
, which is characteristic for a given type of aggregates.
Results show that the breakup rate decreases with increasing threshold. For
small thresholds, it develops a universal scaling among the different flows.
For high thresholds, the breakup rates show strong differences between the
different flow configurations, highlighting the importance of non-universal
mean-flow properties. To further assess the effects of flow inhomogeneity and
turbulent fluctuations, theresults are compared with those obtained in a smooth
stochastic flow. Furthermore, we discuss the limitations and applicability of a
set of independent proxies.Comment: 15 pages, 12 figures, Refinded discussion in Section 2.1, results
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Physics and Modelling of Particle Deposition and Resuspension in Wall-Bounded Turbulence
The objective of this chapter is twofold. First, it provides a general overview of the Eulerian-Lagrangian modelling approach to the numerical simulation of turbulent dispersed flows in the point-particle limit. Second it reviews the phenomenology of particle deposition and resuspension in wall-bounded turbulence as brought to light by Eulerian-Lagrangian studies over the last two decades. Specific interest is devoted to the case of inertial particles, which are ubiquitous in environmental and industrial flow-systems. Effects due to particle shape on deposition and resuspension mechanisms, as well as on numerical modelling are also addressed
Collective dynamics of particles: from viscous to turbulent flows
The book surveys the state-of-the-art methods that are currently available to model and simulate the presence of rigid particles in a fluid flow. For particles that are very small relative to the characteristic flow scales and move without interaction with other particles, effective equations of motion for particle tracking are formulated and applied (e.g. in gas-solid flows). For larger particles, for particles in liquid-solid flows and for particles that interact with each other or possibly modify the overall flow detailed model are presented. Special attention is given to the description of the approximate force coupling method (FCM) as a more general treatment for small particles, and derivations in the context of low Reynolds numbers for the particle motion as well as application at finite Reynolds numbers are provided. Other topics discussed in the book are the relation to higher resolution immersed boundary methods, possible extensions to non-spherical particles and examples of applications of such methods to dispersed multiphase flows
Point-particle Euler-Lagrange simulations of flexible fibers in turbulence
Euler-Lagrange simulations of pointwise particles in turbulence have been widely employed for understanding the fundamental physics of dispersed flows. Most of the times, particles are modelled as isotropic and rigid. In this paper, we investigate the dynamics of elongated flexible particles in turbulent channel flow. We consider particles that are longer than the Kolmogorov length scale of the carrier flow, and their velocity relative to the surrounding fluid is non negligible. Such particles are modelled as chains of sub-Kolmogorov rigid rods connected through ball-and-socket joints that enable bending and twisting under the action of the local fluid velocity gradients. We examine the effect of local shear and turbulence anisotropy on the translational and rotational behaviour of the fibers, considering different elongation (parameterized by the aspect ratio) and inertia (parameterized by the Stokes number). Velocity, orientation and concentration statistics, extracted from one-way and two-way coupled direct numerical simulations, will be presented to give insights into the complex fiber-turbulence interactions that arise when non-sphericity and deformability add to inertial bias. The physical problem considered here provides a useful foundation for exploring the capability of the point-particle approach to capture the macroscopic features of multiphase flows of elongated deformable particles.Non UBCUnreviewedAuthor affiliation: University of UdineResearche