14:00
Multiphase Flows 8
14:00
15 mins
#45
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DNS and modelling dynamics of inertial particles in the under-resolved shear turbulence
Alexis Barge, Mikhael Gorokhovski
Abstract: In a homogeneous shear flow, we considered the motion of a small but heavy particle. What are the effects of strongly intermittent small-scale turbulent dynamics on the acceleration of such a particle, and how to simulate those effects if the turbulence is under-resolved - these are questions addressed in the present work. First, in DNS statistics of a fluid particle acceleration in the periodic box turbulence with uniform mean shear, we observed the following features. The acceleration direction, been correlated on the Kolmogorov time, relaxes stochastically to some preferential direction. The Kadanoff’s block picture of acceleration directions shows that the main contribution to this direction is represented by axial direction of large-scales vortical structures.The acceleration norm, been in turn correlated on the mean shear time-scale, follows very precisely the lognormal distribution with parameters corresponding to the equality between root mean square and mean values of the acceleration norm. On the other hand, the norm of acceleration and its direction behave, practically, in a statistically independent way. Concerning the regenerating cycle, it is shown that the acceleration norm correlation is more persisting during the growth of vortical structures, than during their collapse. On the basis of these observations, two stochastic processes, one for the acceleration norm and another for the acceleration direction, were formulated and implemented on residual scales in our LES simulations of shear turbulence with dragged heavy particles. The assessment of these simulations by DNS shows their high effeciency in predictions of DNS statistics for different Reynolds and Stokes numbers. Stretched tailes in the PDF of the acceleration, long correlations for its norm and short correlations for its direction, these statistics followed fairly good the results from DNS. The normalized Voronoï volumes PDF shows that the prediction of preferential concentration of particles may be also improved if the stochastic subgrid acceleration model is used in LES.
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14:15
15 mins
#50
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Numerical analysis of fully resolved ellipsoidal particle dynamics in isotropic decaying turbulence
Konstantin Fröhlich, Lennart Schneiders, Matthias Meinke, Wolfgang Schröder
Abstract: Turbulent particle-laden flows are encountered in a large number of technical and natural environments and are governed by a vast parameter space. For instance, pulverized biomass and coal combustion involves a wide range of particle and turbulent scales, including Kolmogorov-length-scale size ellipsoidal particles. Accurate models for translational and rotational dynamics are required to predict, e.g., the heat transfer via radiation and the particle ignition process in a highly turbulent combustion chamber. The relevant parameter space, i.e., Kolmogorov-length-scale size non-spherical particles with particle Reynolds number Rep > 0, can not be described by Lagrangian point-particle models, and hence can currently only be studied by a fully resolved approach. Such a fully resolved approach is available with direct particle-fluid simulations (DPFS), where all turbulent scales and particle scales including the particle wakes and boundary layers are resolved.
In this contribution, DPFS of 60,000 ellipsoidal Kolmogorov-length-scale size particles suspended in isotropic decaying turbulence with aspect ratios β = 0.3 to 8 and identical volume-equivalent diameter are presented. The parameters are chosen to study a dilute suspension, i.e., particle collisions have minor statistical relevance, with significant turbulence modulation. The purpose of this analysis is to identify the relevant scales and parameters for the rotational and translational dynamics of the particles using Lagrangian auto-correlation functions.
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14:30
15 mins
#598
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Direct numerical simulations of heat transfer in fluidized beds of spherical particles
Mehdi Mehdi Niazi Ardekani, Christophe Mehdi Duwig, Luca Brandt
Abstract: Controlling heat and mass transfer in particulate suspensions has many important applications such as packed and fluidized bed reactors and industrial dryers. In this work, we study the effect of particle volume fraction and of the Grashof number ($Gr$) on the heat and mass transfer within a suspension of rigid spherical particles in a vertical box, using the immersed boundary method (IBM) to account for the solid-fluid interactions and a volume of fluid (VoF) approach \cite{bib:ArdekaniH1,bib:ArdekaniH2} to resolve the temperature equation both inside and outside the particles. We perform direct numerical simulations of a section of liquid-solid circulating fluidized bed (L-S CFB) with different Grashof numbers and particle volume fractions, aiming to accelerate the process.
Different Grashof numbers $0$, $10000$ and $40000$ are simulated for particle volume fractions $1$, $5$ and $10\%$ where a cold flow and hot particles with density ratio of $1.02$ and Galileo number $Ga = 80$ are introduced at the bottom of the computational domain. The average particle velocity and temperature are monitored, aiming to maximise the heat transfer while the particles are efficiently cooled down during their rise to the top of the box. Our results indicate that increasing the Grashof number increases the average particle velocity for the large volume fractions while its effect on the average temperature is almost negligible. At higher $Gr$ numbers, particle clustering is observed after the flow exhibits a transition to turbulence. Detailed statistics of the fluid and particle phase will be presented at the conference.
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14:45
15 mins
#501
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Momentum and heat transport in multiphase natural convection
Chong Shen Ng, Roberto Verzicco, Detlef Lohse
Abstract: The complex interplay of liquid carriers and bubbles can alter the transport of momentum and heat in a turbulent flow. Bubbly flows are present in many geophysical phenomena, such as in the rising CO2 plumes in lakes [4], and can be utilised in engineering applications, such as to enhance reaction rates of stirred tank reactors in the chemical industry [1]. In this work, we investigate the influence of bubbles on the background turbulence and heat transport of the carrier fluid using direct numerical simulations. The basic configuration is vertical natural convection, which is a buoyancy driven flow confined between two vertical differentially heated walls. We represent the bubbles as finite-size light Lagrangians that are mechanically and thermally coupled to the fluid. Specifically, the deformable droplet interfaces are modelled by immersed boundaries and the interaction potential approach [3, 2]. To distinguish the role of the coupling, we separately enable the mechanical and thermal coupling between the droplets and the carrier fluid and analyse the statistics of the bulk region. It is shown that the degree of turbulence and heat transport can be related to the strength of the droplet-induced turbulence.
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15:00
15 mins
#387
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scaling parameters of subaqueous sediment bedforms in turbulent open channel flow
Markus Scherer, Aman Ghebremichael Kidanemariam, Markus Uhlmann
Abstract: In classical morphodynamics, subaqueous sediment patterns are classified as dunes or ripples depending on their typical length scales, i.e. whether their wavelength is a function of the fluid height or of the particle diameter. However, more recent studies suggest, that this classical distinction might be incomplete. To study the relevant scaling parameters of initial sediment patterns, an immersed boundary method is used in combination with a soft-sphere particle collision model to perform highly-resolved direct numerical simulations of subaqueous sediment patterns in a horizontal channel flow at moderate Reynolds numbers. The particle diameter and the fluid height are varied independently in order to investigate their influence on the scaling properties of the bedforms.
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15:15
15 mins
#605
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Assessment of structural-type subfilter models for particle-laden large eddy simulations
Jacek Pozorski, Maria Knorps, Bogdan Rosa
Abstract: The LES of two-phase disperse flows have been increasingly popular, especially for cases where the large scales of fluid motion control the dynamics of inertial particles. There has been a long debate about the impact of subflter or subgrid scales (SGS), not resolved in LES, on the dynamics of the particulate phase. When dealing with SGS turbulent dispersion, so-called functional and structural models may be distinguished. In the latter, the subfilter velocity field is reconstructed to some extent; hence the name `structural'. A few categories of structural approaches to subfilter particle dispersion in LES are presented and assessed in this work. We report on our recent experience with kinematic simulations applied to particles subject to gravity in the field of isotropic turbulence. This technique, proven useful for the enhancement of the particle collision rates, has difficulties to retrieve the increased settling velocity
of particles due to the preferential sweeping mechanism. Concerning the fractal interpolation, we discuss the representative choice of the interpolation points as well as determination of the stretching parameters. At last, we implement the idea of multifractal enstrophy cascade, proposed as single-phase SGS closure model for LES. We apply it as a structural-type model and demonstrate how such approximately reconstructed subfilter field affects the particle statistics in isotropic turbulence in terms of collision rates and settling velocity, against the DNS and a priori LES results.
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