Lorentz Center - Particles in Turbulence from 14 May 2012 through 16 May 2012
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    Particles in Turbulence
    from 14 May 2012 through 16 May 2012

 

Abstracts

 

Markus Abel

Generalized framework for particle advection using template metaprogramming and MPI

 

In recent times we have sketched a general scheme to combine C++ programming strategies with parallel computation. Our goal is to develop a framework capable to either interface with data or with a CFD code in order to produce particle trajectories for the different numerical codes available on the market. We aim at a high performance for either scientific or applied situations.

 

 

Luca Biferale

Dynamics and breakup of droplets in turbulence

 

 

Eberhard Bodenschatz

The Sling Effect

 

 

Guido Boffetta
Stokes drift for Stokes particles

The motion of inertial particles induced by linear gravity waves
is studied using analytical and numerical methods. The effects of inertia on
the Stokes drift of particles and on sedimentation velocity is discussed.

 

Humberto Bocanegra Evans

Tracking Material Lines of Dispersed Phase in Turbulent Flows

 

We propose a novel diagnostic that will allow the tracking of dispersed phase material lines through droplet tagging. A chamber capable of creating (nearly) isotropic homogeneous flow is densely seeded with droplets. The droplets are created from a phosphorescent solution made out of a lanthanide chelate (Eu+3). The lifetime of the phosphorescent signal is on the order of 1 ms, which is comparable to the Kolmogorov time scale. A well-defined volume of the droplet cloud is tagged through a high-power laser and tracked in time using an intensified high speed camera. The deformation and intensity profile of the tagged volume provide insight into the dynamical aspects of preferential concentration in turbulent flows at the smallest scales, i. e. the Kolmogorov scales.

 

 

Geert Brethouwer

Lattice-Boltzmann simulations of finite-size fibres in turbulent channel flow

Joint work with Minh Do-Quang, Gustav Amberg, Arne V. Johansson

 

In the present study, we investigated the orientation and distribution of finite-size and rigid fibres in turbulent channel flow through fully resolved DNS. The turbulent flow is modelled by an entropy lattice-Boltzmann method and the interaction between fibres and carrier fluid is odelled through an external boundary force (EBF) method. Direct contact and lubrication force models for fibre-fibre, fibre-wall interactions have been implemented as well in this study.

 

Simulations have been carried out in channel flow with a friction Reynolds number of 180 and fibre lengths between 3 and 24 viscous length units. The fibre diameter has been kept at 2 viscous units in all cases and the total number of fibres is up to 65000. The single phase case has been shown to give good agreement with standard simulations with pseudospectral methods for the same Reynolds number.

 

The simulations show that the fibres accumulate in high-speed streaks near the wall. This preferential concentration is caused by the interaction of the fibres with the wall and not by inertial effects because the fibres are almost neutrally buoyant. Since the fibres accumulate in the high-speed streaks, they have a higher mean velocity than the fluid flow.Longer fibres show a more accentuated excess in mean velocity near the wall. We present a conceptual model for this behaviour. Moreover, near the wall, shorter fibres have a higher fluctuation intensities in spanwise and wall-normal directions than longer fibres. Some turbulent statistical quantities of fibres and fluid will be shown in order to understand the new observations.

 

 

Carlo Casciola

A new algorithm for particulate turbulent flows in the two-way coupling regime

Joint work with P. Gualtieri, G. Sardina and F. Picano

 

At increasing mass loads, small particles heavier than the surrounding fluid are able to modify the dynamics of the turbulent suspension by their back reaction on the carrier fluid.

The related phenomenology has been addressed in detail for turbulent anisotropic flows by the authors using standard particle in cell methods. This approach should be used with upmost care, given certain intrinsic difficulties related to the description of the particle as a localized source of momentum for the flow, associated with the Stokes drag the particle experiences in the relative motion with respect to the carrier fluid (slip velocity).

Purpose of the present talk is discussing the main features of a new way to treat the coupling problem currently under development in view of fast and accurate simulations of huge number of sub-Kolmogorov particles. The idea is using in a smart way certain properties of the unsteady Stokeslet to inject in the computation mesh the vorticity generated in the interaction between particles and carrier fluid. Potentially and pitfalls of the new approach will be discussed using known elementary solution for the fluid-particle interaction problem and presenting preliminary results concerning the alteration of the turbulence due to coupling with the suspended phase in comparison with careful simulations made with the standard particle in cell method.

 

 

Massimo Cencini

Clustering of Gyrotactic Swimmers in Turbulent Flows

 

The clustering properties of model gyrotactic organisms are studied in low Re turbulent flows by means of Direct numerical simulations (DNS).

In particular, we show that when turbulent acceleration is large enough gyrotactic organisms clusterize in intense vorticity regions.

However, when turbulent acceleration is much smaller than gravitational acceleration, clustering is still possible via another mechanism. We also show that crucial parameter is the reorientation time of the microorganisms with respect to the fluid vorticity.

 

 

Laurent Chevillard

Lagrangian alignments of vorticity, predictions from a stochastic model and further considerations on pressure Hessian

 

We first review experimental and numerical facts on the preferential alignments of vorticity with the eigenframe of the deformation. We then propose a stochastic model for the velocity gradient tensor able to reproduce realistic alignments, in particular in a Lagrangian fashion. We finally underline limitations of the model to give a realistic picture of pressure effects. A new local-in-space approximation of the pressure Hessian is shown to perform well against DNS.

 

 

Benjamin Devenish*

Multiparticle dispersion in homogeneous isotropic turbulence

 

The dispersion of groups of correlated particles in turbulent flows is of much interest, both because of its relationship with higher order moments of the concentration field and because it allows connections to be made between the statistical and structural descriptions of turbulence. Information on the geometry of a 3-D flow can be obtained by following four particles, a tetrad. In recent years experimental techniques1 2 and numerical simulations 3 4 5 of tetrads have provided much insight into the nature of turbulence. Here, a simple Lagrangian stochastic model (LSM) for tetrads is compared with a direct numerical simulation (DNS) of turbulence. The

model is an extension of Thomson’s model6 for particle pairs. It is constructed to satisfy the well-mixed condition and has as an input the constant of proportionality in the second order Lagrangian velocity structure function, C0. By varying C0, it is possible to make the particles’ motion more (large C0) or less (small C0) diffusive than in real turbulence and thereby assess the relative importance of ballistic versus diffusive motion in real turbulent flows. Furthermore, results will also be presented for a diffusion equation with a separation-dependent eddy diffusivity: K(r) / r4/3 where r is the separation between any two particles. This is the limiting case of the LSM in the limit C0 ! 1 and is an extension of Richardson’s model7 for particle

pairs to tetrads. DNS of tetrads show that they tend to form more elongated shapes than is the

case for tetrads formed from four uncorrelated particles. The results of both the LSM (for values of C0 typical of real turbulence) and Richardson’s diffusion model agree well with the DNS results. This indicates that in the LSM no significant variation of the shape statistics with C0 is expected. The absence of intermittency effects in the two models, and the good agreement with DNS, suggests that intermittency does not play an important role in determining the shape statistics of tetrads in real turbulence. Moreover, the results also suggest that the tendency to form elongated shapes is a kinematic effect, rather than a specific dynamical effect, for any flow field in which the dispersion is reasonably well approximated by a diffusivity of the form

K(r). Indeed, it will be shown that in a generalised Richardson diffusion model, in which the diffusivity is proportional to rm, the ‘degree of elongation’ varies with m: for m > 4/3 the tetrads tend to form more elongated shapes than in real turbulence while the reverse is true for m < 4/3. Results on Lagrangian four-point velocitydifference statistics will also be discussed.

 

*Met Office, FitzRoy Road, Exeter, EX1 3PB, UK

1Xu et al., New J. Phys. 10, 123015 (2008)

2L¨uthi et al.,J. Turb. 8, no. 45 (2007)

3Pumir et al., Phys. Rev. Lett. 85, 5324 (2000)

4Biferale et al., Phys. Fluids 17, 111701 (2005)

5Hackl et al. Phys. Fluids 23, 065103 (2011)

6Thomson, J. Fluid Mech. 210, 113 (1990)

7Richardson, Proc. R. Soc. London, Ser A 110, 709 (1926).

 

 

Herman Clercx

Table-top rotating turbulence: an experimental insight through particle tracking

Joint work with Lorenzo Del Castello and Laurens van Bokhoven

 

The influence of the Earth background rotation on oceanic and atmospheric currents, as well as the effects of a rapid rotation on the flow inside industrial machineries like mixers, turbines, and compressors, are typical examples of fluid flows affected by rotation. Rotating turbulence has often been studied by means of numerical simulations and analytical models, but the experimental data available is scarce and purely of Eulerian nature. In the present study experiments on continuously forced turbulence subjected to different background rotation rates have been performed. Quantitative information on the flow field is obtained by means of stereoscopic Particle Image Velocimetry (allowing to obtain the three velocity components in a 2D plane; Eulerian approach) and 3D Particle Tracking Velocimetry (providing the 3D velocity field in a certain volume of the flow domain; Lagrangian approach). The background rotation is confirmed to induce 2-dimensionalisation of the velocity field. We will illustrate this in the talk with the behaviour of the Lagrangian velocity PDFs with increasing rotation rates and by the presence of the large scales which are dominated by stable counter-rotating vertical tubes of vorticity. Direct effects of the background rotation are also the suppression of high-acceleration events parallel to the (vertical) rotation axis, the enhancement of high-acceleration events for the horizontal acceleration, and the strong amplification of the Lagrangian auto-correlation of the acceleration component perpendicular to both the rotation vector W and local velocity vector u. The Lagrangian auto-correlation of the acceleration component in the plane set up by W and u is only mildly and indirectly enhanced.

 

 

Gerrit Elsinga

Lagrangian measurements at higher seeding densities

 

 

Gregory Falkovich

Stochastic geometry of turbulence

 

Ksenia Guseva

Influence of flow-particle density ratio on fragmentation-aggregation dynamics in turbulent flows

 

In this work we perform a study of the aggregation-fragmentation dynamics in synthetic turbulence. We are interested in the effect of the density of the advected particles on the process. The advection is modeled using a Lagrangian description which incorporates particle inertia, and considers the approximation of spherical particles displaced by the action of forces which include the Stokes drag and an added mass term. Particles of different densities follow distinct trajectories when carried by the flow field. The two distinct well known cases of preferential concentration are bubbles and aerosols, which have particle density lower and higher than the fluid density, respectively. These two particle types segregate in distinct regions of the flow, with high vorticity in the case of bubbles, and low vorticity in the case of aerosols. In our study we are particularly interested in the sizes of formed aggregates which result from the dynamics of collision and break-out of particles during their advection. The collision rate depends on the fractal size of the segregation region, while the fragmentation rate on the other hand depends on the shear forces that the particle experiences in that region. We systematically study the dependence of aggregation and fragmentation on the flow-particle density ratio in both cases of light and heavy particles, and we investigate the influence of the flow properties on the final size distribution of aggregates. Additionally, we consider the aggregation-fragmentation process of two types of primary particles with slightly distinct densities. We obtain the collision rates for particles in such a mixture, and characterise how the size distribution of the aggregates and the aggregate composition depend on the fragmentation rate and on the densities of the primary particles. This part of the study is motivated by the formation and the break up of marine aggregates, which consist of organic and inorganic components having different densities. Depending on their environment their composition can be very different, and their dynamics is an essential part in biological cycles in the ocean.

 

Yannis Hardalupas

Air flow turbulence interaction with spray droplets

 

 

Michel van Hinsberg

Interpolation error in DNS simulations of turbulence: consequences for particle tracking

 

An important aspect in numerical simulations of particle laden turbulent flows is the interpolation of the flow field needed for the computation of the Lagrangian particle trajectories. This study focuses on estimating the interpolation error and compares it with the discretisation error of the flow field. In this way one can balance the errors in order to achieve an optimal result. As a spin-off of the theoretical analysis a practical method is proposed which enables direct estimation of the interpolation error from the energy spectrum. The same is done for the discretisation error. The theory is verified by DNS simulations using a spectral code. Here fluid particle trajectories are computed using several interpolation methods. It is shown that B-spline interpolation has the best accuracy compared with the computational overhead. The accuracy of the interpolation method has direct consequences for the acceleration spectrum of the fluid particle and is also important for the correct calculation of the hydrodynamic forces on neutrally buoyant particles.

 

 

Rudie Kunnen

Tracking glowing particles

Joint work with M.A. Yavuz, H.J.H. Clercx

 

Turbulence-induced coagulation of droplets is an important effect in the growth of raindrops. The effects of turbulence on the coagulation process are not yet fully understood. We are preparing an experiment, which we describe in this presentation. In the proposed experiment we insert fluorescent droplets with a typical diameter of 20 microns into a speaker-driven turbulence chamber. The droplets are excited with a UV laser. The fluorescent light emitted by the particles is captured with four intensified cameras. 3D particle tracking is used to gather droplet trajectories. We also investigate the development in time of the droplet size spectrum with phase-Doppler anemometry. With this experiment we will study the coagulation of droplets in turbulence.

 

 

Detlef Lohse

News from the Twente Turbulent Taylor-Couette (T^3C) facility

Next to Rayleigh-Benard convection, Taylor-Couette flow is the paradigmatic flow in closed systems. This holds in particular for turbulent flows in closed systems. In the last years we have build a facility with which we can reach a Reynolds number of 2e6 and which allows for independent inner and outer cylinder rotation. This is enough to achieve the so-called ultimate regime of turbulence in which the boundary layers become unstable. We will report on latest measurements with this facility and on accompanying numerical simulations.


Marco Martins Afonso

Inertial-particle dispersion and diffusion

 

We analytically investigate the dynamics of inertial particles in incompressible flows in the limit of small but finite inertia, focusing on two specific instances. First, we study the concentration of particles continuously emitted from a point source with a given exit velocity distribution. The anisotropy of the latter turns out to be a necessary factor for the presence of a correction (with respect to the corresponding tracer case) at order square root of the Stokes number. Secondly, by means of a multiple-scale expansion, we analyse the particle effective diffusivity, and in particular its dependence on Brownian diffusivity, gravity effects and particle-to-fluid density ratio. In both cases, we obtain forced advection--diffusion equations for auxiliary quantities in the physical space, thus simplifying the problem from the full phase space to a system which can easily be solved numerically.

 

 

Bernhard Mehlig

Clustering, caustics, and collisions of inertial particles in random flows

 

In this talk I describe recent analytical progress in understanding clustering and collision processes of inertial particles suspended in smooth random flows. Three results are summarised. First, I discuss mechanisms leading to clustering of inertial particles in mixing flows (preferential concentration and multiplicative amplification). I demonstrate under which circumstances one or the other mechanism dominates, and when both are important. Second, I summarise new results concerning the rate-of-caustic formation for inertial particles in random flows. Third, these results determine the distribution of collision velocities. I describe the known properties of this distribution.  I address to which extent these results describe the dynamics of inertial particles suspended in multi-scale turbulent flows, and conclude with a number of open questions. The talk is based on results obtained together with my main collaborators

M. Wilkinson and K. Gustavsson.

 

 

Jean-Pierre Minier

PDF and Lagrangian stochastic modelling of two-phase flows: some theoretical issues and applications for particle deposition

 

The purpose of the presentation is to introduce Lagrangian stochastic models for poly-dispersed two-phase flows. The theoretical

framework will be first recalled so as to emphasize that Lagrangian stochastic approaches are PDF models and to relate present

formulations with respect to various PDF descriptions. Some theoretical issues that may be overlooked in current similar models

will be brought out. The potential and interest of such approaches will be illustrated by showing how chemical effects and

surface properties can be directly and explicitly included in a complete simulation of particle deposition.

 

 

Stefano Musacchio

Control of particle clustering in turbulence by polymer additives

 

We study the clustering properties of inertial particles in a turbulent viscoelastic fluid. The investigation is carried out by means of direct numerical simulations of turbulence in the Oldroyd-B model. The effects of polymers on the small-scale properties of homogeneous turbulence are considered in relation with their consequences on clustering of particles, both lighter and heavier than the carrying fluid. We show that, depending on particle and flow parameters, polymers can either increase or decrease clustering.

 

Vivek Nagendra Prakash

Three-dimensional Lagrangian Voronoï analysis for clustering

of inertial particles in turbulence

Joint work with Y. Tagawa, J. Martinez Mercado, E. Calzavariniy,,C. Sun and D. Lohse

 

We quantify the clustering of inertial particles in homogeneous isotropic turbulence using three-dimensional Voronoï analysis on data sets from numerics in the point particle limit and one experimental data set1. The clustering behavior at different density ratios, particle response times (i.e. Stokes numbers St) and two Taylor-Reynolds numbers Re are investigated. The Probability Density Functions (PDFs) of the Voronoï cell volumes of light and heavy particles show a different behavior from that of randomly distributed particles, implying that clustering is present. The standard deviation of the PDF normalized by that of randomly distributed particles is used to quantify the clustering. Light particles show maximum clustering for St around 1􀀀2.

The experimental dataset shows reasonable agreement with the numerical results.

These results agree well with previous investigations. For a Lagrangian clustering analysis, we calculate the Lagrangian temporal autocorrelation function of Voronoï volumes. We find that the clustering of light particles lasts much longer than that of heavy or neutrally buoyant particles. We also compare the ratio of decorrelation times of Voronoï volume (_V ) and enstrophy (_) and find that this ratio for both light and heavy particles is greater than unity for all St. Remarkably, this means that light and heavy particles remain clustered for much longer times than the ow structures which cause the clustering, due to inertial effects arising from the different density ratios.

 

Nicholas Ouellette
Tracking Active Particles: Dynamics of Insect Swarms

 

Jacek Pozorski

A priori analysis of an anisotropic subfilter model for heavy particle dispersion

Joint work with Maria Knorps

 

Mike Reeks

The Resuspension of Small Particles by a Turbulent Flow

 

Nicolas Rimbert
Crossover between large scale instabilities and small scale intermittencies in liquid drop breakup

Liquid atomization is now a widely studied field of study. It has many applications in many fields: Combustion, Air/Sea Interaction, Agricultural Spraying, Spray drying/cooling/coating...
In this talk, I will show, on a chosen experiment, how first models developed thanks to instability theory, while unwieldy and somewhat inaccurate are still valid to describe the large scale of the droplet field produced. In the meantime, drawn by practical considerations and to cope with discrepancies between experiments and instability models, a turbulent atomization theory, based on Kolmogorov's lognormal distribution, has been developed. While still of practical use to describe the small scales of the droplet distribution, the accuracy of this model can be improved by extending it to log-Lévy stable distribution. This can be related to some classical modelings of turbulent intermittencies which have been rooted more recently into theory thanks to a self-avoiding random vortex stretching scenario.

Monchaux Romain

Heavy particle concentration: a dynamical analysis

 

Bogdan Rosa1

Numerical and experimental investigation of cloud droplet collision-coalescence in turbulent flows.

Joint work with Hossein Parishani2, Orlando Ayala2, Alberto Aliseda3 and Lian-Ping Wang2

1Institute of Meteorology and Water Management – National Research Institute, 61 Podlesna St., 01-673 Warsaw, Poland;

2University of Delaware, Mechanical Engineering, 126 Spencer Laboratory. University of Delaware, Newark, Delaware 19716-3140, United States;

3University of Washington, Mechanical Engineering, 4000 15th Ave NE, Seattle, Washington 98195-2600, United States.

 

In recent years, modeling of atmospheric flows at meso-γ scale has emerged as an important research topic in numerical weather prediction. Systematic progress in this field results from both continuous improvement in the numerical methods and availability of high-performance supercomputers. By employing sub-kilometer grid spacing for operational forecasting, it is expected that severe weather events triggered by deep moist convection can be explicitly resolved. Such a resolution approaches the regime for which the convective processes can be explicitly represented. Nevertheless, moist processes related to cloud physics still need to be better parameterized. Investigation of the moist processes by direct measurements of droplet-droplet and droplet-turbulence interactions in real clouds is difficult due to the short time and small length scales involved. Therefore, in this study we focus on the numerical and laboratory-experimental investigation of collision-coalescence of cloud droplets. Quantitative description of this process is of great importance since the collision-coalescence plays important role in the development of warm rain, that is, the transformation of small cloud droplets to rain drops. Our experimental approach is aimed at developing in the wind tunnel a turbulent flow and droplet distribution similar to those occurring in the real cloud.  Using direct numerical simulations (DNS) we are able to realistically reproduce the conditions that take place in the wind tunnel. Together, we hope to combine these two different tools to gain a better quantitative understanding of turbulent collision-coalescence of cloud droplets.

 


Riccardo Scatamacchia

Particles statistics in turbulent ows from point source emissions: extreme events

Joint work with L. Biferale, A. Lanotte, F. Toschi

 

We present a detailed investigation of particles statistics in homogeneous isotropic turbulence. We use data from a 3D direct numerical simulations at 10243 collocation points and R_ = 300 following the evolution of a huge number of passive tracers and heavy inertial particles, with Stokes numbers in the range St 2 [0:5; 5]. In particular, our simulation aims to investigate extreme events characterising the distribution of absolute and relative dispersions in turbulent ows. To do that, we seeded the ow with hundred millions of particles emitted from localized sources in time and in space (see _gure 1). Thanks to such huge statitics we are able to assess in a quantitative way deviations from Richardson's prediction due to _nite size and _nite time-correlation lenghts in the particle pairs distribution. Furthermore, we present the same kind of

measures for heavy particles.

 

 

Chao Sun

How gravity and size affect the acceleration statistics of bubbles in turbulence

Joint work with V. N. Prakash, Y. Tagawa, E. Calzavarini, J. Martinez Mercado, F. Toschi, and D. Lohse

 

We report results from the first systematic experimental investigation in the regime of very light and large particles in turbulence. Using a traversing camera setup for two-dimensional particle tracking, we study the Lagrangian acceleration statistics of _3 mm diameter (D) air bubbles in water in active-grid generated turbulence. The Reynolds number (Re_) is varied from 145 to 230, resulting in size ratios, _ = D=_ in the range of 7.3{12.5, where _ is the Kolmogorov length scale. Experiments reveal that gravity does not affect the mean value of the vertical acceleration component but has a g2 offset on its variance. Once this gravity offset is subtracted the  variances of both the horizontal and vertical acceleration components are about 5 _ 2 times

larger than the one measured in the same flow for fluid tracers, but still below the estimated upper bound derived from the added-mass effect alone (which is 9 times the tracer value). This is a mark of the finite-sized nature of the bubble. The present experimental acceleration variance measurements are closely matched by numerical simulations of finite-size bubbles1 where no gravity and just the Faxen correction to the added-mass force has been taken into account.

We compare the PDF of the normalized horizontal (x) acceleration component along with the PDF from the DNS simulations with Faxen corrections, to further study the finite-size effects on the bubbles. The finite-sized bubbles show a strongly reduced intermittent shape. It is the first time that such a substantial change in intermittency at growing size ratio is experimentally observed. The DNS appears to underestimate its functional behavior by a factor _2-3 in the size ratio _.

 

Yoshiyuki Tagawa 

Lagrangian statistics of light particles in turbulence

Joint work with J. Martinez Mercado_y, V. N. Prakash_y, C. Sun_y and D. Lohse_y

 

We study the Lagrangian velocity and acceleration statistics of light particles (micro-bubbles in water) in homogeneous isotropic turbulence. Micro-bubbles with a diameter db = 340 _m and Stokes number from 0.02 to 0.09 are dispersed in a turbulent water tunnel operated at Taylor-Reynolds numbers (Re_) ranging from 160 to 265. We reconstruct the bubble trajectories by employing three-dimensional particle tracking velocimetry (PTV). It is found that the probability density functions (PDFs) of the micro-bubble acceleration show a highly non-Gaussian behavior with atness values in the range 23{30. The acceleration atness values show an increasing trend with Re_, consistent with previous experiments1 and numerics2. These acceleration PDFs show a higher intermittency compared to tracers3 and heavy particles4 in wind tunnel experiments. In addition, the micro-bubble acceleration autocorrelation function decorrelates slower with increasing Re_. We also compare our results with experiments in von Karman flows and point-particle direct numerical simulations with periodic boundary conditions.

 

Federico Toschi
On the dynamics of non spherical particles in turbulent flow

Hadar Traugott
Resuspension of particles in an oscillating grid turbulent flow
joint work with A. Liberzon, April 3, 2012, Turbulence Structure Laboratory, School of Mechanical Engineering , Tel-Aviv University, Tel-Aviv 69978, Israel

The incipient motion condition for particulate material exposed to a moving fluid constitutes the central problem for sediment transport in river, coastal areas and atmospheric flows. Furthermore, it is an important mechanism in a variety of engineering applications, such as: silicon wafer cleaning and pneumatic conveying. Despite a significant progress in the field of sediment transport during the past decades, description of the mechanisms responsible for

the initiation of particle motion from a surface and re-entrainment into suspension remains a challenge. This is partially due to the technical difficulties to quantify the forces applied on the particles and the collection of high resolution data of particle displacement simultaneously.

In this study we investigate the necessary conditions for initial entrainment of spherical particles from smooth bed into zero-mean-shear turbulent flow in an oscillating grid chamber. The experiments are not designed to fully mimic the real problem of sediment transport but rather identify key mechanisms, utilizing direct observation and quanti_cation of particle motion at the beginning, during and after lift-off. Particle image velocimetry (PIV) was used to determine the properties of turbulent flow and three-dimensional particle tracking velocimetry (3D-PTV)

is used to examine long duration data that synchronously measure local flow conditions, and track the entrainment of individual test particles through the various phases of the resuspension. The combination of the experimental methods and different types of particles (tracers and test particles) allow to identify the dominant scales within the turbulence spectrum which cause resuspension and to explore the role hydrodnamic forces (lift and drag) play in this process. The results will provide further insight into the resuspension process of spherical particles in the

transitional range of particle size Reynolds numbers 2 < Rep < 500.

 

Lian-Ping Wang

Turbulence modulation by inertial particles

 

Turbulence modulation by inertial particles has been studied computationally by point-particle based simulations (PPS) and more recently by particle-resolved simulations (PRS).  The purpose of this talk is to compare results from PPS with those from PRS, to illustrate effects of finite-particle size and issues related to the validity and limitations of these approaches. A review of theoretical understanding on turbulence modulation by inertial particles will also be presented to help interpret published results and clarify relevant issues.

 

Norbert Warncke

Particle attachment rates to a gas-liquid interface

 

Abstract: We measured the attachment rates of dispersed particles to the surface of an air bubble that is kept stationary in a conical pipe. The experiments show changes in attachment rates during fill-up of the bubble surface with the particles. We discuss these results with regard to particle transport towards the surface and to effects of the thin-film drainage process.

 

 

Michael Wilkinson

Random Tumbling

 

There is a vast literature on how small objects undergo diffusion when subjected to random forcing, but much less has been written about how an object rotates due to a random torque. There is a dimensionless parameter characterising this problem: the persistence angle \beta is the typical angle of rotation during the correlation time of the angular velocity. When \beta is small, the problem is simply diffusion on a sphere. But little is known about models with finite \beta, describing smooth random motion on a sphere. I will discuss the formulation and solution of the simplest model, which is a spherical Ornstein-Uhlenbech process. In two dimensions (circular motion) this is exactly solvable. When \beta is large, the solution has a surprising property, which is analogous to the phenomenon of 'superoscillations'. In three dimensions we obtain asymptotic solutions for large \beta which involve a solving a radial Shroedinger equation where the angular momentum quantum number j takes non-integer values. The case where j=(\sqrt{17}-1)/2 turns out to be of particular significance. As well as discussing random tumbling of a single body, I will also mention some results on the singularities of orientation vector fields of small bodies advected in random flows. This talk reports joint work with Alain Pumir (ENS, Lyon) and Vlad Bezuglyy (Open University).


--
The Open University is incorporated by Royal Charter (RC 000391), an exempt charity in England & Wales and a charity registered in Scotland (SC 038302).

 

Keqing Xia

Experimental Studies of Lagrangian Acceleration and Pair Dispersion in Thermally-Driven Turbulence

 



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