| 1. |
- Scapin, Nicolo
(author)
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Phase-changing flows: numerical methods and fully resolved simulations
- 2022
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Doctoral thesis (other academic/artistic)abstract
- Flows with evaporation and boiling are abundant in different contexts, such as geophysics, the biomedical sectors, and industrial applications. Spray combustion, boiling bubble flows, oceanic sprays, formation and evolution of clouds, spreading of infectious diseases are all relevant examples where a deeper understanding of phase-changing flows is of great importance. Fully resolved simulations may assume a central role of investigation as they can overcome the current limitations of the experimental techniques and complement them.In the first part of this work, we present novel methodologies to perform interface-resolved simulations of phase-changing flows addressing the following three challenges: i) handling abrupt variations of the velocity field across the interface, ii) accurately evaluating the heat and mass interfacial fluxes, iii) incorporate compressibility inevitably present in bounded domains. Both sharp and diffuse interface formulations are considered and the resulting two methods are designed for different classes of multiphase flows. First, we devise a weakly compressible algorithm to describe incompressible evaporating droplets surrounded by a compressible gas medium treated in the low-Mach limit. This approach combines a volume of fluid method and the pressure-splitting techniques of zero-Mach methods to ensure volume conservation of the liquid phase and conservation of the mass of the compressible phase. Next, we develop a fully compressible algorithm for compressible bubbles in boiling flows, where rapid expansions and nonuniformity of the thermodynamic pressure fields make the zero-Mach limit inadequate.In the second part of the thesis, we discuss how these numerical tools can be utilized to study relevant configurations of evaporating flows. Two flow regimes are considered: i) dispersed droplets, and, ii) a horizontal gas-liquid interface. Droplets are first considered in homogeneous shear turbulence in a dilute condition. Here, we benchmark the semi-empirical correlations for the evaporation rate with the data extracted from DNS of finite-size droplets and study the effect of deformation on the global and local evaporation rate. Thereafter, we move then to a denser regime in a triperiodic domain and study the deviation from the d2-law as a function of initial gas temperature and liquid volume fractions. We confirm that even when evaporation is purely driven by diffusion, deviations from the d2-law cannot be characterized only by the initial volume fraction, but also temperature plays a role: high temperature promotes the departure from the d2-law regime at higher volume fractions, while at ambient temperature, this departure occurs at lower volume fraction. Next, we study the evaporation occurring at a gas-liquid interface in Rayleigh–Bénard convection. For this configuration, we develop an analytical prediction of the interface temperature and the global heat transfer modulation and interface-resolved simulations are employed to assess the validity of the models. The excellent agreement opens the possibility to employ the suggested law for those applications where accurate predictions of interface temperature and heat transfer are sought.
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| 2. |
- Banerjee, Indradumna
(author)
-
Point of care microfluidic tool development for resource limited settings
- 2019
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Doctoral thesis (other academic/artistic)abstract
- The development of point of care diagnostics using recent advances in microfluidics have the potential to transform health care in several ways, especially in resource limited settings with limited access to advanced health care infrastructure. However, translating a point of care device to reality is often a challenging task because of the complexities involved in integrating a number of diverse engineering concepts into an easy to use, accurate and portable device. This thesis focuses on miniaturization of crucial diagnostic laboratory tools, that can be used in a portable point of care format without compromising on the accuracy or performance. The first part of the thesis (Paper I-III) focuses on understanding and applying elasto-inertial microfluidics, which is a label-free and passive bio-particle sorting and separation method. A basic understanding of particle trajectories in both inertial (Paper I) and visco-elastic flows (Paper II) is established, followed by an investigation on the combined effects of inertia and elasticity (Paper III). The second part of the thesis (Paper IV-VI) focuses on developing integrated microfluidic platforms, each of which addresses different aspects of point of care diagnostic applications. The applications include neonatal diagnostics using a hand-driven Slipdisc technique (Paper IV), rapid nucleic acid quantification using a novel precipitate-based detection on a centrifugal microfluidics platform (Paper V), and hematocrit level measurement in blood using a portable lab-on- Disc platform operated by a mobile phone (Paper VI). The proof of concept microfluidic tools presented in the scope of this thesis have the potential to replace a number of functions of standard laboratory equipment, at a fraction of the price and without compromising performance. Hence, the different methods developed should contribute towards decentralization of medical testing laboratories, making healthcare accessible to one and all.
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| 3. |
- Bazesefidpar, Kazem
(author)
-
Numerical simulation of non-Newtonian fluids flow over surfaces
- 2023
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Doctoral thesis (other academic/artistic)abstract
- Wetting of surfaces by droplets of non-Newtonian fluids is important for various industrial and natural processes such as coating and cleaning of surfaces and inkjet printing, to name a few. Viscoelastic fluids are compounds of a very small amount of polymers and solvent. They are categorized as non-Newtonian fluids, and they exhibit both elasticity and shear dependent viscosity. Despite their relevance and abundance in our environment, dynamic wetting of viscoelastic fluids has been studied much less than that of the Newtonian fluids. Furthermore, many of the viscoelastic studies make simplifying assumptions of the contact line movement, for example, a constant value of the contact angle independent of the spreading speed of the droplet.In this thesis work, we implement a numerical framework for dynamic contact line problems of viscoelastic fluids, taking into account contact line friction or contact line hysteresis when necessary. We solve the coupled Cahn-Hilliard, Navier-Stokes and viscoelastic constitutive models to reveal detailed information about the flow physics, such as the polymeric stress distributions inside the drops. Especially interesting is the vicinity of discontinuity regions e.g. the contact-line and liquid bridge between the coalescing drops. First, we present the idea of dual-resolution grids to address the high interfacial resolution requirements for a viscoelastic two-phase flow. In particular, a dual-resolution algorithm is presented and validated for the wetting of viscoelastic fluids. Secondly, we apply our algorithm to investigate the effect of non-Newtonian properties on jumping of two merging droplets from a superhydrophobic surface, a problem which might be of interest for self-cleaning surfaces. In the last part, the physical effects of non-Newtonian properties are investigated on both the initial wetting regime on a smooth hydrophilic surface and the pinning and depinning of a droplet in the presence of the contact angle hysteresis.
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| 4. |
- Fornari, Walter, 1989-
(author)
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Suspensions of finite-size rigid spheres in different flow cases
- 2015
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Licentiate thesis (other academic/artistic)abstract
- Dispersed multiphase flows occur in many biological, engineering and geophysical applications such asfluidized beds, soot particle dispersion and pyroclastic flows. Understanding the behavior of suspensionsis a very difficult task. Indeed particles may differ in size, shape, density and stiffness, theirconcentration varies from one case to another, and the carrier fluid may be quiescent or turbulent.When turbulent flows are considered, the problem is further complicated due to the interactionsbetween particles and eddies of different size, ranging from the smallest dissipative scales up to thelargest integral scales. Most of the investigations on the topic have dealt with heavy small particles (typicallysmaller than the dissipative scale) and in the dilute regime. Less is known regarding the behavior ofsuspensions of finite-size particles (particles that are larger than the smallest lengthscales of the fluid phase).In the present work, we numerically study the behavior of suspensions of finite-size rigid spheres indifferent flows. In particular, we perform Direct Numerical Simulations using an ImmersedBoundary Method to account for the solid phase. Firstly is investigated the sedimentation of particles slightly larger than theTaylor microscale in sustained homogeneous isotropic turbulence and quiescent fluid. The results show thatthe mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. By estimatingthe mean drag acting on the particles, we find that non stationary effects explain the increased reductionin mean settling velocity in turbulent environments.We also consider a turbulent channel flow seeded with finite-size spheres. We change the solid volumefraction and solid to fluid density ratio in an idealized scenario where gravity is neglected. The aim isto independently understand the effects of these parameters on both fluid and solid phases statistics.It is found that the statistics are substantially altered by changes in volume fraction, while the main effectof increasing the density ratio is a shear-induced migration toward the centerline. However, at very high density ratios (~100) the two phases decouple and the particles behave as a dense gas.Finally we study the rheology of confined dense suspensions of spheres in simple shear flow. We focus onthe weakly inertial regime and show that the suspension effective viscosity varies non-monotonically with increasingconfinement. The minima of the effective viscosity occur when the channel width is approximately an integernumber of particle diameters. At these confinements, the particles self-organize into two-dimensional frozen layers thatslide onto each other.
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| 5. |
- Ge, Zhouyang, 1991-
(author)
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On droplet interactions and suspension flow
- 2020
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Doctoral thesis (other academic/artistic)abstract
- Micron to millimetre sized droplets, precisely generated or sustained in controlled environment, have great potential in myriads of engineering applications functioning as the basic element to assemble metamaterials, deliver drugs, host surfactant, reduce friction and damp turbulence. The interaction of droplets from pairwise to collective levels is the most important factor in controlling these processes, yet little is known about the detailed mechanisms in various nonideal conditions. The present thesis combines a number of studies aiming to elucidate the physical principles of droplet interactions and suspension flow using both high- and low-fidelity numerical simulations.We first study flow-assisted droplet assembly in microfluidic channels, seeking to harness the droplet interactions to produce photonic bandgap materials. A novel interface-correction level set/ghost fluid method (ICLS/GFM) is developed to directly simulate liquid droplets under depletion forces. Comparing to previous methods, ICLS/GFM conserves the global mass of each fluid using a simple mass-correction scheme, accurately computes the surface tension and depletion forces under the same framework, and has subsequently been applied to investigate the droplet clustering observed in a microfluidic experiment. Our simulations, supported by theoretical derivations, suggest that the observed fast self-assembly arises from a combination of strong depletion forces, confinement-mediated shear alignments of the droplets, and fine-tuned inflow conditions of the microchannel. However, the interplay of these 3D effects negates a simple droplet interaction model of parametric dependence, rendering the design of microfluidic chips for photonic crystal fabrications difficult in practice.The next objective of the thesis is the implementation of a minimal hybrid lubrication/granular dynamics (HLGD) model for simulation of dense particle suspensions. The main ingredients of HLGD include (i) a frame-invariant, short-range lubrication model for spherical particles, and (ii) a soft-core, stick/slide frictional contact model activated when particles overlap. Since contact interactions dominate at high particle concentrations, we expect the methodology to be applicable for probing the jamming of non-spherical particles and the rheology of foams as well.Finally, we include two miscellaneous studies concerning the slippage property of liquid-infused surfaces and droplets statistics in a homogeneous turbulent shear flow. Overall, results of these simulations provide detailed flow visualisations and qualitative dependence of the target functional on various governing parameters, facilitating experimental and theoretical investigations to design more robust drag-reducing surfaces and predict droplet distributions in emulsions.
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| 6. |
- Klinkenberg, Joy
(author)
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Transition in Particle-laden Flows
- 2013
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Doctoral thesis (other academic/artistic)abstract
- This thesis presents the study of laminar to turbulent transition of particle laden flows. When a flow becomes turbulent, the drag increases one order of magnitude compared to a laminar flow, therefore, much research is devoted to understand and influence the transition. Previous research at the Linne Flow Centre at KTH has concentrated on the understanding of the bypass transition process of single-phase fluids. Though there are still questions, the principles of this process are now, more or less, known. However, little is known of the influence of particles on transition. While experiments in the 1960s already showed that particles can reduce the friction in turbulent channel flows significantly. The question explored in this thesis is whether this can be attributed to their influence on transition.The initial onset of transition has been investigated with both modal and non-modal linear stability analysis in a Poiseuille flow between two parallel plates. Particles are introduced as a second fluid and they are considered to be solid, spherical and homogeneously distributed. When the fluid density is much smaller than the particle density, ξ (≡ ρf/ρp) << 1, an increase of the critical Reynolds number is observed. However, transient growth of streamwise vortices resulting in streaks is not affected by inclusion of particles. Particles with ξ ∼ 1 hardly seem to have an effect on stability.Although linear analysis shows that particles hardly influence the transient growth of disturbances, they might affect other (non-linear) stages of transition. To investigate such effects, the full Navier-Stokes equations for 3D Poiseuille flow between two parallel plates are numerically solved and particles are introduced as points with two-way coupling. For particles in a channel flow with ξ<<1, results show that the transition to turbulence is delayed for mass fractions ƒ (=mp N / ρf) larger than 0.1. For a mass fraction of ƒ=0.4 the initial disturbance energy needed to get a turbulent flow increases with a factor of four.Even if lower particle mass fractions ƒ are used, locally there could be large particle mass fractions. Therefore, the next step is to investigate the generation of local large particle mass fractions ƒ. Such particle clusters can be as large as the typical flow structures in the flow, like streak width and vortex size. Then they might change the flow field and (in)stability mechanisms. Numerical simulations of bypass transition in a boundary layer flow are used to determine whether particles cluster and where they tend to cluster. It is found that point particles with ξ<<1 and a large particle relaxation time tend to move in the low speed regions of the flow. In case of streaks, the low speed streaks are most favourable. For smaller particle relaxation times, particles act as tracers and do not have a preferential position and are homogeneously distributed.For particles with ξ∼1 the linear stability analysis showed no transition effect at any ƒ. However, one effect neglected until now is that of particle size. For particles with dimensions of the same order of magnitude of the flow disturbance, particles might influence the flow field. To investigate whether such particles migrate towards positions where they can affect transition some exploratory numerical simulations and experiments are performed.Numerically, the lateral migration of large particles (H/d=5) with ξ=1 in a 3D Poiseuille flow between two parallel plates is investigated. In laminar channel flow, large particles tend to move laterally due to shear to an equilibrium position. For a single large particle some key parameters for migration are identified: the size of the particle and the velocity of the fluid. When multiple particles are present, they tend to form particle trains. If particles are close, they influence each other and the equilibrium position shifts towards the wall, where the final position is dependent on the inter particle spacing. Also, not one steady equilibrium position is present, but particles move around an equilibrium position.Experimentally, migration of particles in bypass transition with ξ=1 is investigated to find out whether neutrally buoyant particles have a preferential position within streaks. The first results with tracer particles (d∼50μm) and few large particles (d∼200μm) do not show detectable preferential positioning.
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| 7. |
- Niazi Ardekani, Mehdi, 1990-
(author)
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Numerical study of non-spherical/spherical particles in laminar and turbulent flows
- 2017
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Licentiate thesis (other academic/artistic)abstract
- The presence of solid rigid particles alters the global transport and rheological properties of the mixture in complex (and often unpredictable) ways. In recent years a few studies have been devoted to investigating the behavior of dense suspensions in the turbulent/inertial regime with the majority of theses analyses limited to mono-disperse rigid neutrally-buoyant spheres. However, one interesting parameter that is rarely studied for particles with high inertia is the particle shape. Spheroidal particles introduce an anisotropy, e.g. a tendency to orient in a certain direction, which can affect the bulk behavior of a suspension in an unexpected ways. The main focus of this study is therefore to investigate the behavior of spheroidal particles and their effect on turbulent/inertial flows.We perform fully resolved simulations of particulate flows with spherical/spheroidal particles, using an efficient/accurate numerical approach that enables us to simulate thousands of particles with high resolutions in order to capture all the fluid-solid interactions.Several conclusions are drawn from this study that reveal the importance of particle's shape effect on the behaviour of a suspension e.g. spheroidal particles tend to cluster while sedimenting. This phenomenon is observed in this work for both particles with high inertia, sedimenting in a quiescent fluid and inertialess particles (point-like tracer prolates) settling in homogenous isotropic turbulence. The mechanisms for clustering is indeed different between these two situations, however, it is the shape of particles that governs these mechanisms, as clustering is not observed for spherical particles. Another striking finding of this work is drag reduction in particulate turbulent channel flow with rigid oblate particles. Again this drag reduction is absent for spherical particles, which instead increase the drag with respect to single-phase turbulence.
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| 8. |
- Schrader, Lars-Uve, 1978-
(author)
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Receptivity of Boundary-Layer Flows over Flat and Curved Walls
- 2010
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Doctoral thesis (other academic/artistic)abstract
- Direct numerical simulations of the receptivity and instability of boundary layers on flat and curved surfaces are herein reported. Various flow models are considered with the aim to capture aspects of flows over straight and swept wings such as wall curvature, pressure variations, leading-edge effects, streamline curvature and crossflow. The first model problem presented, the flow over a swept flat plate, features a crossflow inside the boundary layer. The layer is unstable to steady and traveling crossflow vortices which are nearly aligned with the free stream. Wall roughness and free-stream vortical modes efficiently excite these crossflow modes, and the associated receptivity mechanisms are linear in an environment of low-amplitude perturbations. Receptivity coefficients for roughness elements with various length scales and for free-stream vortical modes with different wavenumbers and frequencies are reported. Key to the receptivity to free-stream vorticity is the upstream excitation of streamwise streaks evolving into crossflow modes. This mechanism is also active in the presence of free-stream turbulence. The second flow model is that of a Görtler boundary layer. This flow type forms on surfaces with concave curvature, e.g. the lower side of a turbine blade. The dominant instability, driven by a vertically varying centrifugal force, appears as pairs of steady, streamwise counter-rotating vortical rolls and streamwise streaks. The Görtler boundary layer is in particular receptive to free-stream vortical modes with zero and low frequencies. The associated mechanism builds on the excitation of upstream disturbance streaks from which the Görtler modes emerge, similar to the mechanism in swept-plate flows. The receptivity to free-stream vorticity can both be linear and nonlinear. In the presence of free-stream turbulence, nonlinear receptivity is more likely to trigger steady Görtler vortices than linear receptivity unless the frequencies of the free-stream fluctuations are very low. The third set of simulations considers the boundary layer on a flat plate with an elliptic leading edge. This study aims to identify the effect of the leading edge on the boundary-layer receptivity to impinging free-stream vortical modes. Three types of modes with streamwise, vertical and spanwise vorticity are considered. The two former types trigger streamwise disturbance streaks while the latter type excites Tollmien-Schlichting wave packets in the shear layer. Simulations with two leading edges of different bluntness demonstrate that the leading-edge shape hardly influences the receptivity to streamwise vortices, whereas it significantly enhances the receptivity to vertical and spanwise vortices. It is shown that the receptivity mechanism to vertical free-stream vorticity involves vortex stretching and tilting - physical processes which are clearly enhanced by blunt leading edges. The last flow configuration studied models an infinite wing at 45 degrees sweep. This model is the least idealized with respect to applications in aerospace engineering. The set-up mimics the wind-tunnel experiments carried out by Saric and coworkers at the Arizona State University in the 1990s. The numerical method is verified by simulating the excitation of steady crossflow vortices through micron-sized roughness as realized in the experiments. Moreover, the receptivity to free-stream vortical disturbances is investigated and it is shown that the boundary layer is most receptive, if the free-stream modes are closely aligned with the most unstable crossflow mode
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| 9. |
- Shahmardi, Armin
(author)
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Numerical study of interface dynamics and phase change
- 2022
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Doctoral thesis (other academic/artistic)abstract
- Multi-phase fluid flows are ubiquitous in natural phenomena and different industrial applications such as in food industry, the medical sector, heat exchangers, power generation systems, to name a few. Understanding the underlining physics of multi-phase flows proved to be a challenging task due to presence of sophisticated dynamics, including the evolution of the interface between any pair of phases, thermodynamics and possibility of phase change, interactions between the fluid phases and a solid phase, etc. Together with theoretical studies and experiments performed on a variety of multi-phase flow problems, numerical simulations have been employed by many researchers to scrutinise different aspects of the problem. During the last decades, a great many studies have been conducted aiming to provide more accurate numerical frameworks for investigating multi-phase flow problems.Among the various complicated aspects of a multi-phase flow, the present thesis is focused on few characteristics of it the understanding of which requires more considerations and demands improvements in the numerical frameworks. First, we elaborate on the different interface tracking approaches suit the study of different multi-phase flows. In particular, a Volume of Fluid method, a compressible formulation of a diffuse interface approach, a Cahn-Hilliard phase field method, and an Immersed Boundary method are employed to study wetting phenomemna and fluxes at the interface. We have initially investigated biological-relevant membranes, extensional dynamics of a Elasto-viscoplastic material, and droplet spreading over rough surfaces. In the second part of the thesis, we propose novel numerical methods and setups to investigate the phase change problems in both nanoscale and mesoscale. In particular, we developed a novel numerical method for the solidification problem, a pressure control setup for studying boiling at nanoscale, and a pressure based algorithm for modelling the boiling and evaporation.
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| 10. |
- Tabaeikazerooni, Seyed Hamid, 1985-
(author)
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Laminar and turbulent particle laden flows : a numerical and experimental study
- 2019
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Doctoral thesis (other academic/artistic)abstract
- Particle laden flows are widespread in nature and in industrial applications. Dust storms, geophysical flows, pharmaceutical processes, fluidized beds and blood flow are only a few examples. Both global and local properties of a particle laden flow may alter significantly in comparison to the corresponding single phase flow. While the bulk behavior of such flows is subject to rheological investigations since many decades, it is just recently that we learn about the detailed dynamics of individual finite-size particles and their influence on the behavior of the carrier phase. The main goal of the present thesis is to employ latest numerical and experimental methods to gain a more fundamental understanding of the dynamics of finite-size particles and associated changes in laminar and turbulent suspension flows at relatively high particle concentrations. To this end, the current research project is conducted in three main steps. First, the inertial migration of finite-size neutrally buoyant spherical particles in a laminar square duct flow is investigated numerically. Next, using fully resolved Direct Numerical Simulations (DNSs), we study a turbulent square duct flow laden with finite-size neutrally buoyant spherical particles. Finally, an experimental framework is developed to explore the spatio-temporal dynamics of finite-size spherical particles in semidilute suspensions in micro-scale systems. Recently, inertial migration of particles in laminar pressure driven flows were successfully utilized to sort and separate particles and cells. While these studies are restricted to very dilute suspensions, little is known about particle separation based on inertial migration for suspensions at high solid volume fractions. In the present study, we investigate the inertial migration of particles by means of an Immersed Boundary Method (IBM), in a laminar square duct flow for suspensions with solid volume fractions between 0.4% to 20% and bulk Reynolds numbers ranging from 144 to 550. The bulk Reynolds number is found to be the key parameter in defining the final distribution of particles over the duct cross section in a dilute suspension (phi=0.4%). However, as solid volume fraction increases, we show that the behavior of particles depends on both the solid volume fraction and the bulk Reynolds number. We also found that the presence of solid particles induces secondary motions in a laminar duct flow regardless of the solid volume fraction. Indeed, we observed similarities to secondary motions of an unladen turbulent duct flow. Previous investigations of turbulent particulate flows have been mostly focused on the behavior of very small particles in canonical flows such as a plane channel flow and flow over a flat plate. However, industrial flow processes commonly take place in more complex geometries. Here, we also study turbulent duct flows laden with finite-size neutrally buoyant spherical particles. We consider a fixed bulk Reynolds number of Re=5600 and suspensions at three different solid volume fractions of 5%, 10% and 20%. We show that the turbulence intensity increases for suspensions up to 10% but then strongly decreases for 20%. This drop is observed to go along with a distribution of particles that mostly accumulate at the duct corners for solid volume fractions of 5% and 10% and in the duct core region for phi=20%. The interaction between particles and turbulent structures as well as particles and turbulent induced secondary flows are documented in detail in the present study. Despite recent advances in computer technology, computational cost of fully resolved numerical simulations of suspension flows are still very high. Hence, experimental studies are essential to access the overall dynamics of suspension flows. In the present study, a measurement framework is developed based on Astigmatism Particle Tracking Velocimetry (APTV) to study the three-dimensional dynamics of finite-size particles in micro-scale flows. The measurement technique is successfully applied to study the behavior of decelerating suspensions flowing over a backward-facing step at low bulk Reynolds number of Re=4. Suspensions of neutrally buoyant PMMA particles with 30 micro meter diameter at three different solid volume fractions of 0.1%, 2% and 10% are considered. We utilize Refractive Index Matching (RIM) to gain optical accessibility to suspensions also at high solid volume fractions. Moreover, a few number of particles are fluorescently labelled to act as suspension tracers. By resolving the 3D particle dynamics, an onset of reduced particle velocities could be evaluated with increasing solid volume fraction for a flow with strong out-of-plane particle displacements.
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