Particle-laden Flow

In this work, we extend the analyses devoted to Newtonian viscous fluids previously reported by Ribe [Physical Review E 68, 036305 (2003)], by investigating shear thickening (dilatant) and shear thinning (pseudoplastic) effects on the development of folding instabilities in non-Newtonian viscous sheets of which viscosity is given by a power-law constitutive equation. Such instabilities are trigged by compression stresses acting on viscous sheets that leave a channel at a very small initial velocity, fall, and then hit a solid surface or a fluid substrate. Our study is conducted through a mixed approach combining direct numerical simulations, energy budget analyses, scaling laws, and experiments. The numerical results are based on an adaptive variational multi-scale method for multiphase flows, while Carpobol gel sheets are considered for the conducted experiments. Two folding regimes are observed: (1) the viscous regime; and (2) the gravitational one. Interestingly, only the latter ...

1 Department of Mechanical, Aerospace and Nuclear Engineering, 2 Center for Multiphase Research, Rensselaer Polytechnic Institute, Troy, NY 12180, USA (*) Tel: 518-276-4000, Fax: 518-276-3055 Email: podowm@ rpi. edu Key words: Membrane filtration; Dean ...

An interaction between boundary layer of a sphere and equivalent Stokes layer generated by a perturbation in solid-air two-phase flow has been investigated using numerical simulation. In this simulation, the sphere is fixed in a cylindrical channel. Particle Reynolds number in this study is around 200 that means the wake of the sphere is steady and axisymmetric, and the friction drag and the pressure drag of the sphere are comparable. As a result of simulation, a drag reduction was found in a uniform flow with single-period perturbation oscillating in the same direction as the uniform flow when Stokes layer thickness is smaller than twice of boundary layer thickness in the sphere surface. A reason of the reduction of drag force is a reduction of friction drag caused by that fluctuation energy generated by the perturbation concentrates in the vicinity of particle and the time averaged velocity gradient becomes smaller.

Particle trapping and deposition around an obstacle occur in many natural and industrial situations and in particular in the nuclear industry. In the steam generator of a nuclear power plant, the progressive obstruction of the flow due to particle deposition reduces the efficiency and can induce tube cracking leading to breaking and damage. The steam generator then loses its role as a safety barrier of the nuclear power plant. From a fundamental standpoint, dilute and concentrated particulate flows have received a growing attention in the last decade. In this study, we investigate the transport of solid particles around obstacles in a confined flow. Experiments were performed in a simplified configuration by considering a laminar flow in a vertical tube. An obstacle was inserted at the middle height of the tube and neutrally-buoyant particles were injected at different locations along the tube. We have investigated first the trajectories of individual particles using particle tracking (PT). Then, the particle trajectories were modeled by using the Boussinesq-Basset-Oseen equation with a flow velocity field either measured using particle image velocimetry (PIV) or calculated by the Code Saturne software in order to account for the three-dimensional (3D) character of the obstacle wake. This paper presents a comparison between the experimental observations and the predictions of the modeling for an obstacle consisting of a rectangular step at a Reynolds number of ≈100 and evidences the importance of accounting for the 3D complex nature of the flow.

Particle trapping and deposition around an obstacle occur in many natural and industrial situations and in particular in the nuclear industry. In the steam generator of a nuclear power plant, the progressive obstruction of the flow due to particle deposition reduces the efficiency and can induce tube cracking leading to breaking and damage. The steam generator then loses its role as a safety barrier of the nuclear power plant. From a fundamental standpoint, dilute and concentrated particulate flows have received a growing attention in the last decade. In this study, we investigate the transport of solid particles around obstacles in a confined flow. Experiments were performed in a simplified configuration by considering a laminar flow in a vertical tube. An obstacle was inserted at the middle height of the tube and neutrally-buoyant particles were injected at different locations along the tube. We have investigated first the trajectories of individual particles using particle tracki...

The motion of exible bres suspended in an incompressible uid is of interest to researchers in a wide variety of elds, including reinforced composite materials, biotechnology and the pulp and paper industry. In this work, we concentrate on the application to pulp bres and demonstrate how the complex hydrodynamic interaction between a exible bre and the surrounding uid can be simulated using the immersed boundary method. The computations involve a single bre suspended in a two{dimensional shear ow at moderate Reynolds number. Previous experimental work di erentiates the observed bre motions into a well{de ned set of \orbit classes," which are reproduced in our simulations for bres with varying exibility. The computed bre orientation angle distributions are compared to classical theoretical results, and shown to exhibit a skewness which is not captured by either the linear theory or other recent numerical computations that ignore the bre{ uid interaction. These simulations set the stage for further work in modeling ows with multiple bres in three dimensions, for the purpose of improving the papermaking process.

This paper reports on original experimental data of mixing in two-phase Taylor-Couette flows. Neutrally buoyant particles with increasing volume concentration enhance significantly mixing of a passive tracer injected within the gap between two concentric cylinders. Mixing efficiency is measured by planar laser induced fluorescence coupled to particle image velocimetry to detect the Taylor vortices. In order to achieve reliable experimental data, index matching of both phases is used together with a second PLIF channel. From this second PLIF measurements, dynamic masks of the particle positions in the laser sheet are determined and used to calculate accurately the segregation index of the tracer concentration. Experimental techniques have been thoroughly validated through calibration and robustness tests. Three particle sizes were considered, in two different flow regimes to emphasize their specific roles on the mixing dynamics. Keywords Taylor-Couette flows • mixing • neutrally buoyant particles • PIV and PLIF techniques.

This study reports the development of a direct simulation code for solid spheres moving through viscoelastic fluids with a range of different rheological behaviors. The numerical algorithm was implemented on an opensource finite-volume solver coupled with an immersed boundary method, and is able to perform fully-resolved simulations, wherein all flow scales associated with the particle motion are resolved. The formulation employed exploits the log-conformation tensor to avoid high Weissenberg number issues when calculating the polymeric extra stress. A number of benchmark flows were simulated using this method, to assess the accuracy of the newlydeveloped solver. First, the sedimentation of a sphere in a bounded domain surrounded by either Newtonian or viscoelastic fluid was computed, and the numerical results were verified by comparison with experimental and computational data from the literature. Additionally, the spatial and temporal accuracies of the algorithm were evaluated, and different transient and advection discretization schemes were investigated. Second, the rotation of a sphere in a homogeneous shear flow was studied, and again the numerical results obtained were compared to those from the literature. Good agreement is obtained for the variation in the particle rotation rate as a function of Weissenberg number, using both the newly implemented algorithm and an alternative fixed-mesh approach. Finally, the cross-stream migration of a neutrally buoyant sphere in a steady Poiseuille flow, consisting of either a Newtonian or viscoelastic suspending fluid was investigated. For the Newtonian fluid good agreement was obtained for the particle equilibrium position when compared to the well known Segré-Silberberg effect, and for the viscoelastic fluid the effect of the retardation ratio on the final particle equilibrium position was studied. Additionally, the newly-developed solver capabilities were tested to study the shear-induced particle alignment in wall-bounded Newtonian and viscoelastic fluids. The role of the fluid rheology and finite gap size on both the rate and approach pathways of the solid particles is illustrated.

For the Oldroyd-B fluid the ML model that presents the best R 2 (as well as the lowest values of RMSE and MAPE) is the Random Forest model. 1.3. COMPARISON WITH CLOSURE DRAG MODEL AND SIMULATED DATA • Relative errors were calculated between the values predicted by the ML models and the actual values given by both a closure drag model [4] and numerical simulations data.

This study reports the development of a direct simulation code for solid spheres moving through viscoelastic fluids with a range of different rheological behaviors. The numerical algorithm was implemented on an opensource finite-volume solver coupled with an immersed boundary method, and is able to perform fully-resolved simulations, wherein all flow scales associated with the particle motion are resolved. The formulation employed exploits the log-conformation tensor to avoid high Weissenberg number issues when calculating the polymeric extra stress. A number of benchmark flows were simulated using this method, to assess the accuracy of the newlydeveloped solver. First, the sedimentation of a sphere in a bounded domain surrounded by either Newtonian or viscoelastic fluid was computed, and the numerical results were verified by comparison with experimental and computational data from the literature. Additionally, the spatial and temporal accuracies of the algorithm were evaluated, and different transient and advection discretization schemes were investigated. Second, the rotation of a sphere in a homogeneous shear flow was studied, and again the numerical results obtained were compared to those from the literature. Good agreement is obtained for the variation in the particle rotation rate as a function of Weissenberg number, using both the newly implemented algorithm and an alternative fixed-mesh approach. Finally, the cross-stream migration of a neutrally buoyant sphere in a steady Poiseuille flow, consisting of either a Newtonian or viscoelastic suspending fluid was investigated. For the Newtonian fluid good agreement was obtained for the particle equilibrium position when compared to the well known Segré-Silberberg effect, and for the viscoelastic fluid the effect of the retardation ratio on the final particle equilibrium position was studied. Additionally, the newly-developed solver capabilities were tested to study the shear-induced particle alignment in wall-bounded Newtonian and viscoelastic fluids. The role of the fluid rheology and finite gap size on both the rate and approach pathways of the solid particles is illustrated.

A recent article (Khair and Kabarowski; Phys. Rev. Fluids 5, 033702) has studied the crossstreamline migration of electrophoretic particles in unbounded shear flows with weak inertia or viscoelasticity. That work compares their results with those reported in two of our previous studies (Choudhary et al. J. Fluid Mech. 874; J. Fluid Mech. 898) and reports a disagreement in the derived analytical expressions. In this comment, we resolve this discrepancy. For viscoelastic flows, we show that Khair and Kabarowski have not accounted for a leading order surface integral of polymeric stress in their calculation of first-order viscoelastic lift. When this integral is included, the resulting migration velocity matches exactly with that reported in our work (J. Fluid Mech. 898). This qualitatively changes migration direction that is reported by Khair and Kabarowski for viscoelastic flows. For inertial flows, we clarify that Khair and Kabarowski find the coefficient of lift to be 1.75π, compared to 2.35π in our previous work (J. Fluid Mech. 874). We show that this difference occurs because Khair and Kabarowski accurately include the effect of rapidly decaying ∼ O(1/r 4) velocity field (a correction to the stresslet field ∼ 1/r 2), which was neglected in our previous work (J. Fluid Mech. 874).

When a submerged sphere propagates along an inclined wall at terminal velocity, it experiences gravity, drag, lift, and friction forces. In the related equations of motion, the drag, lift and friction coefficients are unknown. Experiments are conducted to determine the friction and drag coefficients of the sphere over a range of Reynolds numbers. Through high speed imaging, translational and rotational velocities of spheres propagating along a glass plate are determined in liquids with several viscosities. The onset of sliding motion is identified by computing the dimensionless rotation rate of the sphere. Using drag and lift coefficients for Re <350 obtained from numerical simulations by Rao et al. (JFM, 2012), both static and kinetic friction coefficients are calculated for several materials. The friction coefficients are then employed to estimate the drag coefficient for 350 <Re <2000. The resulting drag curve for a sphere propagating along a wall demonstrates the importance of the frictional force over this Re range.

modelling the motion of spheres submerged in liquid, gravity, drag, lift, and added mass forces have to be taken into account. For spheres contacting bounding surfaces, friction coefficients due to rolling and sliding increase the complexity of the model. In this study, experiments are conducted to investigate the effects of particle density and diameter on the rolling and sliding motion of spheres. Spherical particles with marked surfaces are released from rest on an inclined glass plate in still water at various inclination angles and allowed to accelerate. A 45 • mirror mounted beneath the plate allows simultaneous capture of both longitudinal and spanwise motions of the sphere. Based on sequences obtained by high speed imaging, the translational and rotational velocities are determined. Particle Reynolds numbers at terminal velocity range from 400 to 2500 corresponding with Galileo numbers of 800 to 2800. By comparing the translational and rotational velocities, the occurrence of sliding motion can be identified. The onset of sliding motion is then determined as a function of inclination angle and Galileo number for multiple particle materials. The experimental results are also compared against the existing models from the literature.

Individual magnetic wax spheres with specific gravities of 1.006, 1.054 and 1.152 were released from rest on a smooth wall in water at friction Reynolds numbers, Re τ = 680 and 1320 (d + = 58 and 122 viscous units, respectively). Three-dimensional tracking was conducted to understand the effect of turbulence and wall friction on sphere motions. Spheres subjected to sufficient mean shear initially lifted off of the wall before descending back towards it. These lifting spheres translated with the fluid above the wall, undergoing saltation or resuspension, with minimal rotation about any axis. By contrast, spheres that did not lift off upon release mainly slid along the wall. These denser spheres lagged the fluid more significantly due to greater wall friction. As they slid downstream, they began to roll forward after which small repeated lift-off events occurred. These spheres also rotated about both the streamwise and wall-normal axes. In all cases, the sphere trajectories were limited to the buffer and logarithmic regions, and all wall collisions were completely inelastic. Sphere streamwise velocities fluctuated up to 20% from the mean value even after the sphere had attained an approximate terminal velocity. In the plane parallel to the wall, the spheres migrated in the spanwise direction about 12% of the streamwise distance traveled suggesting that spanwise forces are important. The variations in sphere kinematics were likely induced by high and low momentum zones in the boundary layer, vortex shedding in the sphere wakes, and wall friction. The repeated lift-offs of the forward rolling denser sphere were attributed to a Magnus lift.

Brownsville-The hydrocode SHAMRC has been used in the past to study the formation and growth of the Richtmyer-Meshkov Instability (RMI). While RMI involves impulsively accelerating two continuous fluids of differing densities, a similar class of instabilities has recently been described for multiphase flow. In this scenario, a shock wave passes through a region containing ambient air seeded with particles which have a non-trivial mass and density much greater than that of the surrounding and embedding fluid. In this scenario, no baroclinic vorticity is generated due to the lack of a fluid-fluid density interface. After the shock passage, the particles or droplets lag behind the surrounding gas. Momentum exchange between the embedded phase and the embedding phase leads to non-uniform local equilibrium velocity distribution, and thus to shear and vortex formation. As the primary mechanism for this instability formation is momentum transfer via drag, the morphology of the instability is strongly dependent of the sizes of the particles in the initial conditions. The simulations described here attempt to model the effects of changing the particle size on the morphology and growth rate of this instability.

A new theoretical groundwork for the analysis of wall-bounded turbulent flows is offered, the application of which is presented in a parallel paper. First, it is proposed that the turbulence phenomenon is connected to the onset of an irreversible process – specifically the action of a slip flow – by which a new fundamental model can be derived. Fluid cells with specific dimensions – of length connected with the local slip length and thickness connected with the distance between two parallel slipping flows – can be hypothetically constructed, in which a specific kinetic energy dissipation can be considered to occur. Second, via a maximum entropy production process a self-organized grouping of cells occurs – which results in the distinct zones viscous sublayer, buffer layer, and the log-law region to be built up. It appears that the underlying web structure may take the form of either representing a perfect web structure without any visible swirls, or a partially defect web structure ...

Multiphase flows involving granular materials are complex and prone to pattern formation caused by competing mechanical and hydrodynamic interactions. Here we study the interplay between granular bulldozing and the stabilising effect of viscous pressure gradients in the invading fluid. Injection of aqueous solutions into layers of dry, hydrophobic grains represent a viscously stable scenario where we observe a transition from growth of a single frictional finger to simultaneous growth of multiple fingers as viscous forces are increased. The pattern is made more compact by the internal viscous pressure gradient, ultimately resulting in a fully stabilised front of frictional fingers advancing as a radial spoke pattern.

Numerical simulations are performed for particle-laden flow around two circular cylinders in tandem arrangements. The present study aims to understand the effects of cylinder spacing and particle size on particle dispersion and deposition characteristics. We consider the Reynolds number of 100 at which vortex shedding takes place, and the density ratio of particle to fluid is fixed as 1,000. For a parametric study, we vary the Stokes number from 0.1 to 10 for the center-to-center spacing of 2D, 3D, and 4D, where D is the cylinder diameter. As the spacing decreases from 4D to 2D, vortex shedding becomes weaker and lift fluctuations are reduced, agreeing well with the previous results. The dispersion pattern of particles around the cylinder changes significantly depending on the spacing. The overall deposition efficiency of the upstream cylinder is very similar to that of a single cylinder. However, the efficiency of the downstream cylinder has much smaller values with a strong dependency on the spacing. The local deposition efficiency and the deposition location are also analyzed in detail.

A recent article (Khair and Kabarowski; Phys. Rev. Fluids 5, 033702) has studied the crossstreamline migration of electrophoretic particles in unbounded shear flows with weak inertia or viscoelasticity. That work compares their results with those reported in two of our previous studies (Choudhary et al. J. Fluid Mech. 874; J. Fluid Mech. 898) and reports a disagreement in the derived analytical expressions. In this comment, we resolve this discrepancy. For viscoelastic flows, we show that Khair and Kabarowski have not accounted for a leading order surface integral of polymeric stress in their calculation of first-order viscoelastic lift. When this integral is included, the resulting migration velocity matches exactly with that reported in our work (J. Fluid Mech. 898). This qualitatively changes migration direction that is reported by Khair and Kabarowski for viscoelastic flows. For inertial flows, we clarify that Khair and Kabarowski find the coefficient of lift to be 1.75π, compared to 2.35π in our previous work (J. Fluid Mech. 874). We show that this difference occurs because Khair and Kabarowski accurately include the effect of rapidly decaying ∼ O(1/r 4) velocity field (a correction to the stresslet field ∼ 1/r 2), which was neglected in our previous work (J. Fluid Mech. 874).

Taylor-Couette (TC) flow is the shear-driven flow between two coaxial independently rotating cylinders. In recent years, high-fidelity simulations and experiments revealed the shape of the streamwise and angular velocity profiles up to very high Reynolds numbers. However, due to curvature effects, so far no theory has been able to correctly describe the turbulent streamwise velocity profile for a given radius ratio, as the classical Prandtl-von Kármán logarithmic law for turbulent boundary layers over a flat surface at most fits in a limited spatial region. Here, we address this deficiency by applying the idea of a Monin-Obukhov curvature length to turbulent TC flow. This length separates the flow regions where the production of turbulent kinetic energy is governed by pure shear from that where it acts in combination with the curvature of the streamlines. We demonstrate that for all Reynolds numbers and radius ratios, the mean streamwise and angular velocity profiles collapse according to this separation. We then develop the functional form of the velocity profile. Finally, using the newly developed angular velocity profiles, we show that these lead to an alternative constant in the model proposed by Cheng et al.

We study the three-dimensional structure of turbulent velocity fields around extreme events of local energy transfer in the dissipative range. Velocity fields are measured by tomographic particle velocimetry at the centre of a von Kármán flow with resolution reaching the Kolmogorov scale. The characterization is performed through both direct observation and an analysis of the velocity gradient tensor invariants at the extremes. The conditional average of local energy transfer on the second and third invariants seems to be the largest in the sheet zone, but the most extreme events of local energy transfer mostly correspond to the vortex stretching topology. The direct observation of the velocity fields allows for identification of three different structures: the screw and roll vortices, and the U-turn. They may correspond to a single structure seen at different times or in different frames of reference. The extreme events of local energy transfer come along with large velocity and vorticity norms, and the structure of the vorticity field around these events agrees with previous observations of numerical works at similar Reynolds numbers.

The transport of vorticity in Oldroydian viscoelastic fluid in the presence of suspended magnetic particles is considered. Equations governing the transport of vorticity in Oldroydian viscoelastic fluid in the presence of suspended magnetic particles are obtained from the equations of magnetic fluid flow proposed by Wagh and Jawandhia in their 1996 study on the transport of vorticity in magnetic fluid. From these equations, it follows that the transport of solid vorticity is coupled with the transport of fluid vorticity. Further, it is found that due to thermo-kinetic process, fluid vorticity may exist in the absence of solid vorticity, but when fluid vorticity is zero, then solid vorticity is necessarily zero. A two-dimensional case is also studied.

We investigate experimentally the dynamics of non-axisymmetric fibres in channel flow turbulence, focusing specifically on the importance of the fibre size relative to the flow scales. To this aim, we maintain the same physical size of the fibres and we increase the shear Reynolds number. Experiments are performed in the TU Wien Turbulent Water Channel for three values of shear Reynolds number, namely 180, 360 and 720. Fibres are slender – length to diameter ratio of 120 – rigid, curved and neutrally buoyant particles and their shape ranges from low curvature – almost straight fibres – to moderate curvature. In all cases, fibre size remains small compared with the channel height (${\leqslant }1.5\,\%$). Three-dimensional and time-resolved recordings of the laser-illuminated measurement region are obtained from four high-speed cameras and used to infer fibre dynamics. With the aid of multiplicative algebraic reconstruction techniques, fibre position, orientation, velocity and rotatio...

This paper reports on a two-phase flow Direct Numerical Simulation (DNS) aimed at analyzing the resuspension of solid particles from a surface hit by a transonic jet inside a low pressure container. Conditions similar to those occurring in a fusion reactor vacuum vessel during a Loss of Vacuum Accident (LOVA) have been considered. Indeed, a deep understanding of the resuspension phenomenon is essential to make those reactors safe and suitable for a large-scale sustainable energy production. The jet Reynolds and Mach numbers are respectively set to 3300 and 1. The Thornton and Ning impact/adhesion model is adopted and improved. An advanced resuspension model, which takes into account the dynamics (rolling and slipping) of particles at the wall, is implemented. The use of this model combined with a DNS represents a great novelty in simulating the particle resuspension process. The particles initially deposited at the wall have constant density, whereas their diameters are drawn according to a log-normal distribution, with parameters obtained from experimental data. It has been found that the flow induced motion of wall deposited particles Electronic supplementary material The online version of this article (

The dependence of the barchan dune dynamics on the size of the grains involved is investigated experimentally. Downsized barchan dune slices are observed in a narrow water flow tube. The relaxation time from an initial symmetric triangular heap towards an asymmetric shape attractor increases with dune mass and decreases with grain size. The dune velocity increases with grain size. In contrast, the velocity scaling and the shape of the barchan dune is independent of the size of the grains.

An experimental study was conducted to determine the influence of fluid elastic properties on the critical velocity, frictional pressure drops, and the turbulent-flow characteristics of polymer-fluid flow over a sand bed deposited in a horizontal pipe. Fluids were prepared using a special technique, which allowed for the alteration of fluid elastic properties while keeping the shear viscosity constant. By conducting experiments under controlled conditions, we were able to quantify the individual effect of the fluid elasticity (independent from shear viscosity) on the critical flow rate for bed erosion and the turbulent-flow characteristics of polymer-fluid flow over the stationary sand bed. Results showed that higher critical velocities were required for the onset of the bed erosion when we use the fluid with higher elasticity.

Brownsville-The hydrocode SHAMRC has been used in the past to study the formation and growth of the Richtmyer-Meshkov Instability (RMI). While RMI involves impulsively accelerating two continuous fluids of differing densities, a similar class of instabilities has recently been described for multiphase flow. In this scenario, a shock wave passes through a region containing ambient air seeded with particles which have a non-trivial mass and density much greater than that of the surrounding and embedding fluid. In this scenario, no baroclinic vorticity is generated due to the lack of a fluid-fluid density interface. After the shock passage, the particles or droplets lag behind the surrounding gas. Momentum exchange between the embedded phase and the embedding phase leads to non-uniform local equilibrium velocity distribution, and thus to shear and vortex formation. As the primary mechanism for this instability formation is momentum transfer via drag, the morphology of the instability is strongly dependent of the sizes of the particles in the initial conditions. The simulations described here attempt to model the effects of changing the particle size on the morphology and growth rate of this instability.

The present work deals with the dynamics of turbulent jet in different configurations and geometries. In particular two aspect, important both in the engineering applications and in the scientific research, are stressed. The first one deals with the mixing in the turbulent jets at near-critical thermodynamic conditions. The second addresses the dynamics of inertial particle in turbulent premixed Bunsen flames. In order to perform a Direct Numerical Simulation (DNS) of a turbulent jet at supercritical conditions a suitable method was developed to mimic the gas thermodynamic behavior. The Van der Waals equation of state has been chosen and an Low Mach number expansion of Navier Stokes equations has been performed. This approach is completely original in the context of real gas equation, and it is considered as useful as the whole Navier Stokes system in the fully compressible formulation especially at very Low Mach number. The new equations are implemented in a numerical code in order to perform the first, in our knowledge, DNS of a fully turbulent coaxial jet in supercritical thermodynamic conditions. The configuration adopted is similar to the coaxial injectors of the liquid rocket engines and consists in an inner jet with liquid-like density and low velocity and in an outer jet characterized by a gas-like density and high velocity. Aim of the simulation is to observe high-density finger-like structures observed in previous experimental visualizations, the so-called "ligaments", and to understand the mechanism of their formation. In particular these finger-like structure formation is ascribed to the joint effects of the jet dynamics and the thermodynamics condition. In fact the Kelvin-Helmholtz structures, which generates by the peculiar jet configuration, contributes to the "ligaments" formation while the thermodynamic conditions allow these high-density structures to persist in low-density field. In the real gas jet the interface between the high and low density fluid is observed to be thinner than the perfect gas jet, hence the diffusion occurs at smaller and smaller scales. The obtained data are considered useful for the study of the mixing and combustion processes in the near critical conditions. In the LES/RANS context, in addition, DNS data are necessary for the evaluation of sub-grid terms and for development of new models. The dynamics of the inertial particles in turbulent premixed flame is addressed with the same code. The DNS of a reactive Bunsen jet laden with inertial particle is performed. The simulation reproduces a lean Methane/Air premixed flame in the "flamelet" regime. The flow is seeded with four particle population of different inertia with the mass density much larger than the fluid one and diameter much smaller than the Kolmogorov length scale. In these conditions, the particle dynamic equation is forced by only the Stokes drag and the one-way coupling regime can be assumed (no fluid-particle or particle-particle interaction occur). A suitable Stokes number is defined as a function of the particle features and of the laminar flame speed and thickness and burned/unburned gas temperature ratio. It is shown that the so-defined "flamelet" Stokes number is the suitable to describe the particle dynamics in the premixed flames. The DNS data are analyzed to address the effects of particle inertia on Particle Image Velocimetry measurements, in particular is observed that the particle inertia induces a time lag in particle to follow the v fluid acceleration across the flame front. This time lag generate a mismatch in the prediction of fluid velocity through the particle velocity. It is shown that the evaluation of high order velocity statistics needs particle with smaller and smaller flamelet Stokes number. The data are analyzed also to address the effects of the interaction between particle inertia and fluctuating flame front on the particle spatial distribution. Two statistical tools are used to this purpose, the Clustering Index, K, and the radial distribution function, g(r). K measures the departure of the actual distribution from the Poissonian distribution, g(r) is the probability to find a couple of particle at a certain distance r. An important outline consists in the presence of particle clusters in the flame brush. In particular also quasi-Lagrangian particles are characterized by not Poissonian spatial distribution as a consequence of the intermittent fluctuation of the instantaneous thin flame front separating two regions with different particle concentration. With the increasing of the Stokes number the cluster intensity increases experiencing a maximum value for order one flamelet Stokes number. All results concerning the particle dynamics are confirmed by experimental measurement on a Bunsen Methane-Air turbulent reacting jet. The obtained results are considered important both for experimental measurements and for soot dynamics and growth strongly influenced by the interaction of particle with the flame front and by their collision. vi The flame induced turbulent dynamics of the particles is characterized by rather peculiar particle distributions. Recently, the issue of the anomalous spatial arrangement of inertial particles has been studied in a certain detail for simple incompressible flows. They are known to accumulate in intermittent aggregates, the clusters, which are separated by voids. The typical dimensions of the clusters depend on the characteristics of the turbulent field and on the parti

Viscous streaming refers to the rectified, steady flows that emerge when a liquid oscillates around an immersed microfeature. Relevant to microfluidics, the resulting local, strong inertial effects allow manipulation of fluid and particles effectively, within short time scales and compact footprints. Nonetheless, practically, viscous streaming has been stymied by a narrow set of achievable flow topologies, limiting scope and application. Here, by moving away from classically employed microfeatures of uniform curvature, we experimentally show how multicurvature designs, computationally obtained, give rise, instead, to rich flow repertoires. The potential utility of these flows is then illustrated in compact, robust, and tunable devices for enhanced manipulation, filtering, and separation of both synthetic and biological particles. Overall, our mixed computational/experimental approach expands the scope of viscous streaming application, with opportunities in manufacturing, environment...

Particle transfer in the wall region of turbulent boundary layers is dominated by coherent structures which i) control the turbulence regeneration cycle, ii) bring particles toward and away from the wall and iii) favour particle segregation in the viscous region giving rise to nonuniform particle distribution profiles which peak close to the wall. In this work, we focus on the transfer mechanism of inertial particles and on the influence of gravity on transfer, segregation and deposition mechanisms. By tracking ¢ ¡ ¤ £ ¦ ¥ § ¦ © particles in Direct Numerical Simulation (DNS) of a turbulent channel flow at £ ¥ , we find that particles reach the wall mainly by two different mechanisms: free-flight, stronger for larger particles, and diffusional deposition, more significant for smaller particles. For dominant diffusional deposition, particles are segregated very near the wall and are then slowly driven to the wall.

In this paper, the rotation of rigid fibers is investigated for the reference case of turbulent channel flow. The aim of the study is to examine the effect of local shear and turbulence anisotropy on the rotational dynamics of fibers with different elongation and inertia. To this aim, statistics of the fiber angular velocity, Ω, are extracted from direct numerical simulation of turbulence at shear Reynolds number Re τ = 150 coupled with Lagrangian tracking of prolate ellipsoidal fibers with Stokes number, St, ranging from 3 to 100 and aspect ratio, λ, ranging from 1 to 50. Accordingly, the fiber-to-fluid density ratio ranges from S 7 (for St = 1, λ = 50) to S 3,470 (for St = 100, λ = 1). Statistics are compared one to one with those obtained for spherical particles to highlight effects due to elongation. Results for mean and fluctuating angular velocities show that elongation is important for fibers with small inertia (St ≤ 5 in the present flow-fiber combination). For fibers with larger inertia, elongation has an impact on fiber rotation only in the streamwise and wall-normal directions, where mean values of Ω are zero. It is also shown that, in the center of the channel, the Lagrangian autocorrelation coefficients of Ω and corresponding rotational turbulent diffusivities match the exponential behavior predicted by the theory of homogeneous dispersion. In this region of the channel, the probability density function of fiber angular velocities is generally close to Gaussian, indicating that particle rotation away from solid walls can be modeled as a diffusion process of the Ornstein-Uhlenbeck type at stationary state. In the strong shear region (comprised within a distance of 50 viscous units from the wall in the present simulations), fiber anisotropy adds to flow anisotropy to induce strong deviations on fiber rotational dynamics with respect to spherical particles. The database produced in this study is available to all interested users at https://www.fp1005.cism.it.

1 Department of Mechanical, Aerospace and Nuclear Engineering, 2 Center for Multiphase Research, Rensselaer Polytechnic Institute, Troy, NY 12180, USA (*) Tel: 518-276-4000, Fax: 518-276-3055 Email: podowm@ rpi. edu Key words: Membrane filtration; Dean ...

The possibility of using hydrocyclone plants for cleaning used oils is considered. The dependence of the degree of purification of waste oil on insoluble impurities is determined theoretically, determined by the mass content of particles in the oil flow removed through the lower drainage hole, from the time of the particle in the hydrocyclone and the radius of the surface of zero axial velocity, which determines the separation of the oil flow and the entrainment of particles through the upper or lower discharge holes.

Brownsville-The hydrocode SHAMRC has been used in the past to study the formation and growth of the Richtmyer-Meshkov Instability (RMI). While RMI involves impulsively accelerating two continuous fluids of differing densities, a similar class of instabilities has recently been described for multiphase flow. In this scenario, a shock wave passes through a region containing ambient air seeded with particles which have a non-trivial mass and density much greater than that of the surrounding and embedding fluid. In this scenario, no baroclinic vorticity is generated due to the lack of a fluid-fluid density interface. After the shock passage, the particles or droplets lag behind the surrounding gas. Momentum exchange between the embedded phase and the embedding phase leads to non-uniform local equilibrium velocity distribution, and thus to shear and vortex formation. As the primary mechanism for this instability formation is momentum transfer via drag, the morphology of the instability is strongly dependent of the sizes of the particles in the initial conditions. The simulations described here attempt to model the effects of changing the particle size on the morphology and growth rate of this instability.

Falling particle curtains are important in many engineering applications, including receivers for concentrating solar power facilities. During the formation of such a curtain, we observe a multiphase analog of Rayleigh-Taylor instability (RTI). It was originally described in 2011 for a situation when air sparsely seeded with glycol droplets was placed above a volume of unseeded air, producing an unstably stratified average density distribution that was characterized by an effective Atwood number 0.03. In that case, the evolution of the instability was indistinguishable from single-phase RTI with the same Atwood number, as the presence of the droplets largely acted as an additional contribution to the mean density of the gaseous medium. Here, we present experiments where the volume (and mass) fraction of the seeding particles in gas is considerably higher, and the gravity-driven flow is dominated by the particle movement. In this case, the evolution of the observed instability appears significantly different.

The work presents an experimental investigation into the motion of and hydrodynamic forces along a single flexible stem in regular waves. The experiment covers a large range in relevant non-dimensional parameters: the drag-to-stiffness ratio $CaL\in [0.003,3.8]$, the inertia-to-stiffness ratio $CaL/KC\in [4\times 10^{-5},14.8]$, the Keulegan–Carpenter number $KC\in [3.8,145]$ and the Reynolds number $Re\in [230,2900]$. The two first parameters relate to the response of the stem in waves and thus account for material properties, while the two last parameters are relevant for hydrodynamic forces on the stem. The displacement of the stem was captured with a digital video camera and the displacement along the stem was captured for every 2.5 mm at 25 Hz. This unique laboratory data set allowed for the following analyses: (i) Determination of the relevant non-dimensional parameter to predict the stem motion and shape. (ii) A direct comparison between the measured force for mimics of two l...

Non-spherical particles transported by an anisotropic turbulent flow preferentially align with the mean shear and intermittently tumble when the local strain fluctuates. Such an intricate behaviour is here studied for small, inertialess, rod-shaped particles embedded in a two-dimensional turbulent flow with homogeneous shear. A Lagrangian stochastic model for the rods angular dynamics is introduced and compared to the results of direct numerical simulations. The model consists in superposing a short-correlated random component to the steady large-scale mean shear, and can thereby be integrated analytically. Reproducing the single-time orientation statistics obtained numerically requires to take account of the mean shear, of anisotropic velocity gradient fluctuations, and of the presence of persistent rotating structures that combine together to bias cumulative Lagrangian statistics. The model is then used to address two-time statistics. The notion of tumbling rate is extended to diffusive dynamics by introducing the stationary probability flux of the rods unfolded angle, which provides information on the overall, cumulated rotation of the particle. The model is found to reproduce the long-term effects of an average shear on the mean and the variance of the fibres angular increment. Still, for intermediate times, the model fails catching violent fluctuations of the rods rotation that are due to trapping events in coherent, long-living eddies.

1 Department of Mechanical, Aerospace and Nuclear Engineering, 2 Center for Multiphase Research, Rensselaer Polytechnic Institute, Troy, NY 12180, USA (*) Tel: 518-276-4000, Fax: 518-276-3055 Email: podowm@ rpi. edu Key words: Membrane filtration; Dean ...

We study the rotational dynamics of inertial disks and rods in three-dimensional, homogeneous isotropic turbulence. In particular, we show how the alignment and the decorrelation timescales of such spheroids depend, critically, on both the level of inertia and the aspect ratio of these particles. These results illustrate the effect of inertia-which leads to a preferential sampling of the local flow geometry-on the statistics of both disks and rods in a turbulent flow. Our results are important for a variety of natural and industrial settings where the turbulent transport of asymmetric, spheroidal inertial particles is ubiquitous.

Adding polymers or particles to a flow can alter the drag experienced by it. For instance, polymers in a flow reduce drag but their ability is bounded by a limit, the maximum drag reduction asymptote. However, the effect of particles on drag is ambiguous, with studies reporting contradicting observations; even in cases where particles are reported to reduce drag no asymptotic limit is know. The ambiguity arises because in addition to particle concentration, particle shape, size and density affect the drag. Hence, various particles behave differently in distant ranges of Re and concentration. In the present study, we experimented in a pipe flow setup with neutrally buoyant spherical and elongated particles, covering wide ranges of both Re and concentration. Based on friction factor, we found that spherical particles do not show drag reduction at any Re while the elongated particles do within an specific interval of Re. This interval strongly depends on particle concentration and relative size of pipe to particle, and within this interval the friction factor reaches a minimum value. These drag reduction maxima fall onto a distinct curve which can be considered the maximum drag reduction asymptote for a given particle shape, irrespective of the pipe diameter or concentration.

of a shock wave with planar and perturbed curtain of massive particles is studied experimentally. To form the curtain, solid soda lime particles (30-50 micron diameter) are dropped from a hopper fitted with mesh sieves and vibrated with a motor. The curtain forms when the particles move through a rectangular slot in the top of the test section of the shock tube used in experiment. The curtain can be either planar or perturbed in the horizontal plane (parallel to the shock direction) based on the shape of the slot. This setup generates a particle curtain with a volume fraction varying between 2 and 8 percent along its vertical height. A laser illuminates the curtain in vertical and horizontal planes. When the diaphragm separating the driver and the driven section is ruptured, shock waves with Mach numbers ranging from 1 to 2, depending on the pressure, propagate down the driven section and into test section. The phenomena following the shock wave impingement on the particle curtain are captured using an Apogee Alta U42 camera.

Brownsville-The hydrocode SHAMRC has been used in the past to study the formation and growth of the Richtmyer-Meshkov Instability (RMI). While RMI involves impulsively accelerating two continuous fluids of differing densities, a similar class of instabilities has recently been described for multiphase flow. In this scenario, a shock wave passes through a region containing ambient air seeded with particles which have a non-trivial mass and density much greater than that of the surrounding and embedding fluid. In this scenario, no baroclinic vorticity is generated due to the lack of a fluid-fluid density interface. After the shock passage, the particles or droplets lag behind the surrounding gas. Momentum exchange between the embedded phase and the embedding phase leads to non-uniform local equilibrium velocity distribution, and thus to shear and vortex formation. As the primary mechanism for this instability formation is momentum transfer via drag, the morphology of the instability is strongly dependent of the sizes of the particles in the initial conditions. The simulations described here attempt to model the effects of changing the particle size on the morphology and growth rate of this instability.

We present a jointed experimental and numerical study examining the influence of vortical structures on the settling of solid spherical particles under the action of gravity at low Stokes numbers. The two-dimensional model experiment uses electroconvection to generate a two-dimensional array of controlled vortices which mimics a simplified vortical flow. Particle image-velocimetry and tracking are used to examine the motion of the particles within this vortical flow. Particle motion is compared to the predictions of a numerical simulation inspired by the model equation developed by Maxey ["The motion of small spherical particles in a cellular flow field," Phys. Fluids 30, 1915 (1987)].

Numerical studies into the dynamics of non-spherical particles in turbulent channel flow have, until now, been mostly confined to low Reynolds numbers. In this paper, we investigate the dynamics of tracer non-spherical particles in a channel flow at Re τ ≈ 1000 for the first time. Tracer spheroidal particles suspended in a turbulent channel flow are computed by means of direct numerical simulation coupled with a Lagrangian point-particle approach. We examine the rotational dynamics and alignment of tracer spheroids of different aspect ratio in both the near-wall region and, in particular, the quiescent core which is not observed at lower Reynolds numbers. In the near-wall region, the effect of Reynolds number on preferential alignment is negligible with particle rotation exhibiting a weak dependence for all aspect ratios investigated. However, the particle rotation is strongly damped in the quiescent core demonstrating that the motion of tracer spheroids becomes quiescent. The striking difference in spheroids' rotational dynamics observed inside and outside of the quiescent core is correlated with the local Kolmogorov time scale in the core and outside of it.

Axis-symmetric spheroids, such as rod-like and disk-like particles, have been found to orient preferentially in near-wall turbulence by both experiment and numerical simulation. In current work we examined the orientation of inertialess spheroids in a turbulent channel flow at medium friction Reynolds number given based on the half of channel height. Both elongated prolate spheroid and flat oblate spheroid are considered and further compared with the reference case of spherical particle. The statistical results show that in near wall region the prolate spheroids tend to align in the streamwise direction while the oblate spheroids prefer to orient in the wallnormal direction, which are consistent with earlier observation in low Reynolds number () wall turbulence. Around the channel center we found that the orientation of spheroids is not fully isotropic, even though the fluid vorticity are almost isotropic. The mechanism that gives rise to such particle orientations in wall-turbulence has been found to be related to fluid Lagrangian stretching and compression (Zhao and Andersson 2016). Therefore, we computed the left Cauchy-Green strain tensor along Lagrangian trajectories of tracer spheroids in current flow field and analyzed the fluid Lagrangian stretching and compression. The results indicated that, similar to the earlier observations, the directions of the Lagrangian stretching and compression in near-wall region are in the streamwise and wall-normal directions, respectively. Furthermore, cross over the channel the prolate spheroids aligned with the direction of Lagrangian stretching but oblate spheroids oriented with the direction of Lagrangian compression. The weak anisotropy of orientations of fluid Lagrangian stretching and compression observed at the channel center could be the reason for the aforementioned modest anisotropic orientation of spheroids in channel central region.

We study the three-dimensional structure of turbulent velocity fields around extreme events of local energy transfer in the dissipative range. Velocity fields are measured by tomographic particle velocimetry at the centre of a von Kármán flow with resolution reaching the Kolmogorov scale. The characterization is performed through both direct observation and an analysis of the velocity gradient tensor invariants at the extremes. The conditional average of local energy transfer on the second and third invariants seems to be the largest in the sheet zone, but the most extreme events of local energy transfer mostly correspond to the vortex stretching topology. The direct observation of the velocity fields allows for identification of three different structures: the screw and roll vortices, and the U-turn. They may correspond to a single structure seen at different times or in different frames of reference. The extreme events of local energy transfer come along with large velocity and vorticity norms, and the structure of the vorticity field around these events agrees with previous observations of numerical works at similar Reynolds numbers.