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Particle image velocimetry and particle tracking velocimetry have been employed to investigate the dynamics of finite-size spherical particles, slightly heavier than the carrier fluid, in a horizontal turbulent square duct flow. Interface resolved direct numerical simulations (DNSs) have also been performed with the immersed boundary method at the same experimental conditions, bulk Reynolds number Re2H=5600, duct height to particle-size ratio 2H/dp=14.5, particle volume fraction Φ=1%, and particle to fluid density ratio ρp/ρf=1.0035. Good agreement has been observed between experiments and simulations in terms of the overall pressure drop, concentration distribution, and turbulent statistics of the two phases. Additional experimental results considering two particle sizes 2H/dp=14.5 and 9 and multiple Φ=1%, 2%, 3%, 4%, and 5% are reported at the same Re2H. The pressure drop monotonically increases with the volume fraction, almost linearly and nearly independently of the particle size for the above parameters. However, despite the similar pressure drop, the microscopic picture in terms of fluid velocity statistics differs significantly with the particle size. This one-to-one comparison between simulations and experiments extends the validity of interface resolved DNS in complex turbulent multiphase flows and highlights the ability of experiments to investigate such flows in considerable detail, even in regions where the local volume fraction is relatively high., QC 20190215

We report experimental observations of turbulent flow with spherical particles in a square duct. Three particle sizes, namely 2H/d(p) = 40, 16 and 9 (2H being the duct full height and d(p) being the particle diameter), are investigated. The particles are nearly neutrally buoyant with a density ratio of 1.0035 and 1.01 with respect to the suspending fluid. Refractive index matched-particle image velocimetry (RIM-PIV) is used for fluid velocity measurement even at the highest particle volume fraction (20 %) and particle tracking velocimetry (PTV) for the particle velocity statistics for the flows seeded with particles of the two largest sizes, whereas only pressure measurements are reported for the smallest particles. Settling effects are seen at the lowest bulk Reynolds number R-e2H approximate to 10 000, whereas, at the highest R-e2H approximate to 27 000, particles are in almost full suspension. The friction factor of the suspensions is found to be significantly larger than that of single-phase duct flow at the lower R-e2H investigated; however, the difference decreases when increasing the flow rate and the total drag approaches the values of the single-phase flow at the higher Reynolds number considered, R-e2H = 27 000. The pressure drop is found to decrease with the particle diameter for volume fractions lower than (sic) = 10% for nearly all R-e2H investigated. However, at the highest volume fraction (sic) = 20 %, we report a peculiar non-monotonic behaviour: the pressure drop first decreases and then increases with increasing particle size. The decrease of the turbulent drag with particle size at the lowest volume fractions is related to an attenuation of the turbulence. The drag increase for the two largest particle sizes at (sic) = 20 %, however, occurs despite this large reduction of the turbulent stresses, and it is therefore due to significant particle-induced stresses. At the lowest Reynolds number, the particles reside mostly in the bottom half of the duc, QC 20181121

We study the settling of rigid oblates in a quiescent fluid using interface-resolved direct numerical simulations. In particular, an immersed boundary method is used to account for the dispersed solid phase together with lubrication correction and collision models to account for short-range particle-particle interactions. We consider semi-dilute suspensions of oblate particles with aspect ratio AR = 1/3 and solid volume fractions (Phi = 0.5-10%. The solid-to-fluid density ratio R = 1.02 and the Galileo number (i.e. the ratio between buoyancy and viscous forces) based on the diameter of a sphere with equivalent volume Ga = 60. With this choice of parameters, an isolated oblate falls vertically with a steady wake with its broad side perpendicular to the gravity direction. At this Ga, the mean settling speed of spheres is a decreasing function of the volume Phi and is always smaller than the terminal velocity of the isolated particle, V-t. On the contrary, in dilute suspensions of oblate particles (with Phi <= 1 %), the mean settling speed is approximately 33 % larger than V-t. At higher concentrations, the mean settling speed decreases becoming smaller than the terminal velocity V-t between (Phi = 5 % and 10%. The increase of the mean settling speed is due to the formation of particle clusters that for Phi = 0.5-1 % appear as columnar-like structures. From the pair distribution function we observe that it is most probable to find particle pairs almost vertically aligned. However, the pair distribution function is non-negligible all around the reference particle indicating that there is a substantial amount of clustering at radial distances between 2 and 6c (with c the polar radius of the oblate). Above Phi = 5 %, the hindrance becomes the dominant effect, and the mean settling speed decreases below V-t. As the particle concentration increases, the mean particle orientation changes and the mean pitch angle (the angle between the particle axis of symmetry and gravity, QC 20180731

We study the settling of finite-size rigid spheres in quiescent fluid and in sustained homogeneous isotropic turbulence (HIT) by direct numerical simulations using an immersed boundary method to account for the dispersed solid phase. We consider semi-dilute and dense suspensions of rigid spheres with solid volume fractions ϕ= 0.5 - 10 % , solid-to-fluid density ratio R= 1.02 , and Galileo number (i.e., the ratio between buoyancy and viscous forces) Ga= 145. In HIT, the nominal Reynolds number based on the Taylor microscale is Re λ ≃ 90 , and the ratio between the particle diameter and the nominal Kolmogorov scale is (2 a) / η≃ 12 (being a the particle radius). We find that in HIT the mean settling speed is less than that in quiescent fluid for all ϕ. For ϕ= 0.5 % , the mean settling speed in HIT is 8 % less than in quiescent fluid. However, by increasing the volume fraction the difference in the mean settling speed between quiescent fluid and HIT cases reduces, being only 1.7 % for ϕ= 10 %. Indeed, while at low ϕ the settling speed is strongly altered by the interaction with turbulence, at large ϕ this is mainly determined by the (strong) hindering effect. This is similar in quiescent fluid and in HIT, leading to similar mean settling speeds. On the contrary, particle angular velocities are always found to increase with ϕ. These are enhanced by the interaction with turbulence, especially at low ϕ. In HIT, the correlations of particle lateral velocity fluctuations oscillate around zero before decorrelating completely. The time period of the oscillation seems proportional to the ratio between the integral lengthscale of turbulence and the particle characteristic terminal velocity. Regarding the mean square particle displacement, we find that it is strongly enhanced by turbulence in the direction perpendicular to gravity, even at the largest ϕ. Finally, we investigate the collision statistics for all cases and find the interesting result that the collision frequency is, QC 20190304

We study the turbulent square duct flow of dense suspensions of neutrally buoyant spherical particles. Direct numerical simulations (DNS) are performed in the range of volume fractions phi = 0-0.2, using the immersed boundary method (IBM) to account for the dispersed phase. Based on the hydraulic diameter a Reynolds number of 5600 is considered. We observe that for phi = 0.05 and 0.1, particles preferentially accumulate on the corner bisectors, close to the corners, as also observed for laminar square duct flows of the same duct-to-particle size ratio. At the highest volume fraction, particles preferentially accumulate in the core region. For plane channel flows, in the absence of lateral confinement, particles are found instead to be uniformly distributed across the channel. The intensity of the cross-stream secondary flows increases (with respect to the unladen case) with the volume fraction up to phi = 0.1, as a consequence of the high concentration of particles along the corner bisector. For phi = 0.2 the turbulence activity is reduced and the intensity of the secondary flows reduces to below that of the unladen case. The friction Reynolds number increases with phi in dilute conditions, as observed for channel flows. However, for phi = 0.2 the mean friction Reynolds number is similar to that for phi = 0.1. By performing the turbulent kinetic energy budget, we see that the turbulence production is enhanced up to phi = 0.1, while for phi = 0.2 the production decreases below the values for phi = 0.05. On the other hand, the dissipation and the transport monotonically increase with phi The interphase interaction term also contributes positively to the turbulent kinetic energy budget and increases monotonically with phi, in a similar way as the mean transport. Finally, we show that particles move on average faster than the fluid. However, there are regions close to the walls and at the corners where they lag behind it. In particular, for phi = 0.05, 0.1, the slip vel, Not duplicate with DiVA 1157360QC 20180803

To study the dynamics of particles in turbulence when their sizes are comparable to the smallest eddies in the flow, the Kolmogorov length scale, efficient and accurate numerical models for the particle-fluid interaction are still missing. Therefore, we here extend the treatment of the particle feedback on the fluid based on the volume-averaged fluid equations (VA simulation) in the previous study of the present authors, by estimating the fluid force correlated with the disturbed flow. We validate the model against interface-resolved simulations using the immersed-boundary method. Simulations of single particles show that the history effect is well captured by the present estimation method based on the disturbed flow. Similarly, the simulation of the flow around a rotating particle demonstrates that the lift force is also well captured by the proposed method. We also consider the interaction between non-negligible size particles and an array of Taylor–Green vortices. For density ratios ρd /ρc ≥ 10, the results show that the particle motion captured by the VA approach is closer to that of the fully-resolved simulations than that obtained with a traditional two-way coupling simulation. The flow disturbance is also well represented by the VA simulation. In particular, it is found that history effects enhance the curvature of the trajectory in vortices and this enhancement increases with the particle size. Furthermore, the flow field generated by a neighboring particle at distances of around ten particle diameters significantly influences particle trajectories. The computational cost of the VA simulation proposed here is considerably lower than that of the interface-resolved simulation., QC 20180517

We study the inertial migration of finite-size neutrally buoyant spherical particles in dilute and semidilute suspensions in laminar square duct flow. We perform several direct numerical simulations using an immersed boundary method to investigate the effects of the bulk Reynolds number Re-b, particle Reynolds number Re-p, and duct to particle size ratio h/a at different solid volume fractions phi, from very dilute conditions to 20%. We show that the bulk Reynolds number Re-b is the key parameter in inertial migration of particles in dilute suspensions. At low solid volume fraction (phi = 0.4%), low bulk Reynolds number (Re-b = 144), and h/a = 9 particles accumulate at the center of the duct walls. As Re-b is increased, the focusing position moves progressively toward the corners of the duct. At higher volume fractions, phi = 5%, 10%, and 20%, and in wider ducts (h/a = 18) with Re-b = 550, particles are found to migrate away from the duct core toward the walls. In particular, for phi = 5% and 10%, particles accumulate preferentially at the corners. At the highest volume fraction considered, phi = 20%, particles sample all the volume of the duct, with a lower concentration at the duct core. For all cases, we find that particles reside longer times at the corners than at the wall centers. In a duct with lower duct to particle size ratio h/a = 9 (i.e., with larger particles), phi = 5%, and high bulk Reynolds number Re-b = 550, we find a particle concentration pattern similar to that in the ducts with h/a = 9 regardless of the solid volume fraction phi. Instead, for lower Bulk Reynolds number Re-b = 144, h/a = 9, and phi = 5%, a different particle distribution is observed in comparison to a dilute suspension phi = 0.4%. Hence, the volume fraction plays a key role in defining the final distribution of particles in semidilute suspensions at low bulk Reynolds number. The presence of particles induces secondary cross-stream motions in the duct cross section, for all phi. Th, QC 20171116

We study the rheology of confined suspensions of neutrally buoyant rigid monodisperse spheres in plane-Couetteflow using Direct Numerical Simulations.We find that if the width of the channel is a (small) integer multiple of the spherediameter, the spheres self-organize into two-dimensional layersthat slide on each other and the effective viscosity of the suspension issignificantly reduced. Each two-dimensional layer is found to be structurallyliquid-like but its dynamics is frozen in time., QC 20160205

Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows, yetlittle is known about the behavior of finite-size particles inhomogeneous isotropic turbulence. To fill this gap, we perform Direct Numerical Simulations of sedimentation in quiescent and turbulent environments using anImmersed Boundary Method to accountfor the dispersed rigid spherical particles. The solid volume fractions considered are 0.5-1%,while the solid to fluid density ratio 1.02.The particle radius is chosen to be approximately 6 Komlogorov lengthscales. Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows, yet little is known about the behaviour of finite-size particles in homogeneous isotropic turbulence. To fill this gap, we perform direct numerical simulations of sedimentation in quiescent and turbulent environments using an immersed boundary method to account for the dispersed rigid spherical particles. The solid volume fractions considered are phi = 0.5-1%, while the solid to fluid density ratio rho(p)/rho(f) = 1.02. The particle radius is chosen to be approximately six Kolmogorov length scales. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. The reductions with respect to a single particle in quiescent fluid are approximately 12 % and 14% for the two volume fractions investigated. The probability density function of the particle velocity is almost Gaussian in a turbulent flow, whereas it displays large positive tails in quiescent fluid. These tails arc associated with the intermittent fast sedimentation of particle pairs in drafting kissing tumbling motions. The particle lateral dispersion is higher in a turbulent flow, whereas the vertical one is, surprisingly, of comparable magnitude as a consequence of the highly intermittent behaviour observed in the quiescent fluid. Using the concept of mean relative velocity we estimate the, QC 20160220

We study the effect of varying the mass and volume fraction of a suspension of rigid spheres dispersed in a turbulent channel flow. We performed several direct numerical simulations using an immersed boundary method for finite-size particles changing the solid to fluid density ratio R, the mass fraction χ, and the volume fraction φ. We find that varying the density ratio R between 1 and 10 at constant volume fraction does not alter the flow statistics as much as when varying the volume fraction φ at constant R and at constant mass fraction. Interestingly, the increase in overall drag found when varying the volume fraction is considerably higher than that obtained for increasing density ratios at same volume fraction. The main effect at density ratios R of the order of 10 is a strong shear-induced migration towards the centerline of the channel. When the density ratio R is further increased up to 1000, the particle dynamics decouple from that of the fluid. The solid phase behaves as a dense gas and the fluid and solid phase statistics drastically change. In this regime, the collision rate is high and dominated by the normal relative velocity among particles., QC 20160520

We study the sedimentaion of finite-size particles in a quiescent wall-boundedNewtonian and shear-thinning fluids. The problem is studied numerically bymeans of direct numerical simulations with the presence of the particles ac-counted for with an immersed boundary method. The supensions are Non-Brownian rigid spherical particles with particle to fluid density ratio ρ p /ρ f =1.5; three different solid volume fractions Φ = 1%, 5% and 20% are considered.The Archimedes number is kept constant to Ar = 36 for all shear-thinning fluidcases, while it is changed to Ar = 97 for the Newtonian fluid to reproduce thesame terminal velocity of a single particle sedimenting in the shear-thinningfluid. We show that the mean settling velocities decrease with the particle con-centration as a consequence of the hindering effect and that the mean settlingspeed is always larger in the shear thinning fluid than in the Newtonian one.This is due to the decrease of the mean viscosity of the fluid which leads to alower drag force acting on the particles. We show that particles tend to formaggregates in the middle of the channel in a shear-thinning fluid, preferentiallypositioning in the wake of neighboring particles or aside them, resulting in lowerlevels of fluctuation in the gravity direction than in a Newtonian fluid., QC 201912010

We study the effect of varying the mass and volume fraction of a suspension of rigid spheres dispersedin a turbulent channel flow. We performed several Direct Numerical Simulations using an Immersed Boundary Method forfinite-size particles changing the solid to fluid density ratio R, the mass fraction and the volume fraction. We find that varying the density ratio R between 1 and 10 at constant volume fraction does not alter the flow statisticsas much as when varying the volume fraction at constant R and at constant mass fraction. Interestingly, the increase in overall drag found when varying the volume fraction is considerablyhigher than that obtained for increasing density ratios at same volume fraction. The main effect atdensity ratios R of the order of 10 is a strong shear-induced migration towards the centerline of the channel. When thedensity ratio R is further increased up to 100 the particle dynamics decouple from that of the fluid. The solid phase behaves as a dense gas andthe fluid and solid phase statistics drastically change. In this regime, the collisionrate is high and dominated by the normal relative velocity among particles., QS 2015