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In the present paper, the three dimensional (3D) settlement process of two side-by-side free-falling spheres in a water tank was numerically investigated using coupled Smoothed Particle Hydrodynamics (SPH) - Discrete Element Method (DEM) method. The maximum particle Reynolds number, calculated based on the terminal velocity and sphere diameter, is R e m = 2 . 4 × 1 0 4 . The accuracy of the model was firstly validated by simulating the single sphere water-entry, settlement of two circular cylinders arranged in series and two identical spheres settlement side by side problems, respectively. The simulation results exhibited a satisfactory agreement with the available numerical and experimental data in the literatures. Then, the settlement process of two spheres with different sizes and transverse distances were simulated. In particular, the motion patterns of two interacting spheres with different initial gaps and diameter ratios, including motion trajectory, vertical falling distance and flow field, were systematically investigated. The results demonstrate that when initial gap S 0 ∗ S 0 ∗ > 0.75, the interaction between the two spheres are negligible. In addition, the interaction between particles would slow down the settling velocity and affect the terminal settling distance.

Empirical correlations and experimental works relating the drag coefficient (CD) of spheres to the Reynolds number (Re) and Re to the Archimedes number (Ar) were reviewed with a focus on the correlations applicable to the entire Re range. In addition, experiments with various spherical particles and fluids for the entire range of Re were conducted using high-speed video and added to the literature data. One of the correlations from the literature for Re-Ar and a modified correlation for CD–Re were found to best fit the experimental results. The analysis also established new correlations for Re–Ha (a non-dimensional number for velocity not including particle size) and CD–Ar. The latter is easier to use than the common CD–Re curve.

The management of fluids in space is complicated by the absence of relevant buoyancy forces. This raises significant technical issues for two-phase flow applications. Different approaches have been...

Study of drag force on dendritic particles in highly viscous fluids is essential for understanding the dynamic sinking/floating behavior of particles in metallurgical systems. In this study, experiments were carried out to investigate the drag coefficient of simplified three-dimensional dendritic particles in silicone oil with a range of viscosities (20, 100 and 500 cSt). The experimental system was designed to represent the basic oxygen steelmaking (BOS) slag system based on a dynamic similarity approach. The drag coefficient of each dendritic particle was calculated from the measured terminal velocity based on a force balance and the known drag coefficient of its volume equivalent sphere. The drag coefficients were determined over a range of Reynolds numbers (~ 0.035 to 84) differing by two orders of magnitude accounting for various particle shape factors that included aspect ratio, sphericity, and particle orientation. Correlations between the drag coefficient and the shape factors were proposed for each viscosity.

An experimental study for the drag of solid spheres in drag reducing Fluids is presented. Polyacrylamide solutions of different concentrations are considered. The drag coefficient is measured by tie terminal velocity of the spherical particle falling in stagnant fluid using high speed camera, Compared to Newtonian Fluid results, the drag of solid particle in polymer solutions exhibit reduced values. However, a drag increase Is exhibited at low Reynolds number. Analysis of the data show the presence of a critical Reynolds number that distinguish between the drag increase and drag reduction regions. It has been postulated that the measured drag is a result of two opposing effects of polymer additives. One is flow resistance increase due to increased extensional viscosity. The other, is the reduction in the form drag due to the influence of these additives on vortex shedding and boundary layer separation. It has been found that the experimental data of drag coefficient is best correlated with the polymer -particle parameter ᴓ or Weissenberg number Ws as well as Reynolds number.

The results of experimental studies of the terminal velocity of peat dust in an air flow with different size distributions of dust particles are presented, and the particle sizes and shapes were examined by microscopic methods. The aerodynamic drag of peat particles of various sizes in an air flow was studied, and a criterion relationship between the drag coefficient CD and the Reynolds number (Re) was determined. The experimental results on the terminal velocity of peat dust were compared with data calculated using well-known methods, and recommendations for their application in the design of pneumatic fuel supply systems were given.

This paper provides an alternative Lambert solution based on the terminal velocity vector. The chordal terminal velocity component is selected as the independent variable since it is the final requ...

A novel theoretical model which takes into account the particle property, equipment parameters, particle terminal velocity and Reynolds number was established to describe the separation of particles under critical conditions. Meanwhile, this established model was used to predict the superficial velocity of particle separation in the different spacing of inclined channels. A system was developed to test and verify the model. The correlation coefficient (R2) and Sum of Squares for Error (SSE) were adopted to evaluate the experimental results. When the ratio of fluid Reynolds number to particle Reynolds number was less than 43, the novel model was proved to be highly accurate in the prediction of the superficial velocity. The results suggested that the novel model provided much better conclusions than does a tradition empirical model. This study provides a theoretical and experimental foundation for the design of industrial inclined channel structure and operating parameters.

Knowledge on regime transition from conventional to circulating fluidization is important to scale-up and operation of liquid-solid fluidized systems in industrial applications. Previous experimental studies reported that the onset velocity measured by bed empty time test is pertinent to the physical properties of particle and liquid, and independent of solid inventory, bed geometry, and configuration. The effect of liquid properties is, however, not clear yet since tap water is usually used as the working fluid in laboratory test for convenience. To address this problem, a Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) model is applied to numerically measure the onset velocity of particles in different liquid media. The model is first validated in terms of the experimental data of bed expansion in literature. Then several kinds of particles and three kinds of liquid media of different densities and viscosities are further simulated. We find that the Reynolds number based on onset velocity and Archimedes number follows a power-law relationship. With the decrease of Archimedes number, the discrepancy of Reynolds number based on onset velocity and that on particle terminal velocity becomes highly significant. The ratio of the onset velocity to particle terminal velocity for various particles in one liquid medium is not a constant, and instead dependent on both the particle and liquid properties.

The motion of solids in a viscous fluid with nonlinear complex behavior has important applications in different industries and engineering sciences. This work experimentally investigated the free-fall behavior of rotating and non-rotating spherical particles in a viscous fluid. The effects of physical parameters, including particle size, particle density, fluid viscosity, and rotational velocity, on the terminal velocity of the falling spherical particles were examined. A high-speed camera was employed to capture the phenomena and study the free-fall behavior of the spherical solid particles in a viscous fluid. The studied spherical particles had five different densities of 2800, 7800, 7900, 8660, and 8960 kg/m3 and three varying dimensions of 4, 5, and 6 mm. To explore the effect of fluid viscosity on the terminal velocity of particles, two types of fluids with the viscosities of 0.012 and 0.014 Pa.s were used. The initial rotational velocities were 0, 600, 900, and 1200 rpm for the falling particles. The obtained results revealed that increases in the particle diameter, density, and rotational velocity, along with a decrease in fluid viscosity, accelerated the terminal velocity and shortened the time of reaching this velocity. Furthermore, an increase in the Reynolds number gave rise to a decrease in the lift coefficient. On the other hand, the accelerated dimensionless rotational velocity increased the lift coefficient.

A new theoretical formula can be used to calculate superficial velocity for particle separation with a critical condition, and it is based on the size, density, terminal velocity, Reynold number of the particle and equipment parameters. In a continuous system, a series of fractionation experiments were conducted to quantify the separation performance for 0–2 mm coal particles under various factors, including channel spacing, solids throughput and split fluidization rate. The separation performance considers the ash content and yield of products varying with the particle size. A great fluidization environment can be created by the 6 mm channel spacing. For the solids throughput of 10.20 t/(m2h) provides a well effective separation in the narrow channels. In addition, the split fluidization of 0.0058 m/s can produce a higher shear induce force in the inclined channels to prevent the low-density particles from being lost in the underflow. The theoretical superficial velocity calculated by the new formula is 0.042 m/s, which can report the particles with the upper and lower size reach at 9-fold to the overflow, it is almost the same as the actual separation fluidization velocity of 0.04 m/s. Meanwhile, the data obtained under each separation condition is basically consistent with data of the sink-float mothed, which shows a good separation performance for the Reflux Classifier.

Various dimensionless characterizations of the solid-fluid situation during settling of a spherical particle are surveyed and related to the Archimedes number. The new landmark drag correlation developed recently is rearranged into a quadratic equation of the terminal Reynolds number. Its solution is an explicit equation of the terminal velocity of a settling sphere depending only on the Archimedes number. This new relationship is compared with a few other explicit relations existing in the literature to predict the terminal Reynolds number of spheres.

Ionic liquids (ILs) have been proved to be excellently potential catalysts and solvents in a few chemical processes. In order to provide fundamental data for the reactor design and process optimization, it is essential to investigate the hydrodynamics and the mass transfer phenomena of liquid-solid two-phase systems containing ILs. In this work, we focus on the investigation of the hydrodynamic properties of a spherical particle in ILs, and a series of experiments are implemented in a cylindrical bubble column with three different diameters of spherical particles made of borosilicate glass (2 mm, 3 mm and 4 mm) in three kinds of ILs ([Bmim][BF4], [Bmim][PF6] and [Bmim][Tf2N]). Moreover, Two key physical quantities (terminal velocity of particle and contact angle) are measured with a high-speed camera system and an automatic contact angle meter. Based on dimensional analysis, two novel empirical models of the drag coefficient (CD) between a spherical particle and ILs are developed in the range of particle’s Reynolds number (Re) from 0.1 to 85. The first model is simple and the calculated CD agrees fairly well with the experimental data. Furthermore, in order to evaluate the effects of surface properties on CD, a newly defined dimensionless number (Mo′) is introduced into IL-particle flow studies for the first time, which comes into being a more rigorous model. The average predicted deviations of the two models are ±1.1% and ±1.5% respectively, which is much lower than that of contrastive models in the same situations (about 13.3% or more). These two models are significant for studying the hydrodynamics and mass transfer characteristics of IL-particle systems as well as the fluid dynamics simulation in related reactors.

Nanobubbles have many potential applications depending on their types. The long-term stability of different gas nanobubbles is necessary to be studied considering their applications. In the present study, five kinds of nanobubbles (N2, O2, Ar + 8%H2, air and CO2) in deionized water and a salt aqueous solution were prepared by the hydrodynamic cavitation method. The mean size and zeta potential of the nanobubbles were measured by a light scattering system, while the pH and Eh of the nanobubble suspensions were measured as a function of time. The nanobubble stability was predicted and discussed by the total potential energies between two bubbles by the extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The nanobubbles, except CO2, in deionized water showed a long-term stability for 60 days, while they were not stable in the 1 mM (milli mol/L) salt aqueous solution. During the 60 days, the bubble size gradually increased and decreased in deionized water. This size change was discussed by the Ostwald ripening effect coupled with the bubble interaction evaluated by the extended DLVO theory. On the other hand, CO2 nanobubbles in deionized water were not stable and disappeared after 5 days, while the CO2 nanobubbles in 1 mM of NaCl and CaCl2 aqueous solution became stable for 2 weeks. The floating and disappearing phenomena of nanobubbles were estimated and discussed by calculating the relationship between the terminal velocity of the floating bubble and bubble size. Validerad;2021;Nivå 2;2021-04-16 (alebob);Finansiär: Natural Science Foundation of China (NSFC 21976039)

Recently the thermal friction on an expanding bubble from the cosmic first-order phase transition has been calculated to all orders of the interactions between the bubble wall and thermal plasma, leading to a $\gamma^2$-scaling instead of the previously estimated $\gamma^1$-scaling for the thermal friction exerted on a fast-moving bubble wall with a Lorentz factor $\gamma$. We propose for the first time the effective equation of motion (EOM) for an expanding bubble wall in the presence of an arbitrary $\gamma$-scaling friction to compute the efficiency factor from bubble collisions, which, in the case of $\gamma^2$-scaling friction, is found to be larger than the recently updated estimation when the bubble walls collide after starting to approach a constant terminal velocity, leading to a slightly larger signal of the gravitational waves background from bubble collisions due to its quadratic dependence on the bubble collision efficiency factor, although the $\gamma^2$-scaling friction itself has already suppressed the contribution from bubble collisions compared to that with $\gamma^1$-scaling friction. We also suggest a phenomenological parameterization for the out-of-equilibrium term in the Boltzmann equation that could reproduce the recently found $(\gamma^2-1)$-scaling of the friction term in the effective EOM of an expanding bubble wall, which merits further study in future numerical simulations of bubble expansion and collisions., Comment: v1, 26 pages, 2 figures; v2, typos corrected and references added; v3, minor improvements in presentation

We investigate the turbulence statistics in a multiphase plume made of heavy particles (particle Reynolds number at terminal velocity is 450). Using refractive-index-matched stereoscopic particle image velocimetry, we measure the locations of particles whose buoyancy drives the formation of a multiphase plume, together with the local velocity of the induced flow in the ambient salt–water. Measurements of the mean axial flow in the plume centreplane follow Gaussian profiles and that of the mean radial flow is consistent with integral plume theory. The turbulence characteristics resemble those measured in a bubble plume, including strong anisotropy in the normal Reynolds stresses. However, we observe structural differences between the two multiphase plumes. First, the skewness of the probability density function of the axial velocity fluctuations is not that which would be predicted by simply reversing the direction of a bubble plume. Second, in contrast to a bubble plume, the particle plume has a non-negligible fluid-shear production term in the turbulent kinetic energy (TKE) budget. Third, the radial decay of all measured terms in the TKE budget is slower than those in a bubble plume. Despite these dissimilarities, a bigger picture emerges that applies to both flows. The TKE production by particles (or bubbles) roughly balances the viscous dissipation, except near the plume centreline. The one-dimensional power spectra of the velocity fluctuations show a power law that puts both the particle and bubble plume in a category different from single-phase shear-flow turbulence.

To describe bubbly flows, an accurate prediction of the bubble rise velocity is crucial. For non-Newtonian fluids, closures for the bubble rise velocity provided in the literature are usually empirical and rather restricted in their applicability. In this work, a Front-Tracking Computational Fluid Dynamics model has been used to investigate the behaviour of a single bubble rising in a power-law fluid, for a very wide range of viscosities covering both shear-thinning and shear-thickening behaviour, where the power-law exponent n was varied between 0.5 and 1.5 and for three different bubble diameters (viz. 0.5 mm, 2 mm and 4 mm). The non-Newtonian behaviour of the continuous phase strongly influences the shape of the single rising bubbles caused by the viscosity profiles that develop in the flow field. As a consequence, large non-spherical bubbles become more spherical in shear-thickening fluids (in comparison to the same bubble in a Newtonian liquid), whereas small spherical bubbles lose their sphericity in shear-thinning fluids. To determine the bubble rise velocity for bubbles in non-Newtonian fluids with a power law behaviour, the drag closure derived for bubbles rising in Newtonian liquids proposed by Dijkhuizen et al. (2010), which combines viscous drag and shape-induced drag in a single correlation, is adapted using a modified Reynolds number. In this work it is shown that this adapted correlation is able to predict the terminal rise velocity of single bubbles rising in non-Newtonian power-law fluids within 20% accuracy for the majority of the investigated cases, provided that the drag regime does not change.

A series of numerical simulations are performed for the sedimentation process of a particle cloud in shear-thinning fluids using lattice Boltzmann and discrete element methods. The initial particle concentration, c 0 , and the power-law index of the fluid, n, and Reynolds number, Re , are varied in these simulations. For 1.0 ⩽ Re ⩽ 10.9 , the particle cloud size grows in the longitudinal direction as the cloud settles, leading to reduced particle concentration and a quasi-steady settling velocity, w ¯ ∞ . The velocity ratio, w ¯ ∞ / w ∞ , where w ∞ is the corresponding single-particle terminal velocity, is found to decrease with both n and Re . This velocity ratio is only weakly dependent on the initial concentration ( 0.05 ⩽ c 0 ⩽ 0.20 ) due to particle dispersion. For 0.071 ⩽ Re 1.0 , the cloud loses its initial shape and disintegrates while settling, with particles escaping from the cloud due to differential particle settling velocities.

A casted leaded-bronze block was impinged by a circular water jet. A 30-fps digital camera was used to video record the wet front propagation process on the surface of the block. The video frames were analyzed to illustrate the effect of different key parameters on the wetting front propagation. Regression correlations were developed to describe the effect of the jet Reynolds number (Red), the ratio of the degree of jet subcooling to the initial temperature of the block Δ T sub T i and the nozzle-to-surface spacing (H) on the wetting front propagation. The correlation is applicable for the experimental ranges investigated; 12,155 ≤ Red ≤ 36,460, 0.085 ≤ T sub T i ≤ 0.107 and H ≥ 30 mm. Good agreement was found between the experimental results and the correlation predictions with a maximum deviation of 25%. At high Reynolds number, the wetting front propagation was faster due to the high velocity and flow inertia. As the ratio Δ T sub T i increased, the wetting front propagation was accelerated. The effect of the nozzle-to-surface height on the propagation speed was negligible for H ≥ 30 mm. The effect of the increased liquid head on the terminal velocity was weakened by the drag forces on the jet as well as the jet instabilities associated with the large spacing.

The spectral reflectivity in terms of Doppler velocity obtained by micro rain radar (MRR) at a tropical location can reveal the splitting of Doppler spectrum of falling rain drops caused by strong downdraft. The phenomenon, known as aliasing, occurs in Doppler spectrum of MRR during intense convective events. In this case, the rain drop velocity exceeds the unambiguous Doppler velocity range that can be sensed by MRR. The downdraft affecting the raindrop velocity significantly causes an ambiguity in the Doppler spectrum of the radar signal scattered from raindrops. The aliasing effect is most prominent near the boundary layer height (0.8–2 km) for convective rain. Also at this altitude range, the resultant height gradient obtained from ECMWF vertical velocity of air mass data and drop terminal velocity, is maximum. The importance of the present study lies in the fact that the split in Doppler spectrum can be utilized to estimate downdraft velocity during rain. The de-aliasing technique has been applied to the raw Doppler spectrum of MRR to retrieve the rain drop size distribution conforming to ground based measurements.

Lagrangian stochastic models for simulation of tracer-particle trajectories in turbulent flows can be adapted for simulation of particle trajectories. This is conventionally done by replacing the zero-mean fall speed of a tracer-particle with the terminal speed of the particle. Such models have been used widely to predict spore and pollen dispersal. Here I show that this modification predicts that particles become uniformly distributed throughout the air column, which is at variance with the seminal experimental studies of Hirst et al. (1967) that demonstrated spore concentrations (and pollen concentrations) declined exponentially with height in unstable air. This discrepancy arises because the terminal speed, which is a Lagrangian property of a particle, has always been treated as if it were an Eulerian property of an ensemble of particles. In this study models are formulated correctly. I show that the mean acceleration of a tracer-particle should be replaced by the mean acceleration of a particle. Model predictions for aerial density profiles then agreed with the observations of Hirst et al. (1967) and with observations of ground-level concentrations but differed significantly from predictions obtained using conventional models. In accordance with the results of numerical simulations, the models also predict that particles are moving downwind marginally more slowly than the wind itself. Finally, the new modelling approach can be extended to predict the dispersal of small insects with active flight behaviours.

We present an optical spectroscopic study of the black hole X-ray transient V4641 Sgr (=SAX J1819.3-2525) covering the 1999, 2002 and 2004 outbursts. The spectra were taken over 22 different epochs during the low luminosity phases that follow the sharp and bright outburst peaks displayed by the system. The data reveal the frequent presence of wind-related features in H (Balmer) and He I emission lines in the form of P-Cygni profiles and strong emission lines with broad wings. The terminal velocity of the wind, as measured in the blue-shifted absorption (P-Cygni) components, is in the range of ~ 900-1600 km/s, while the broad emission line wings (so-called nebular phases) imply outflow velocities of up to ~3000 km/s. We show that, at least for several of the wind detections, the radio jet was active and the system was likely in the hard state. This, together with previous detections reported in the literature, shows that V4641 Sgr is the second source of this class, after V404 Cyg, where the presence of these cold wind outflows has been clearly established. We discuss the similar phenomenology observed in both systems as well as the possible nature of the outflow and its impact on the accretion process., Accepted for publication in MNRAS

To study the hydrodynamics of rising droplets (especially less than 1 mm) in quiescent water, a microfluidic device with co-flowing configuration was integrated to prepare micron-sized droplets. Soybean oil and toluene droplets of size from 100 to 600 µm were obtained by five co-flowing devices with different capillary sizes. It is found that the capillary device with smaller tip size produces smaller droplets and, for a given device, the droplet size decreases with the increase of continuous phase flow rate. Alternatively, the increase of dispersed phase flow rate has little influence on the droplet size. To predict the droplet size, a correlation for dimensionless droplet diameter as a function of Capillary number (Ca), Reynolds number (Re) and viscosity ratio was proposed. Then, the hydrodynamics of a single droplet and of droplet swarms rising in quiescent water were extensively investigated. The experimental results show that the terminal velocity of a single micro-droplet is consistent with that of rigid spheres with the same size and density, while the terminal velocity of a droplet swarm is obviously higher than that of a single droplet. Experimental observation shows that the motion of a droplet swarm in static water is such a manner that each droplet alternately accelerates by chasing others, causing a higher terminal velocity. Furthermore, the terminal velocities of millimeter-sized toluene droplets rising in water were measured. Compared with rigid spheres, the droplets of dp=1~4.6 mm have higher terminal velocities, while the droplets of dp>4.6 mm have lower terminal velocities. Finally, by taking into account the interfacial tension, internal circulation and shape deformation, a comprehensive explanation is proposed to describe the variation of the drag coefficient with the droplet size ranging from microns to millimeters.

In this study, the effects of surfactants on the hydrodynamic characteristics of bubbles in shear-thinning fluids at low Reynolds number (Re

This work included the experimental studies of the Newtonian fluid flow over hollow frustum both in the normal and inclined channel. It encompasses terminal velocity and drag coefficient, CD data for 0.13 ≤ Re ≤ 8.41, 0.19 ≤ do/D ≤ 0.33, 0.22 ≤ di/do ≤ 0.83 and 40° ≤ θ ≤ 90o. The effect of inner diameter to outer diameter ratio, di/do and the outer diameter to the flow channel diameter ratio, do/D on terminal velocity is reported for the specified range of angle of inclination, θ for several hollow frustums in a series of high viscous Newtonian fluids. The terminal velocity shows an increasing trend with increasing and decreasing do/D and di/do ratios, respectively. The terminal velocity also increases with increasing the angle of inclination, θ of the flow channel. Predictive equations are developed for the estimation of CD as a combined function of Re, di/do, do/D and θ. The statistical sensitivity analysis shows a large variation of the drag coefficient of the hollow frustum with the Reynolds number than with di/do and do/D ratios. The dependency of CDRe term on the fluid viscosity and θ are expressed in terms of the developed correlations. Linear variation of CD with 1/Re confirms that flow regime is laminar in the present study. The experimental drag coefficients are then predicted excellently through numerical approximation using Reynolds Stress Model (RSM) available in Ansys-15.

This paper presents a study of the terminal fall velocity and drag coefficient of natural particles of highly irregular shapes in a wide range of Reynolds number (0.01–3700). The study is based on experimental measurements of the terminal fall velocities of calcareous sand particles settling in liquids of different densities and viscosities. A total of 133 calcareous sand particles of highly irregular shapes were measured and 521 experimental data points were obtained. To characterize the particle shapes, a shape descriptor which can take into account both sphericity and inverse circularity, were introduced and it can be measured via image particle analysis techniques. This shape descriptor is particularly suitable for non-spherical and highly irregular particles, such as calcareous sand particles. The drag coefficient has been correlated to the particle Reynolds number and the shape descriptor, and a new model for predicting drag coefficient has been correlated; the correlation has the functional form of a power law. Its simple explicit form makes it easy to use in computer models and other applications. The performance of seven widely-used correlation laws has been examined; and the fitting of experimental measurements with the new correlation model shows that the new correlation model predicts the drag coefficients and the terminal fall velocities of irregularly-shaped particles, i.e., calcareous sand particles, more accurately than other shape-dependent drag laws. In addition, the influence of presence of neighboring particles on drag coefficient and terminal velocity was investigated under conditions where the test particle is surrounded by other equal-sized and similar-shaped particles arranged in different configurations. Furthermore, through proposing a new parameter which can consider the effects of the number of neighboring particle, particle separation distance and relative position of the particles, the determination of drag coefficient ratio for the settling of multiple particle was correlated based on the results of our settling tests. These findings provide a basis for further improvement for the general polydisperse particle system and analysis of flow through assemblage of numerous particles.

In the process of pomegranate arils pneumatic separation, the aerodynamic characteristics of pomegranate aril, rind and locular septa are essential. The main aim of this study was to measure and compare the aerodynamic characteristics of these materials to provide the data and to facilitate the design and adjustment of machines that perform separation of pomegranate arils from rind and locular septa based on aerodynamic characteristics (terminal velocity, drag coefficient and Reynolds number). To achieve this objective, Ashraf variety pomegranate fruit during its maturity stages was studied. The obtained results showed that the variation in maturity stage significantly influenced the terminal velocity, drag coefficient and Reynolds number (P

We investigate numerically the dynamics of freely rising gas bubbles driven by buoyancy at low Reynolds numbers, focusing on two distinct characteristics: the clustering morphology and the probability density functions (p.d.f.) of the velocity fluctuations for bubbles. A modified Volume-of-fluid (VOF) method is implemented in our direct numerical simulations to circumvent the nonphysical coalescence between bubbles. Four values of the gas fraction are studied: 5%, 10%, 15%, and 20%. The Eotvos number and the Galileo number are fixed at Eo = 2 and Ga = 855, respectively. For a single rising bubble, the Reynolds number according to the terminal velocity and the bubble diameter is around 27, displaying an ellipsoidal shape. For a swarm of bubbles, both irregular and regular clustering configurations are observed in our numerical simulations as we vary the gas fraction, which can only be realized at high Reynolds numbers in the previous experiments due to the low gas fraction limit. Moreover, we find that the p.d.f.s for bubble velocity fluctuations exhibit distinctly different behavior from the high Reynolds number experiments. The horizontal components for these p.d.f.s approach Gaussian distributions only when the gas fraction is high, i.e., α=15% and 20%. It suggests that the strong bubble–bubble interaction through their flow wakes is an efficient way to trigger the flow into turbulent states.

In this study, the sedimentation of a torus is studied numerically over a range of Reynolds number spanning 10 ≤ Re T ≤ 90. The three-dimensional governing equations are solved using the fluid–structure interaction version of the SVD-GFD method on a hybrid Cartesian-cum-meshless grid. The torus has a fixed aspect ratio AR = 2, and is set to fall along the negative x direction at an initial inclination angle θ 0 , in the range of 0 ≤ θ 0 ≤ 80°. The flow behavior induced by the falling torus is characterized by the far field vortex sheet and the near field recirculation zone. At a given Re T , the flow reaches the same terminal state and the torus maintains the same final state, orientation and terminal velocity, regardless of θ 0 . The torus initially experiences a zigzag motion (along the x and z directions) in the global frame and the pitch motion in its body frame, which are more obvious at larger Re T and θ 0 . The offset of the torus centroid in the z direction between the initial and terminal states increases with increasing Re T and θ 0 because of the θ 0 effect. The falling time ( T f ) for the torus traveling along the same distance in the x direction decreases with increasing θ 0 when Re T ≤ 50. However, when Re T > 50, T f initially increases and then decreases with increasing θ 0 , with the maximum T f occurring at θ 0 ≈ 60°. A lift-like force is generated in the positive x direction due to the translational torus motion in the z direction, which originates from the pressure. This force is most pronounced at θ 0 ≈ 60° for Re T > 50, which thus significantly decelerates the falling speed of the torus and increases T f .

The nonequilibrium properties and strong spatiotemporal long-range correlations in gas-solid suspensions were analyzed systematically by discrete particle simulation in periodic domains. The effective interphase slip velocity is significantly larger than the particle terminal velocity, the particle velocity distribution function is non-Maxwellian with a high-energy tail and the solid concentration distribution function is far from the discrete Poisson distribution, all of which manifest strong deviation from the equilibrium or homogeneous state in gas-solid flow. The solid concentration, gas and solid velocities are spatiotemporally correlated and show a clear scale-dependence. More importantly, all characteristic correlation lengths scale linearly with the size of simulation domain, although the correlation length of solid concentration is much smaller than those of gas and solid velocities. On the other hand, the characteristic correlation times at first increase with increasing domain size and then seem to approach to asymptotic values. Those results indicate that the heterogeneity and spatiotemporal long-range correlations should be considered properly in continuum theory of heterogeneous gas-solid flow.

The molecular nature of the drag force exerted by a fluid on a body is revealed from a fundamental perspective. To do this, the Newtonian and Brownian theories are employed to simulate the falling of a body with atomic structure in a liquid with the explicit and implicit presence of the fluid molecules, respectively. The body reaches a terminal velocity in the way predicted for macroscopic spherical bodies by the Stokes law. In this respect, the present approach shows the hydrodynamic limit for very small objects. The values of the viscosity are found in good agreement with the experimental data. The present formulation not only complements the continuous-medium approach, traditionally employed to study fluids, but also opens new ways in the investigation of fluids and their effects on bodies with different shapes, sizes, and compositions, due to the atomic nature of the simulations.

In this study, a two-dimensional fully coupled computational model is developed for simulation of proppant settlement in hydro-fractures with the use of the extended finite element framework. The porous domain is governed by the well-known $$(\mathbf{u}-p)$$ formulation, which consists of the momentum balance equation of the bulk, in conjunction with the momentum balance and continuity equations of the pore fluid. The hydro-fracture inflow is modeled as a 1D flow on the basis of the Darcy law, in which fracture permeability is incorporated by means of the cubic law. Contact constraints are elaborated to eliminate the overlap of fracture edges and the leak-off flow. Proppant settlement is conducted on the basis of Stokes’ law for particle terminal velocity, in which the effects of fracture walls, concentration, viscosity and bridging are incorporated into the model. A fixed-point algorithm is introduced to achieve the optimum combination for the proppant injection. Using the extended finite element method, the strong discontinuity in the displacement field due to crack body, as well as the weak discontinuity in the pressure field due to leakage, is included in the model with the use of the Heaviside and modified level set enrichment functions, respectively. The robustness and versatility of the proposed numerical algorithm in determining the optimum proppant injection is examined through several numerical simulations.

An analytical approach is developed to show how particles sparsely distributed in steady and laminar channel flows can be transported for long distances or conversely deposited inside the channel due to the relative importance between the flow-induced drag and gravity forces. More precisely, we establish a rather simple particle trajectory equation which demonstrates that when particles’ inertia is negligible, their behavior is characterized by the channel geometry and by a dimensionless number W that relates the ratio of the particles sedimentation terminal velocity to the flow mean velocity. The proposed particle trajectory equation is verified by comparing its predictions to particle tracking numerical simulations taking into account particle inertia and fully resolving equations. The equation is shown to be valid under the conditions that flow inertial effects are limited. Based on this trajectory equation, we build a regime diagram that can predict the behavior of particles entering closed channel flows. This diagram, by relating W to the ratio of the channel mean aperture to its total length, enables to forecast if the particles entering the flow will be either deposited or transported along the channel. The influence of the channel geometry on the particle behavior is then investigated by considering channels with straight and sinusoidal walls. In particular, the effect of the corrugation amplitude, of the asymmetry and of the phase lag between the walls on the extent of the transport and deposition zones is evaluated and verified against numerical experiments. Firstly, it is shown that the regime diagram for straight channels can be used for wavy channels with in-phase walls. Secondly, it is found that increasing the phase lag between the two walls and/or the walls corrugation amplitude leads to an increase of both the transport and sedimentation zones. Finally, it is demonstrated that increasing the lower wall corrugation amplitude relatively to the upper wall corrugation enhances particle transport.

Liquid metal is an important type of energy transport carrier in nuclear reactors, such as in accelerator-driven sub-critical systems, fusion reactors and spallation neutron source devices. It is necessary to conduct research for bubbles rising in a liquid metal under different magnetic field intensities. The Perspex container is positioned concentrically inside a transverse magnetic field, which provides a homogeneous DC longitudinal magnetic field that passes through the fluid district. The coils are supplied with maximum field strength of 1.97 T. The equivalent diameter of the bubble is 3.1–5.6 mm. The Ultrasonic Doppler Velocimetry (UDV) method is used to evaluate the internal flow velocity of opaque liquid metals. Research shows that the influence of the Lorenz force on the bubble ascension velocity is not simply positive or negative. The magnetic field inhibits the ascension velocity of small bubbles with diameters of 3.1 mm and 3.4 mm. The terminal velocity for large bubbles with diameters of 4.57 mm, 5.15 mm and 5.6 mm is higher under a weak magnetic field than without a magnetic field. The positive effect happens under strong magnetic intensity. The target is to obtain the hydro-dynamical relationships between the terminal velocity, drag coefficient, the Eotvos number, Reynolds number, and Stuart number in a strong magnetic field using a multiple regression method to reveal that the mechanism of the induced current's restraining influence determines the ascension velocity of the bubble in viscous electric liquids with a strong magnetic field.

In the buoyant rise of two bubbles arranged in an inline configuration, the trailing bubble tends to accelerate beyond its terminal velocity, due to its interaction with the wake of the preceding bubble. It is demonstrated that several different interactive behaviours could be obtained due to this acceleration. Firstly, at low values of Reynolds numbers (Re=ρlVtD/μ⩽35)(Re=ρlVtD/μ⩽35), the inline configuration was found to be stable, and the two bubbles would collide and coalesce due to the velocity difference between the two. Secondly, at higher Re values (Re > 50), vorticity development around the trailing bubble causes it to deviate away from the inline configuration, thus preventing the occurrence of a head-on collision between the two bubbles. The deviation of the bubble was found occur if a certain critical velocity is exceeded by the trailing bubble. The value of the critical velocity was found to decrease with increasing Re values. Further, an increase in Eotvos number (Eo=ρlgD2/σ)(Eo=ρlgD2/σ) tends to increase the critical velocity, indicating that enhanced bubble deformability actually improves the path stability of the trailing bubble. The interaction of bubbles can therefore significantly influence the tendency of the bubbles to collide and coalesce.

Magnets are attached to a falling object, which is allowed to fall through a conducting ring. A magnetic impulsive force produced by the ring reduces (very slightly) the velocity of the object. The impulsive force is fully characterized using a novel method for measuring the velocity and acceleration of the object, which allows for determination of the gravitational field strength, to within 0.2% of the published local value. The experimentally obtained impulsive force is well explained by the calculated Lorentz force acting on the ring.

The present study aims to investigate the characteristics of airflow inside the room through 5-blade square louver face ceiling diffuser using CFD. Several diffuser models are installed inside 3D room model by changing blade angles (60°, 65° and 45°) and lip angles (0°, 5°, 10° and 15°). Besides, the model with 60° blade angle and 0° lip angle is installed inside different room models with various return air inlet positions. The realizable k–e turbulence model is used in simulation process. The results indicated that the diffuser model with 65° blade angle provide jet and terminal air velocities beside airflow patterns cling with ceiling higher than the models 60° and 45°. Therefore, it is suitable to install in the space with larger size and relatively higher noise level to reaches to the comfort criteria faster than the other models to reduce the energy consumption. On the other hand the model with 45° blade angle is applicable to installed in the spaces with larger height due to airflow patterns out in cone shape. In addition, increasing lip angle to 15° leads to relatively reduce the cling of airflow patterns with room ceiling, and reach to comfort conditions with less energy consumption faster than 0° lip angle. Moreover, the room geometry model with two return air inlets on the ceiling with adequate size is produced ideal design of a fully mixed air distribution system with relatively constant temperature gradient.

The drag coefficient has a significant effect on the calculation of bubble motion. This paper experimentally investigated the drag coefficient of a single bubble rising in deionized water with the help of a high-speed video system. First, it is found that the terminal velocity presented periodical fluctuation, indicating that the drag coefficient is not a constant value. Then the measured drag coefficient was compared with correlations available in literatures. The comparison shows that these correlations cannot give fully satisfactory results in predicting the fluctuated drag coefficient. Based on massive data analysis, a new correlation combined Reynolds number, Eotvos number and Weber number was proposed to calculate the fluctuation of drag coefficient. The drag coefficient predicted by this new model is in excellent agreement with the experimental results over the range of 550 ⩽ Re ⩽ 1700, and the largest relative error is less than 0.4%. Comparing with data of water from existing literature under the condition of 8

We quantitatively investigated nanoparticle deposition (density: 1.0 g/cm3, diameter: 0.1–1.0 μm) in a pulmonary acinus using computational fluid dynamics. We assumed that acinar flow was induced by the expansion and contraction of the acinar model, with the volume changing sinusoidally with time as the boundary condition. We based the complicated acinar model and volume change on a mammalian lung, and evaluated the distribution of particle depositions through acinar generation based on three dimensional thinning. The maximum Reynolds numbers at the inlet and terminal alveoli were 0.191 and 1.93 × 10−3, respectively. With only gravitational force, particle deposition was strongly affected, except for the 0.1- and 0.2-μm particles, and the 0.8- and 1.0-μm particles were deposited around the inlet due to sedimentation after the beginning of inflation. We found that the 0.1- to 0.4-μm particles were deposited at the distal alveoli and that the deposition was enhanced by both the gravitational and Brownian forces, despite the small terminal velocity. For the 0.1- and 0.2-μm particles in particular, the particles were deposited throughout the acinus. The maximum deposition fraction of each acinar segment reached 1.2% after a single breath, and was higher in the middle generations and at the terminal alveoli.

The aerodynamic properties of crop grains influence the selection of the design and operational parameters of equipment. These properties are crop variety specific and moisture dependent. Measurement of terminal velocity is important to determine other aerodynamic properties. A terminal velocity measuring apparatus with vertical tapering rectangular air duct was developed which can measure velocity from 0 to 15 m/s. The sesame (Sesamum indicum Linn.) seeds of variety N-8 were dipped in water to obtain test samples of moisture contents 9.50, 13.46, 18.89, 24.17 and 28.99 per cent. The terminal velocity measured with the newly developed apparatus was found increased from 3.05 to 5.25 m/s with corresponding increase in moisture contents from 9.50 to 28.99 per cent (w.b.), respectively. In other aerodynamic properties, the values of drag co-efficient were found decreased from 2.9 to 0.6 and Reynolds number increased from 3939.9 to 11043.6 with corresponding increase in moisture contents from 9.50 to 28.99 per cent (w.b.), respectively. Simple linear regression equations were fitted to the obtained data to predict the aerodynamic properties of sesame seeds.

Mass transfer and terminal velocity of toluene drops rising in an extraction column was studied experimentally. The impact of packing was investigated by inserting a structured packing in three different heights of 10, 25 and 40 cm inside the column. The effect of packing height, flow rate and drop size on terminal velocity and Sherwood number were investigated and the results were also compared with the case of non-packed column. It was discussed that a 30% increase in Sherwood number could be achieved using the packing with 40 cm in height. It was also found that for Eotvos number, Eo, higher than 0.4, the deformation of drops and velocity oscillation appeared, which affected the terminal velocity. Finally, two correlations were proposed, which permitted the prediction of deformation and extraction ratio versus the dimensionless numbers of Reynolds, Eotvos and the packing height ratio.

New experiments of falling volcanic particles were performed in order to define terminal velocity models applicable in a wide range of Reynolds number Re. Experiments were carried out with fluids of various viscosities and with particles that cover a wide range of size, density and shape. Particle shape, which strongly influences fluid drag, was measured in 3D by High-resolution X-ray microtomography, by which sphericity Φ3D and fractal dimension D3D were obtained. They are easier to measure and less operator dependent than the 2D shape parameters used in previous papers. Drag laws that make use of the new 3D parameters were obtained by fitting particle data to the experiments, and single-equation terminal velocity models were derived. They work well both at high and low Re (3 × 10− 2 < Re < 104), while earlier formulations made use of different equations at different ranges of Re. The new drag laws are well suited for the modelling of particle transportation both in the eruptive column, where coarse and fine particles are present, and also in the distal part of the umbrella region, where fine ash is involved in the large-scale domains of atmospheric circulation. A table of the typical values of Φ3D and D3D of particles from known plinian, subplinian and ash plume eruptions is presented. Graphs of terminal velocity as a function of grain size are finally proposed as tools to help volcanologists and atmosphere scientists to model particle transportation of explosive eruptions.

Observation of clouds and precipitations with radars in millimeter waves is one of the most fruitful method to investigate those interior structures because the sensitivities for small particles and droplets are much better than those in longer wavelengths. We have developed the cloud profiling FMCA (Frequency Modulated Continuous Wave) 95 GHz Doppler Radar, named “FALCON-I” (Fig.1; FALCON=FMCW Radar for Cloud Observations), at Chiba University. FALCON-I consists of two 1-m diameter antennas and has a spatial resolution of 0.18 degree (which corresponds to 15m at the height of 5km) and ranging resolution of typically 50m. We make regular observations at zenith with the temporal interval of 10 seconds and make scanning observations with +/- 5-degree in one direction from the zenith with the interval of 15 seconds when we want to observe spatial extent of clouds. Fig.2 shows clouds and precipitations observed with FALCON-I on 15 August 2017 at Chiba University. Time-height intensity map from 00:00-02:00 UT (09:00-11:00 JST; Fig.2a) and Doppler profile map at 00:50 UT (Fig.2b) are presented. The intensity map shows that precipitations started around 00:35 UT at the height of 5 km. The Doppler profile map shows the cloud at 5-7km in height has Doppler velocities about 0 to -2 m/s, which negative velocity means downward motion. At the bottom of the cloud i.e. 5km in height, rain droplets with the Doppler velocities of up to -6 m/s are abruptly produced within a few hundred meters in height. Terminal velocity of 6 m/s corresponds to about 2 mm diameter droplets. Rain droplets were produced intermittently in this case and one of the gropes of droplets is marked with a solid line which just started falling below the cloud bottom as shown in Fig.2b. We can recognize several groups of droplets whose inclines in Fig.2b are getting steeper with falling down. We can derive droplet number distribution N(D) from the Doppler spectra by assuming the observed velocities are terminal velocities. Successive Doppler profile maps and movies from 00:30 to 01:00 UT show dynamic phenomena in the beginning phase of precipitations such as production and acceleration of rain droplets at the bottom of the cloud, evaporation of droplets, changes of number distributions during the falling way, and so on. These results of analysis show that observations of millimeter-wave Doppler radar FALCON-I are powerful methods to investigate micro processes in clouds and precipitations. Fig.1. Cloud Profiling Doppler Radar FALCON-I consists two 1-m antennas and observes clouds and precipitations at 95GHz with high spatial resolution of 0.18 degree. Fig.2. Clouds and precipitations observed with FALCON-I on 15 August 2017 at Chiba University. Time-height intensity map from 00:00-02:00 UT (a) and Doppler profile map at 00:50 UT (b) are presented. The intensity map shows that precipitations occurred around 00:35 UT at the height of 5 km. The Doppler profile map shows the cloud at 5-7km in height has Doppler velocities about 0 to -2 m/s, which negative velocity means downward motion. At the bottom of the cloud i.e. 5km in height, rain droplets with the Doppler velocities of up to -6 m/s are abruptly produced within a few hundred meters. Terminal velocity of 6 m/s corresponds to about 2 mm diameter droplets. Rain droplets were produced intermittently in this case and falling as shown in (b).

We propose a simple noninvasive technique called holographic deflection imaging (HDI) to measure electric charge on individual aerosol particles. Particles in many flows such as atmospheric turbulence can carry electric charges, which add forces that compete with turbulence and affect particle dynamics. To measure particle charges, we devised a novel technique for polydispersed particles with bipolar charge distributions. The technique uses common optical hardware and a novel particle sizing algorithm for polydispersed size distributions. A charged particle sample is introduced into a vertical channel air flow and a horizontal electric field. For each particle, In-line digital holography is used to simultaneously track particle positions over successive holograms and decode size information from hologram interference fringes. This enables measurement of the electric-force-induced horizontal component of terminal velocity and the particle size. From velocity and size, particle charge is calculated using a force balance, assuming Stokes drag on spherical particles. Two experiments were conducted, one to validate HDI measurements of a bipolarly charged particle sample against an electrometer, and the other to demonstrate particle size effect on triboelectric charging of aerosols in a fan-driven isotropic turbulence chamber. The HDI setup was configured for each experiment, and measurement uncertainty was estimated. Experiment 1 found good agreement (

International audience; The current article presents a regularization procedure of the Lagrangian point-force approach commonly used to account for the perturbation of a fluid phase by a dispersed particle phase. The regularization procedure is based on a nonlinear diffusion equation to naturally ensure parallel efficiency when the regularization length scale extends over several grid cells. The diffusion coefficient thus becomes a function of the particle source term gradient and expressions allowing to approximately adjust the regularization length scale according to the local particle to mesh size ratio are proposed, so that mesh refinement or polydisperse sprays may be handled. Elementary numerical test cases confirm the convergence of the present procedure under mesh refinement and its ability to locally adapt the regularization length scale. Furthermore, the chosen regularization length scale allows to match the leading order term of the perturbation flow field set by the particle beyond approximately two particle diameters in the Stokes regime. When applying the presented source term regularization procedure, the terminal velocity of a particle settling under gravity in the Stokes regime becomes relatively insensitive to mesh refinement. However, errors with respect to the theoretical settling velocity remain substantial and removal of the particle's self induced velocity appears necessary to recover the undisturbed fluid velocity at the particle location and correctly evaluate the drag force. As the current regularization procedure yields source terms that are close to c 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Gaussian, an analytic expression from the literature is used to estimate the particle's self induced velocity. When combining source term regularization and removal of the particle's self induced velocity, good results are obtained for the terminal settling speed in the Stokes regime. Results obtained for horizontally separated particle pairs settling under gravity in the Stokes regime show equally good agreement with theoretical results. Because analytic expressions for the particle's self-induced velocity are no longer available at finite particle Reynolds numbers, correlations recently proposed in the literature are used to obtain correct settling velocities beyond the Stokes regime.

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 $$\phi =0.5{-}10\%$$ , 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 $$Re_{\lambda } \simeq 90$$ , and the ratio between the particle diameter and the nominal Kolmogorov scale is $$(2a)/\eta \simeq 12$$ (being a the particle radius). We find that in HIT the mean settling speed is less than that in quiescent fluid for all $$\phi $$ . For $$\phi =0.5\%$$ , 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 $$\phi =10\%$$ . Indeed, while at low $$\phi $$ the settling speed is strongly altered by the interaction with turbulence, at large $$\phi $$ 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 $$\phi $$ . These are enhanced by the interaction with turbulence, especially at low $$\phi $$ . 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 $$\phi $$ . Finally, we investigate the collision statistics for all cases and find the interesting result that the collision frequency is larger in quiescent fluid than in HIT for $$\phi =0.5{-}1\%$$ . This is due to frequent drafting–kissing–tumbling events in quiescent fluid. The collision frequency becomes instead larger in HIT than in still fluid for $$\phi =5{-}10\%$$ , due to the larger relative approaching velocities in HIT, and to the less intense drafting–kissing–tumbling events in quiescent fluid. The collision frequency also appears to be almost proportional to the estimate for small inertial particles uniformly distributed in space, though much smaller. Concerning the turbulence modulation, we find that the mean energy dissipation increases almost linearly with $$\phi $$ , leading to a large reduction of $$Re_{\lambda }$$ .

With this comment we want to clarify a number of aspects of the paper recently published by Dioguardi, Mele, and Dellino “A New One-Equation Model of Fluid Drag for Irregularly Shaped Particles Valid Over a Wide Range of Reynolds Number” (hereafter referred to as DMD2018). In particular, we show that contrary to the conclusions of DMD2018, the model of Bagheri and Bonadonna (2016, https://doi.org/10.1016/j.powtec.2016.06.015), hereafter referred to as BB2016, is the best model in predicting the drag and terminal velocity of particles measured by DMD2018, as demonstrated here by comparison of estimation errors. The discrepancy is mainly due to a production error (misplaced parentheses) introduced in BB2016 during the publication process and partly due to the incorrect methodology used by DMD2018 to calculate particle terminal velocity. Here we present the correct sets of equations and methodology to show that typo-free model of BB2016 outperforms all existing drag models including the new model suggested by DMD2018.

The settling process and wall impact of large spherical particles in a stagnant, highly viscous fluid has been observed by means of high-speed shadow imaging. The particles included in this study vary in size and material properties: steel, polytetrafluorethylen (PTFE), polyoxymethylen (POM), or rubber. The corresponding terminal Reynolds numbers range from 333 to 4012, covering in principle the transitional and Newton regime for drag forces. However, most particles do not reach the terminal velocity before colliding with the impact object. Therefore, the main focus of this study is set on particle settling and collision in the transitional regime. For collision studies, the Stokes number just before impact is also relevant, and lies in the range 50 St 2250 . The settling curves obtained experimentally (characterized by vertical position and vertical velocity component) are compared with numerical and analytical solutions. The latter has been derived on the basis of nominal terminal velocity and relaxation time for the Stokes and Newton regimes. The numerical model takes into account the side walls and the corresponding correction of the drag coefficient. A deviation between experimental results and analytical solution was observed in all cases where the terminal Reynolds number is larger than 300 and smaller than 1100. It appears that, in this flow regime, the settling process of the spheres is already affected at a long distance from the impact object, leading to an early but significant deceleration. Moreover, a reduced settling velocity was observed along the whole trajectory for the PTFE particles with the lowest terminal Reynolds number. All these effects are captured in the numerical model and the corresponding results agree fairly well with the experiments. There is one exception, induced by particle rotation, which is not considered in the current model. In that case, it is not possible to correctly predict the settling process. All processed datasets are available via the Mendeley Data repository Hagemeier (2020). Two additional effects have been observed during this study. First, a bright region was detected around all PTFE spheres. It finally was found to be due to total light reflection around the sphere, but, to the best of our knowledge, this peculiarity has never been reported before. Being purely optical, this does not effect the settling behavior for PTFE. On the other hand, partial absorption of liquid at the particle surface was observed for rubber, leading to a reduced sedimentation velocity. This property, already documented for homogeneous porous particles, is found here for a rubber particle with almost impermeable core.