Particle Volume Fraction Research Articles

Electromagnetic non-orthognal flow of variable viscosity nanofluid over a rotating riga disk with prescribed thermal slip

Abstract This paper investigates the complex dynamics of a 3D oblique magneto hydrodynamic (MHD) flow of Ag-water Nanofluid across a rotating Riga disk. Temperature dependent viscosity along with prescribed momentum and thermal slip on the surface of rotating Riga disk are taken into account. Permanent magnets are attached to a flat surface of the electrode array on Riga plate, which is oriented span-wise. This configuration produces an exponentially decaying Lorentz force parallel to the array. The interaction of magnetic fields, rotational effects, and boundary slip phenomena in a three-dimensional flow is the main topic of the study. The study uses the Shooting algorithm to investigate the effects of the Riga disk's rotation and the presence of a magnetic field on heat transfer and flow characteristics. Furthermore, the addition of momentum and thermal slip boundary conditions sheds more light on differences between realistic situations and standard no-slip models. With implications for better design and optimization in engineering systems involving rotating disks and magnetic fields, this work advances our understanding of complex fluid dynamics in MHD applications. Velocity profiles f ′ ( η ) and g(η) drops down while enhancing values of nano particle volume fraction ϕ whereas g(η) reduces by ehancement of Hartman number Ha and velocity slip parameter ω 1. The temperature θ(η) rises with increasing thermal slip ω 2 velocity slip parameter ω 1 and Hartmann number Ha. Additionally, it increases with the volume fraction of solid nanoparticles ϕ and the width parameter δ. As width parameter δ increases, the local heat flux − K n f K f . θ ′ ( 0 ) decreases. Skin friction coefficient f″(0) increases with viscosity parameter m and decreases with variations in thermal slip parameter ω 2. 3D flow patterns exhibits the significant influence of momentum slip parameter ω 1 on the location of stagnation point.

Influence of matrix stiffness on microstructure evolution and magnetization of magneto-active elastomers.

Field-induced microstructure evolution can play an important role in defining the coupled magneto-mechanical response of Magneto-Active Elastomers (MAEs). The behavior of these materials is classically modeled using mechanical, magnetic and coupled magneto-mechanical contributions to their free energy function. If the MAE sample is fully clamped so it cannot deform, the mechanical coupling is reduced to the internal microscopic deformations caused by the particles moving and deforming the elastic medium that surrounds them. In the present study, we build on a unified mean-field theoretical approach which takes the microscopic elastic energy into account. Combined with experiment, this approach reveals how microstructure evolution affects the magnetization behavior of isotropic MAEs. MAE disks with various matrix stiffness and volume fraction of particles were fabricated and the magnetization curves were measured by vibrating sample magnetometry. We demonstrate that the idea of columnar structures forming from randomly distributed particles upon the application of an external magnetic field provides an effective approach in modeling microstructure evolution in these materials. Our unified mean-field model, using few and physically meaningful parameters, shows good quantitative agreement with the experimental data on magnetization and magnetic differential susceptibility of MAE samples. More importantly, our model can estimate microstructure evolution in highly filled samples, for which measurements are very challenging. Since changes in magnetization and stiffness are both driven by microstructural evolution, a quantitative relationship can be established between the two effects, as they represent different macroscopic manifestations of the same microscopic process. Therefore, our model can be used in conjunction with magnetization measurements to predict the mechanical modulus of MAEs without the need for elastic testing.

Nano‐Silica‐Particles Filling Effect on Mechanical Properties of Silicone‐Rubber Composites

ABSTRACTThe mechanical properties of silicone‐rubber composites filled with nano‐ or micro‐silica‐particles are discussed experimentally and theoretically. The matrix rubbers cured under various ratios of the main component to the curing agent are prepared to change the crosslinking density of the matrix rubber. Because the interphase layer made of the matrix rubber in the glassy state is formed around the nano‐particle, the nano‐particle behaves as an apparent single particle together with the interphase layer and the interphase layers apparently increase the nano‐particle volume fraction. The Young's moduli of the nano‐ and micro‐composites with a larger (apparent) particle volume fraction are greater than that of the matrix rubber. However, the proportional limit strains are smaller, although the fracture strains are slightly smaller. The Young's modulus and proportional limit strain can be analyzed theoretically with the (apparent) volume fraction. The fracture toughness of the composites increases as the (apparent) volume fraction increases, but it increases little at the (apparent) volume fraction above 0.2. However, the fracture toughness monotonically increases for the nano‐composites with low crosslinking density of the matrix rubber. Finally, the mechanical properties can be designed by using the matrix rubber with the different crosslinking density and the interphase layer.

Microstructure response of concentrated suspensions to flow reversal.

We study experimentally at the macroscopic and microstructure scale a dense suspension of non-Brownian neutrally buoyant spherical particles experiencing periodic reversals of flow at constant rate between parallel plates and tracked individually. We first characterize the quasi-steady state reached at the end of half periods. The volume fraction of particles increases from the walls to the center as a result of migration induced by the nonuniform strain rate. Except very close to the walls and the center, the particle pair distribution is fore-aft asymmetric with depletions of pairs in the extensional quadrants, similar to that reported for shear flows of same volume fraction as the local one. The dynamics of the periodic rearrangements occurring after each flow reversal are characterized by a microstructure tensor component. The relaxation time characterizing the reorganization increases from the walls to the center due to the inhomogeneous strain rate. On the other hand, the local accumulated strain required for this reorganization decreases with the volume fraction, like for viscosity measurements in uniform strain rate conditions. However, the variation of the microstructure with the accumulated strain is faster than that of the viscosity, showing the complementarity of the two measurements.

Effective Thermal Conductivity of Nanofluids Containing Silicon Dioxide, Titanium Dioxide, Copper Oxide, Polystyrene, or Polymethylmethacrylate Nanoparticles Dispersed in Water, Ethylene Glycol, or Glycerol

The present study represents a continuation of our investigations on the effective thermal conductivity λeff of nanofluids by systematically varying the types of base fluids and particles. For the spherical nanoparticles with mean diameters between (20 and 175) nm, the metal oxides silicon dioxide (SiO2), titanium dioxide (TiO2), and copper oxide (CuO) as well as the polymers polystyrene (PS) and polymethylmethacrylate (PMMA) were selected to cover a broad range for the particle thermal conductivity λp from about (0.1 to 30) W⋅m–1⋅K–1. The corresponding polar base fluids water, ethylene glycol, and glycerol allow to not only vary their thermal conductivity λbf by a factor of more than two, but also their dynamic viscosity by about three orders of magnitude. For the measurement of λeff of the twelve different particle–fluid combinations, i.e., TiO2 or CuO with all three liquids as well as SiO2, PS, or PMMA with water or ethylene glycol, a steady-state guarded parallel-plate instrument (GPPI) associated with an expanded (k = 2) relative uncertainty between 0.022 and 0.032 was used at atmospheric pressure over a temperature range from (283 to 358) K at varying particle volume fractions up to 0.31. The results for the thermal-conductivity ratio λeff·λbf–1 are independent of temperature and show a moderate and relatively linear change as a function of the particle volume fraction. For similar ratios λp·λbf–1, the experimental data for λeff·λbf–1 are also very similar, which are above, close to, or below 1 if λp is larger than, comparable to, or smaller than λbf, respectively. For all nanofluids investigated, the Hamilton–Crosser model can describe the present measurement results and reliable experimental data reported in the literature for λeff·λbf–1 typically within ± 5 %. Overall, the measurement results from this work contribute to an extension of the database for λeff of nanofluids with respect to the investigated wide ranges of systems, temperature, and particle volume fraction.

Auto-Deposited Microparticle Composite Coating for Low-Cost and Efficient Daytime Radiative Cooling.

Radiative cooling is an excellent strategy for mitigating global warming, by enhancing heat fluxes away from the Earth, thus balancing the Earth's heat flow. However, for randomly particle-dispersed radiative cooling materials, the particle content as high as 94-96 wt % or 60 vol %, far exceeds the critical pigment percentage (40-50%) of traditional coatings, preventing its large-scale application. Here, inspired by particle deposition under gravity in solution, we demonstrate an auto-deposited SiO2 composite radiative cooling coating (ADRC) which reduces the amounts of particles required and lowers costs. This particle density gradient structure enhances the local volume fraction of particles, thereby the coating exhibits high solar reflectance (93%) and infrared emissivity (89%), contributing to a daytime subambient temperature drop of 12 °C. The cooling energy-saving potential of buildings in China utilizing the ADRC as roofs ranges from 6.42 to 13.52%. This low-cost and scalable radiative cooler is expected to reduce energy consumption and carbon emissions, addressing global warming issues.

NIR-Reflective Black Photonic Films Designed for Effective LiDAR Recognition.

Conventional dark-tone paints absorb both visible light and near-infrared (NIR) wavelengths, posing a challenge for light detection and ranging (LiDAR) recognition in autonomous driving. To overcome this issue, various chemical and structural coating materials have been explored to selectively reflect NIR. In this study, we newly propose colloidal photonic crystals with a stopband in the NIR range, fabricated through the spontaneous formation of crystalline arrays of silica particles dispersed in a photocurable resin, as a potential solution. The stopband position is finely adjusted by controlling the volume fraction of particles, and the translucent films are rendered black by incorporating carbon black nanoparticles into the interstitial regions. By optimizing the concentration of carbon black, we achieve a balance of low visible light and high LiDAR intensity. These black photonic films demonstrate superior LiDAR intensity compared to commercial dark-tone paints and perform on par with the perylene black-deposited white coating, one of the most promising LiDAR-assistive materials currently available, under normal reflection conditions. Additionally, the photonic films are highly durable, thermally stable, nontoxic, and environmentally friendly, making them promising candidates for LiDAR-assistive coatings.

Experimental Investigation of Dispersant on Dynamics of Impact of Al2O3 Nanofluid Droplet.

Spray cooling, of which the essence is droplet impacting, is an efficient thermal management technique for dense electronic components in unmanned aerial vehicles (UAVs). Nanofluids are pointed as promising cooling dispersions. Since the nanofluids are unstable, a dispersant could be added to the fluid. However, the added dispersant may influence the droplet, thereby impacting behaviors. In this work, the effects of dispersant on the nanofluid droplet-impacting dynamics are studied experimentally. The base fluid is deionized water (DI water), and Al2O3 is the selected nanoparticle. Sodium dodecyl sulfate (SDS) is used as the dispersant. Five different concentrations of nanofluids are configured using a two-step method. Droplet impacting behaviors are observed by high-speed imaging techniques. The other effects, i.e., the nanofluid particle volume fraction and the Weber number on droplet impact dynamics, are also systematically investigated. The results illustrate that the surface tension of the Al2O3 nanofluid increases with increased nanofluid concentrations. The surface tension of Al2O3 nanofluid with SDS is lower than that of DI water. And the increase in droplet impact velocity increases the spreading morphology. Nanofluid droplets exhibit spreading and equilibrium process when SDS is added. Furthermore, as the concentration of the nanofluid increases, the spreading process is inhibited. Whereas without SDS, the droplets undergo spreading, receding, and equilibrium processes. Moreover, there is no appreciable change in the impacting process with concentration increase. The empirical models of maximum spreading factor should be established without SDS and with SDS, respectively. This study can provide theoretical basis and specific guidance for experimental characterization of UAVs' electronic devices based on the mechanism of nanofluid droplet impact on the wall.

Full-Spectrum Mechanochromic Photonic Films with Large Interparticle Distance.

Non-close-packed crystalline arrays of colloidal particles in an elastic matrix exhibit mechanochromism. However, small interparticle distances often limit the range of reversible color shifts and reduce reflectivity during a blueshift. A straightforward, reproducible strategy using matrix swelling to increase interparticle distance and improve mechanochromic performance is presented. Photonic composites are initially prepared with silica particle arrays embedded in an elastomer matrix at volume fractions of 0.35-0.5. To increase interparticle distance, the composites are immersed in an elastomer-forming monomer, causing the matrix to swell, followed by photopolymerization, thereby producing liquid-free composites. The degree of swelling is controllable up to 3.16, depending on monomer choice, matrix volume fraction, and crosslinking density. The process can be repeated to further increase swelling up to 10.36. This method can reduce the volume fraction of silica particles from 40% to 3.8%, while interparticle distance increases from 53 to 257nm. The swollen photonic composites exhibit a full visible spectrum under compression, while minimizing reflectivity loss. This allows red-colored photonic composites to be transformed into vivid multicolor patterns when compressed with stamps featuring spatial height variations.

A new three-dimensional microscopic modeling strategy in studying coupled electro-mechanical properties for piezoelectric composites

In this study, an innovative micromechanical modeling framework based on the three-dimensional finite-volume direct averaging micromechanics theory is presented to investigate mechanical properties of 0–3 type piezoelectric composites subjected to a coupled electro-mechanical environment. To this end, the electro-mechanical coupling constitutive relation is presented. Moreover, the parametric elements are further constructed to improve the calculation precision. In order to verify the proposed micromechanical model, the numerical results of local stress and electric field distribution are also obtained by finite element method. The convergence and calculation accuracy of the proposed model are demonstrated by excellent consistency of the numerical results. On this basis, The elastic modulus, piezoelectric coupling coefficient and dielectric permittivity coefficient are obtained by three-dimensional FVDAM model at different particle volume fractions. By applying different loading conditions, the local stress and electric field intensity distributions on the interface between particle and matrix are investigated. It can be concluded that the proposed micromechanical model offers a useful tool in studying the effective properties and local responses of the piezoelectric composites.

Particle Sedimentation in a Fluid at Low Reynolds Number: A Generalization of Hindered Settling Described by a Two‐Phase Continuum Model

AbstractParticle‐fluid separation by settling is a ubiquitous process in Earth and Planetary Sciences. The settling velocity of particles is controlled by a balance between buoyancy and drag forces. Since the seminal work of George Gabriel Stokes, parameterizations for the reduction of particle velocities caused by viscous dissipation due to mutual interactions have been described by a non‐linear mapping between particle volume fraction and separation velocity. We argue that these parameterizations neglect important physical behavior at high particle volume fractions (>80% of the maximum packing) and are only appropriate when considering suspensions where the particle volume fraction does not evolve in space or time. We introduce a more general model that accounts for the energy dissipation caused by changes in local particle volume fraction, which introduces a new term similar to the compaction term at higher particle volume fraction. This term depends on a consolidation/compaction viscosity that measures the resistance to changes in solid volume fraction. We derive closure equations for this compaction viscosity under dilute and concentrated particle volume fraction limits. Numerical simulations show that the extended hindered settling model predicts two significant differences compared to traditional hindered settling. First, while the steepening of particle volume fraction fronts remains, a dynamic instability is also generated at the front. Second, the rate of growth and structure of a cumulate layer growing above a no‐flux boundary is affected by the compaction‐like term and predicts the trapping of a higher volume fraction of interstitial melt in a correspondingly thicker cumulate layer.

Flow of SWCNT and MWCNT based hybrid nanofluids in a semi-circular enclosure with corrugated wall

Heat transfer mechanisms participate to fulfill the necessities of modern technologies. The phenomenon of heat transport has implementations in multiple fields including metal working, heating systems, thermal management in spacecraft, solar energy, and automobile engines. In the current novel work, heat transfer mechanism in a semi-circular enclosure with corrugated circular wall is studied numerically. A hybrid nanofluid consisting of water as the base fluid and solid particles of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) is considered. Prevailing mathematical equations are explained by finite element method with the help of COMSOL Multiphysics. The study examines various volume fractions of solid particles over a broad range. The SWCNT volume fraction is adjusted from 0.02% to 0.1%, while the MWCNT volume fraction ranged from 0.01% to 0.04%. Velocity, temperature, and pressure contours are imagined. Local and average Nusselt numbers are studied for each of the cases. The results indicate that heat transfer in the semi-circular enclosure improves with higher volume fractions of solid particles. As the volume fraction of solid particles increases, the average Nusselt number over the heated surface decreases.

COMPARATIVE EXPERIMENTAL INVESTIGATION ON VISCOSITY AND STABILITY OF W/EG-BASED NON-NEWTONIAN HYBRID NANOFLUIDS

This research explores the stability and rheological characteristics of hybrid nanofluids made from water-ethylene glycol (W/EG) and incorporating nanoparticles such as SiC, Al<sub>2</sub>O<sub>3</sub>, and multi-walled carbon nanotubes (MWCNT). The preparation involved a two-step method, and the nanoparticles were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Stability assessment showed that Al<sub>2</sub>O<sub>3</sub>-MWCNT hybrid nanofluids are optically more stable than SiC-MWCNT as W/EG-based Al<sub>2</sub>O<sub>3</sub>-MWCNT hybrid nanofluids took longer to sediment. Al<sub>2</sub>O<sub>3</sub>-MWCNT hybrid nanofluids exhibited superior stability in visual tests over a period of 19-21 days while SiC-MWCNT nanofluid took 12-14 days to sediment. The rheological analysis revealed that higher particle concentrations resulted in increased viscosity, with SiC-MWCNT and Al<sub>2</sub>O<sub>3</sub>-MWCNT hybrid nanofluids showing viscosity increases of 3.56 and 3.98 times, respectively, in comparison to the base fluid. Conversely, raising the temperature from 25°C to 55°C led to a decrease in shear stress, with reductions of 72.8% and 64.8% observed for SiC-MWCNT and Al<sub>2</sub>O<sub>3</sub>-MWCNT hybrid nanofluids, respectively. Furthermore, the viscosity versus shear rate trends indicated a pseudoplastic or shear-thinning nature for both hybrid nanofluids with particle volume fraction above or equal to 0.1%.

Low-Velocity Impact Behaviors of B4C/SiC Hybrid Ceramic Reinforced Al6061 Based Composites: An Experimental and Numerical Study

This study studied the low-velocity impact behavior of silicon carbide (SiC), boron carbide (B4C), and hybrid B4C/SiC reinforced aluminum 6061 alloy (Al 6061) matrix composites experimentally and numerically. The effect of hybridization, reinforcement volume fraction, and reinforcement type on low-velocity impact was investigated. 16 different metal-matrix composite materials were manufactured for the study, including 9 hybrid samples, 6 non-hybrid samples for each reinforcing ceramic particle, and 1 unreinforced sample. The powder metallurgy - hot press method was used in experimental production and then low-velocity impact tests were carried out. Numerical study was carried out using the non-linear finite element method (FEM). A Python algorithm running on ABAQUS/Explicit was utilized to determine ceramic particle distribution based on volume fraction, hybridization ratio, and random particle arrangement. The Johnson-Cook Plasticity Model was used for the non-linear relationship between stress and strain in the Al 6061 metal-matrix during impact. The results were interpreted with the contact force-time, contact force-displacement, and energy-time data obtained from low-velocity impact tests. In addition, the amount of collapse that occurred during the impact was measured and compared, and finally, SEM/EDX analysis was performed for the microstructure characterizations of the composite sample. Impact tests revealed that the collapse of composite samples ranged from 3.06mm to 3.58mm, with the unreinforced S6 sample collapsing 4.18mm, indicating that increased reinforcement elements reduce ductility, while a lower B4C ratio or higher SiC ratio in hybrid composites leads to greater collapse. Keeping the hybridization rate constant while increasing the reinforcement ratio leads to an increase in contact force and a decrease in contact time. Hybrid composites exhibited higher stiffness as SiC content increased, while B4C reinforced non-hybrid composites showed more significant stiffness. There is good agreement between the numerical and experimental results. Non-hybrid samples have a better match between numerical predictions and experimental results than hybrid samples.

Computational analysis of the effect of interaction heterogeneity on fluid-crystal coexistence in micron-scale colloidal systems.

Micron-scale colloidal particles with short-ranged attractions, e.g., colloids functionalized with single-stranded DNA oligomers, have emerged as a powerful platform for studying colloidal self-assembly phenomena with the long-term goal of identifying routes for metamaterial fabrication. Although these systems have been investigated extensively both experimentally and computationally, the role of "real world" features that may impact self-assembly in unexpected ways has been largely ignored. One such example of an important, yet underappreciated, feature is interaction heterogeneity (IH), i.e., variations in interparticle interaction strengths, which can arise from variability in the DNA strand areal density on particle surfaces during fabrication. A previous study demonstrated that IH can modulate nucleation and gelation kinetics under non-equilibrium conditions. Here, we investigate in detail the dependence of bulk fluid-crystal coexistence on IH. Using a multicomponent coexistence tracing approach, we compute phase diagrams for both Gaussian and bidisperse IH distributions, revealing that IH shifts the fluid-side coexistence boundaries outward, promoting crystallization at lower particle volume fractions while also resulting in crystals that are enhanced in the stronger binding species. Our results demonstrate that IH significantly influences crystallization behavior even under equilibrium conditions and provide a new perspective on tuning IH as a controllable parameter for optimizing colloidal self-assembly.

Beyond the Average: Computation of Vertical Profiles in Dilute Pyroclastic Density Currents and Their Use in Shallow‐Water Models

AbstractPyroclastic density currents (PDCs) present significant hazards due to their high temperatures and dynamic pressures. Accurate estimation of dynamic pressure, vital for assessing potential damage, requires knowledge of the vertical variations of velocity and particle concentration within the PDC, particularly in the first few meters of the flow above the ground. Existing approaches to dynamic pressure calculations used in hazard assessment are often based on average values for velocity and particle volume fraction. These average values may misrepresent the flow dynamics, especially near the base of the flow where the gradients of flow variables are larger. Here, we present a new, physically based approach that allows for the calculation of the vertical profiles of velocity and concentration from a combination of depth‐averaged values for these properties and non‐dimensional flow parameters. Finally, we demonstrate the use of these profiles within an existing shallow‐water model and show its potential applications toward probabilistic hazard assessment.

Influence of Aging Treatment and Volume Fraction on Nano-Indentation Behavior of Ni-Based Single Crystal Superalloys.

The effects of aging treatment and the volume fraction of precipitation particles on the nano-hardness and nano-indentation morphology of Ni-based single crystal superalloys are systematically investigated. Using nano-indentation tests and atomic force microscopy (AFM), this study examined the mechanical properties and related physical mechanisms of Ni-based superalloys that have two volume fractions of precipitation particles and four aging treatment times. Results analyzed using the Oliver-Pharr method indicate that prolonging the aging time or increasing the volume fraction of particles enhances the nano-hardness and creep resistance of Ni-based single crystal superalloys and reduces the indentation-affected area. Additionally, the nano-hardness and elastic modulus decrease gradually with increasing applied force, revealing an obvious indentation size effect. These variations are closely linked to the size and density of particles and work hardening rate, as well as to the topologically close-packed (TCP) phases, which influence dislocation movement and accumulation within the material and lead to various nano-indentation behavior in Ni-based single crystal superalloys. The related study provides theoretical guidance and experimental data to support the design and application of superalloys.

Hardness Degradation Behaviors of Ni-Base Superalloys after Phase Coarsening

Ni-based superalloys are commonly utilized in the production of hot-end components in the aerospace, energy, and other sectors. Due to the extreme environment, phase coarsening is prone to occur within the alloys, leading to degradation in the mechanical properties and thus posing a risk to equipment safety. To investigate the influences of volume fraction of precipitation particles and phase coarsening on hardness of materials, and explore the relationship between their microstructure and macroscopic hardness, based on the finite element method simulations, a two-dimensional Vickers hardness model is constructed for testing. Results show that hardness is closely linked to the particle shape, size, and distribution features; with the increase of the volume fraction of particles or the degree of phase coarsening, the hardness of alloys gradually weakens; and when the volume fraction of particles is larger, the decrease in the hardness is more obvious along with the degree of phase coarsening. Related research makes a good reference for the design of Ni-base superalloy structures as well as the evaluation or prediction of their mechanical properties during the service.

Experimental and Numerical Investigations of the Sediment Abrasion Mechanism at the Leading Edge of an Airfoil

Multiple engineering projects have confirmed that hydraulic machinery operating in sediment-laden rivers undergoes sediment abrasion. Guide vanes are among the most severely worn flow-passing components and have long been a key research focus in hydraulic machinery. In this research, a wear test of the NACA0012 cascade under a 10° incoming flow angle was carried out in the Venturi test system, and the evolution process of the wear was analyzed. The three-dimensional flow channel of the cascade was constructed, and the Finnie wear model was adopted for computational fluid dynamics (CFD) simulations to analyze the wear mechanism at the initial stage. The results indicate that abrasion primarily occurs at the airfoil’s leading edge and progresses through three stages: initiation, development, and stabilization. The calculated results closely matched the latest wear outcomes: In the initial stage, the wear rate density was influenced by the particle impact velocity, angle, volume fraction, and y-direction shear stress. A low-velocity zone near the impact point, combined with rebounding particles causing secondary impacts, increases the particle volume fraction and wear rate density. These secondary impacts are the primary causes of erosion on both the upstream and downstream surfaces. Furthermore, flow separation downstream from the leading edge makes this region highly susceptible to wear. This study provides valuable insights for addressing wear in hydraulic machinery for practical engineering applications.

Prolate spheroids settling in a quiescent fluid: clustering, microstructures and collisions

In this study we investigate the sedimentation of prolate spheroids in a quiescent fluid by means of the particle-resolved direct numerical simulation. With the increase of the particle volume fraction $\phi$ from $0.1\,\%$ to $10\,\%$ , we observe a non-monotonic variation of the mean settling velocity of particles, $\langle V_s \rangle$ . By virtue of the Voronoi analysis, we find that the degree of particle clustering is highest when $\langle V_s \rangle$ reaches the local maximum at $\phi =1\,\%$ . Under the swarm effect, clustered particles are found to preferentially sample downward fluid flows in the wake regions, leading to the enhancement of the settling speed. As for lower or higher volume fractions, the tendency of particle clustering and the preferential sampling of downward flows are attenuated. The hindrance effect becomes predominant when the volume fraction exceeds 5 % and reduces $\langle V_s \rangle$ to less than the isolated settling velocity. Particle orientation plays a minor role in the mean settling velocity, although individual prolate particles still tend to settle faster in suspensions when they deviate more from the broad-side-on alignment. Moreover, we also demonstrate that particles are prone to form column-like microstructures in dilute suspensions under the effect of wake-induced hydrodynamic attractions. The radial distribution function is higher at a lower volume fraction. As a result, the collision rate scaled by the particle number density decreases with the increasing volume fraction. By contrast, as another contribution to the particle collision rate, the relative radial velocity for nearby particles shows a minor degree of variation due to the lubrication effect.