Fluid-particle Interaction Research Articles

Synthesis and characterization of polyvalent high-performance synthetic base oil for drilling operations in recalcitrant and unconventional oil and gas reservoirs

Synthetic ester fluid is one of the most widely recommended drilling fluids for unconventional and recalcitrant reservoirs owing to its numerous technical advantages, high sustainability, non-toxic and inherently biodegradable. However, there are serious concerns about its thermal and hydrolytic stability at high temperatures and pressures (HTHP) as well as its viscosity and flow characteristics in low temperature climates such as deep-water wells. Hence, its inability to maintain stable borehole and flat rheology characteristics may invariably threaten its application under these aforementioned scenarios.In this study, a high-performance ternary mixture of synthetic ethyl esters of plants oil (SEEP mixture) fluid was synthesized by a non-catalyzed supercritical ethanol transesterification process using coconut, castor seed and linseed oil as starting materials. The nature, chemistry and fluid particle interactions of the individual components that make up the synthetic ester fluid were critically studied. The process equipment consists of a continuous flow reactor equipped with a PID controlled furnace as heat source to the reactor and a high- pressure transducer. The conversion efficiency was monitored by an Agilent 5500a FTIR machine with attenuated total reflectance (ATR) system while a GC-EIMSD system was used for fatty acid ester compositional analysis. The oil was extracted by a Soxhlet apparatus mechanism. The extracted Plant oil and the synthetic esters were characterized under standard recommended laboratory procedures by API, ASTM and the European Union (EN) standard protocols. Also, fluid's toxicity and biodegradability tests were conducted according to OECD recommended practices. The results were compared with existing synthetic hydrocarbon fluids (SHFs) for HTHP and deep-water wells.The conversion efficiency of the non-catalyzed transesterification process was 96.50%. The amount of oil yield from the plant oil after Soxhlet extraction is commendable. Hence, synthetic ester fluid raw materials are readily available in large quantities and in a sustainable manner. The GC-MS analysis shows that the coconut oil contains 92% saturated fatty acid. Whereas, the castor seed oil contains 98.31% unsaturated fatty acid with 89.75% of mono-unsaturated ricin-oleic acid (D-12-hydroxyoctadec-cis-9-enoic acid). The linseed oil has high content of linolenic acid with 89.60% unsaturated fatty acid. The designed fluid has excellent lubricity characteristics, extremely low cloud and pour point temperatures, higher oxidative stability, low peroxide value, lower copper strip corrosion value and lower kinematic viscosity and density variations with temperatures which signals flat rheology when used to form a synthetic based mud. Also, its flash point and auto-ignition temperatures are high and thus forestall possible fire hazard in high temperature zones. Finally, the fluid is non-toxic and inherently biodegradable.

Flow regimes and characteristics of dense particulate flows with coarse particles in inclined pipe

This paper presents a numerical simulation for dense particulate flows with coarse particles in inclined pipe to identify different flow regimes by means of computational fluid dynamics-discrete element method. The drag, gravitational, pressure gradient and virtual mass forces on particles, as well as the effect of particle–particle collisions are considered. Two flow regimes and their transitions are observed and described. The influence of Stokes number St, Froude number Fr, inclination angle β, etc., on critical flow regimes is analyzed to identify the dependence of flow regimes on these parameters. A new diagram for recognizing regime transition is given. The temporal variations of flow fields are also analyzed to illustrate propagating of kinematic waves. The wave velocity increases with Fr, β, and particle concentration increasing. Two dimensionless numbers, collision stress and fluid–particle interaction stress, are defined to explain the regime transition mechanism. The maximum pressure drop occurs at approximately β = 60°.

Gilded vaterite optothermal transport in a bubble

Laser beams, capable of controlling the mechanical motion of micron-scale objects, can serve as a tool, enabling investigations of numerous interaction scenarios under full control. Beyond pure electromagnetic interactions, giving rise to conventional gradient forces and radiation pressure, environment-induced thermal effects can play a role and, in certain cases, govern the dynamics. Here we explore a thermocapillary Marangoni effect, which is responsible for creating long-range few hundreds of nano-Newton forces, acting on a bubble around a ‘gilded vaterite’ nanoparticle. Decorating calcium carbonate spherulite (the vaterite) with gold nanoseeds allows tuning its optical absorption and, as a result, controlling its temperature in a solution. We demonstrate that keeping a balance between electromagnetic and thermal interactions allows creating of a stable micron-scale bubble around the particle and maintaining its size over time. The bubbles are shown to remain stable over minutes even after the light source is switched off. The bubbles were shown to swim toward a laser focus for over 400-µm distances across the sample. Optothermal effects, allowing for efficient transport, stable bubble creation, and particle–fluid interaction control, can grant nano-engineered drug delivery capsules with additional functions toward a theragnostic paradigm shift.

Erosion mechanism for submarine pipeline on graded sediments using a coupled PORO-CFD-DEM model

To investigate the meso-mechanism of scouring process, a numerical model that couples the porous medium model, computational fluid dynamics and discrete elements method (PORO-CFD-DEM model) is established and validated. Two pipeline conditions, suspended and partially buried, are considered. The porous medium is used to replace most of the discrete elements, significantly reducing the computational requirements. Two particle generation methods, namely fictious grid method (FGM) and breath-first search method (BFM), are proposed, and three kinds of particle samples are generated with different gradations: Gaussian, gap and continuous. Detailed analyses of flow, fluid-particle interaction and sediment motion is conducted with an increasing inlet velocity. When the pipeline is suspended, the seepage flow is induced by the gap flow beneath the pipeline, with the pressure dropping along the seepage path. Around partially buried pipeline, the seepage flow is much weaker and the pressure concentration appears. The seepage force fluctuates, and the loose bed is disturbed, leading to randomly collapses. Different particle movement patterns, including the jumping, suspended transportation and the formation of sand slope, are captured during the simulations. It’s confirmed that the PORO-CFD-DEM model can capture the meso-movement of the particles and provides insights into the theory of sediment transportation.

Sedimentation of particles with various shapes and orientations in a closed channel using smoothed particle hydrodynamics

The sedimentation of particles with various shapes and orientations in a closed channel using smoothed particle hydrodynamics is investigated in this paper. The continuity and momentum equations of both fluid and solid are discretized using kernel approximation in the Lagrangian frame. The sedimentation behavior of different general shapes, including circle, pentagon, square, ellipse, rectangle, and triangle, at various initial orientations in the suspending fluid is simulated. The stable equilibrium orientation (SEO) of these shapes is examined, excluding the circle which serves as a validation case. Specifically, the major axis of the ellipse and rectangle tends to align horizontally, whereas the orientations of the pentagon and square seem to be random due to the lack of a major axis and the finite channel height. The settling behavior of the three types of triangles is also discussed, and the von Mises stress of these shapes during their settling is presented. This study offers valuable insights into fluid-particle interactions, specifically regarding the SEO and internal stress of settling particles with varying shapes and orientations.

On the influence of the natural shape of particles in multiphase fluid systems: Granular collapses

The collapse of natural granular particles is encountered in various scenarios, including landslides, tsunamis, avalanches, and industrial processes. However, our understanding of these phenomena is still limited due to the complex interactions and the wide range of parameters that influence such systems. Particle shape is one of the factors that can significantly impact granular collapse behavior. In this paper, we studied the role of particle morphology in the interaction between fluids and particles in collapse problems and wave generation in multiphase fluid systems. To achieve this, we employed an image-based Discrete Element Method coupled with the Volume-of-Fluid approach. Initially, the proposed method is validated using experimental data for spherical particles. Then, we increased the irregularity of the spherical particles in four incremental steps. Our study establishes a clear relationship between particle morphology and important characteristics such as displacement, velocity, inter-particle forces, and kinetic energy. We observed that as irregularity increases, interlocking between particles becomes more prominent, leading to reduced particle travel distances. Additionally, our findings demonstrate that interlocking also influences particle-fluid interactions, resulting in significant alterations in the formation of generated waves.

Signed distance field enhanced fully resolved CFD-DEM for simulation of granular flows involving multiphase fluids and irregularly shaped particles

It is challenging to model granular particles with arbitrary shapes and related complications to fluid–particle interactions for granular flows which are widely encountered in nature and engineering. This paper presents an improved framework of the immersed boundary method (IBM)-based fully resolved computational fluid dynamics (CFD) and discrete element method (DEM), with an emphasis on irregular-shaped particles and the implications to particle–fluid interactions. The improved CFD-DEM framework is featured by two novel enhancements with signed distance field (SDF). First, an SDF-based formulation is employed to enable handling of granular particles with arbitrary shapes in DEM robustly and efficiently. Second, the IBM is modified to be consistent with SDF to fully resolve fluid–particle interactions in the presence of non-spherical particles. Such treatments leverage SDF as a generic interface to furnish a new SDF-CFD-DEM framework for universal modeling of arbitrarily shaped particles interacting with multiphase fluids with desired resolutions. Exemplified particle shape models include super-quadrics, spherical harmonics, polyhedron and level set, and new shape models can be flexibly developed by implementing the unified SDF-based shape interface. The proposed SDF-CFD-DEM is validated and showcased with examples including particle settling, drafting–kissing–tumbling, immersed granular collapse, and mudflow. The results demonstrate the good accuracy and robustness of the SDF-CFD-DEM and its potential for efficient computational modeling of multiphase granular flows involving granular particles with arbitrary shapes.

CFD-DEM modeling of turbidity current propagation in channels with two different topographic configurations

Submarine turbidity currents are a special type of sediment gravity flow responsible for turbidite deposits, attracting great interests from scientists and engineers in marine and petroleum geology. This paper presents a fully coupled computational fluid dynamics (CFD) and discrete element method (DEM) model to quantitatively analyze the turbidity current propagation in channels with two different topographic configurations. An appropriate drag force model is first incorporated in the CFD-DEM scheme, and two benchmark cases, including a single-particle sedimentation case and an immersed granular collapse case, are conducted to verify the accuracy of the developed CFD-DEM model. The model is then employed to investigate the fluid and particle dynamics of turbidity currents flowing over a flat bed (FB), and three obstacle-placed beds with different heights (OPB, OPB_1 and OPB_2). The CFD-DEM results indicate that the front position of turbidity current in the FB case is well consistent with the classic lock-exchange experiment. Results also show that the presence of the obstacle can clearly diminish the inter-particle collisions and the particle kinetic energy, weaken the particle-fluid interactions, and further make more sediment particles settle in front of the obstacle. Increase of obstacle height can result in diverse flow morphology of particles and fluids, and intensify the influences of obstacle on particle dynamics of turbidity currents. We show that our models enable reproducing the typical process of turbidity current propagation, and further can provide more valuable insights in understanding the turbidite-related geological phenomena from the point of view of particulate flow.

A spheropolyhedral-based discrete element lattice Boltzmann method for simulation of non-spherical adhesive particulate flow

Non-spherical particulate flow is commonly encountered in nature, industry and our daily life. But up to now, there still lack effective model to describe the adhesive elastic collision between non-spherical microparticles. To solve this problem, a spheropolyhedral-based discrete element method (DEM) combined with Johnson-Kendall-Roberts (JKR) model is proposed. The contact between non-spherical particles is attributed to the contacts between three geometry features, including vertice-vertice (V-V), vertice-edge(V-E), vertice-face (V-F) and edge-edge (E-E). The JKR adhesive model is modified to calculate the contact force of the contacting geometry features based on the actual contact area. Meanwhile, the lattice Boltzmann coupled with the immersed moving boundary (LB-IMB) method is employed to simulate the fluid flow and fluid-particle interaction. The novel spheropolyhedral-based DE-LBM is verified to be accurate and effective by several experiments and the data in references. Finally, an application test for simulating the packing process of non-spherical microparticles is presented. The method well predicts the packing structure of both adhesive and non-adhesive non-spherical particles, meaning that it can be easily extended to simulate the massive non-spherical microparticles with high effectiveness and efficiency. Program summaryProgram Title: adhesive contact force model for non-spherical particleCPC Library link to program files:https://doi.org/10.17632/vswvv7ppz2.1Developer's repository link:https://github.com/QIAN-C/adhesive-force-model-for-non-sphericalLicensing provisions: GNU General Public License v3Programming language: C++Nature of problem: Various methods have been proposed to handle the complex shapes of non-spherical particles in the discrete element method. However, there is still a lack of effective models for calculating the contact force between the non-spherical fine particles, particularly the adhesive forces.Solution method: Based on the sphere-polygon algorithm and the JKR model, this program has developed a contact force model that can accurately calculate adhesive forces between non-spherical fine particles within the framework of the open source library MechSys (URL: http://mechsys.nongnu.org/index.shtml).

Experimentelle und numerische Untersuchung von Fluid-Partikel-Interaktionen in Wasser

ZusammenfassungFür die Entwicklung verbesserter Modelle zur Beschreibung des Sedimenttransports in Fließgewässern ist das grundlegende Verständnis der Interaktion zwischen den festen Partikeln und dem bewegten Fluid (Wasser) bedeutend. In diesem Beitrag werden aktuelle Entwicklungen im Bereich der Fluid-Partikel-Interaktion anhand von zwei konkreten Forschungsarbeiten von Gold et al. (2023) und Worf et al. (2022) vorgestellt. Die experimentelle Forschungsarbeit von Gold et al. (2023) untersucht mithilfe von Messmethoden auf dem neuesten Stand der Technik die Strömung rund um Kugeln verschiedener Dichte, die in anfangs ruhendem Wasser oszillieren. Über den gesamten Dichtebereich (m^{*}=rho _{S}/rho _{F}= 1,14; 14,95; Dichteverhältnis von Solid und Fluid) konnte für das Kugelpendel eine vergleichbare Charakteristik der Wirbelablösung festgestellt werden. Ebenso wird eine neue Methode der Objektverfolgung (DOT) vorgestellt, welche eine zeitlich und räumlich aufgelöste Analyse der Strömungsstrukturen im Fluidfeld ermöglicht. Gold et al. (2023) zeigen, dass es während der ersten Periode zur Wirbelablösung kommt. Dieser Wirbel pflanzt sich in Richtung Beckenboden fort und dissipiert letztendlich. Weiters wurde ein Dämpfungsoptimum des Kugelpendels im Bereich von m^{*}=2{,}50 beobachtet. Außerdem wurde ein Experiment von einem Zylinderpendel mit m^{*}=4{,}98 mittels Immersed-Boundary-Methode numerisch untersucht. Dabei wurde der Vorgang der Bildung und Ablösung bis hin zum Zerfall eines Wirbelrings beschrieben. Zusätzlich wurde bei dieser Untersuchung von Worf et al. (2022) die Bildung von Randwirbeln beobachtet, beschrieben und mit der anfänglichen Entwicklung der dreidimensionalen Strömung und „zusätzlichen Masse“ in Verbindung gebracht.

A multiple-time-step integration algorithm for particle-resolved simulation with physical collision time

In this paper, we present a multiple-time-step integration algorithm (MTSA) for particle collisions in particle-resolved simulations. Since the time step required for resolving a collision process is much smaller than that for a fluid flow, the computational cost of the traditional soft-sphere model by reducing the time step is quite high in particle-resolved simulations. In one state-of-the-art methodology, collision time is stretched to several times the flow solver time step for the fluid to adapt to the sudden change in particle motion. However, the stretched collision time is not physical, the hydrodynamic force may be severely underestimated during a stretched collision, and the simulation of sediment transport may be sensitive to the stretched collision time. The proposed MTSA adopts different time steps to resolve fluid flow, fluid-particle interaction, and particle collision. We assessed the MTSA for particle-wall collisions as well as particle-particle collisions, determined the optimal iteration number in the algorithm, and obtained excellent agreements with experimental measurements and reference simulations. The computational cost of the MTSA can be reduced to about one order of magnitude less than that using the traditional soft-sphere model with almost the same accuracy. The MTSA was then implemented in a particle-resolved simulation of sediment transport with thousands of particles. {By comparing the results obtained using the MTSA and a version of the stretching collision time algorithm similar to Costa et al.(2015), we found that stretching the collision time reduced particle stiffness, weakened particle entrainment, and affected some turbulence and particle statistics.

Proppant transport in rough fractures of unconventional oil and gas reservoirs

A method to generate fractures with rough surfaces was proposed according to the fractal interpolation theory. Considering the particle-particle, particle-wall and particle-fluid interactions, a proppant-fracturing fluid two-phase flow model based on computational fluid dynamics (CFD)-discrete element method (DEM) coupling was established. The simulation results were verified with relevant experimental data. It was proved that the model can match transport and accumulation of proppants in rough fractures well. Several cases of numerical simulations were carried out. Compared with proppant transport in smooth flat fractures, bulge on the rough fracture wall affects transport and settlement of proppants significantly in proppant transportation in rough fractures. The higher the roughness of fracture, the faster the settlement of proppant particles near the fracture inlet, the shorter the horizontal transport distance, and the more likely to accumulate near the fracture inlet to form a sand plugging in a short time. Fracture wall roughness could control the migration path of fracturing fluid to a certain degree and change the path of proppant filling in the fracture. On the one hand, the rough wall bulge raises the proppant transport path and the proppants flow out of the fracture, reducing the proppant sweep area. On the other hand, the sand-carrying fluid is prone to change flow direction near the contact point of bulge, thus expanding the proppant sweep area.

Interface-resolved simulations of particles in active nematics

An accurate coarse-grained simulation of an active fluid is invaluable as a tool to understand its hydrodynamic behaviors. The study on the dynamics of particles immersed in an active fluid also requires accurate resolution of the fluid–particle interaction. In this paper, we propose a robust direct forcing fictitious domain method to study the dynamics of suspended particles in an active fluid modeled by “active nematics.” This method serves as both a coarse-grained approach and an accurate model of fluid–particle interaction. We first validate the method by computing the kinetic energy spectrum for the bulk active nematics and find that it accurately reproduces the scaling laws reported theoretically and experimentally. By utilizing these interface-resolved simulations, we illustrate that the model's activity parameter cannot be simply considered as the concentration of bacterial suspensions. Moreover, we find that the diffusion coefficient DT of an individual disk is relevant to the length scale lc of the active nematics, following a power-law scaling DT ∼ lc−1.5. Regarding collective dynamics, we discover a self-organized length scale of approximately 7.5 times the disk's diameter in the active nematics. Additionally, the disks modify the kinetic energy spectrum of the active nematics at both the self-organized length scale and the individual disk's diameter scale, respectively.

Ultrasonic Velocity, Viscosity and Thermal Conductivity Studies on Barium Oxide: Silicone Oil Nanofluids

In this study, we focused on the preparation and characterization of Barium oxide (BaO): Silicone oil nanofluids with the assistance of ultrasonication. The purpose was to investigate the potential impact of these nanofluids on solar radiation absorption. To achieve this, six different concentrations (ranging from 0.01 g to 0.06 g) of BaO: Silicone oil nanofluids were prepared. The nanofluids were subjected to various characterization techniques to evaluate their properties. Ultrasonic velocity measurements were conducted to assess the dispersion quality and stability of the nanofluids. Fourier transform infrared (FTIR) spectroscopy was utilized to examine any potential interactions between the nanoparticles and the fluid medium. Ultraviolet-Visible (UV-Visible) spectroscopy was employed to investigate the optical properties of the nanofluids, particularly their ability to absorb solar radiation. Additionally, electron microscopy analysis provided insights into the morphology and size distribution of the BaO nanoparticles. The results obtained from the UV-Visible analysis provided valuable information regarding the solar radiation absorption efficiency of the BaO: Silicone oil nanofluid systems. These findings contribute to our understanding of the potential application of these nanofluids in solar energy harvesting. Furthermore, the ultrasonic studies and FTIR analysis confirmed that there were no significant particle-fluid interactions, indicating the stability of the nanofluids. Thermal conductivity measurements were carried out to determine the heat transfer efficiency of the BaO: Silicone oil nanofluid system at different concentrations. The results revealed an optimal concentration that exhibited the highest heat transfer efficiency, suggesting the potential of these nanofluids for enhancing heat transfer processes. In conclusion, this study successfully prepared and characterized BaO: Silicone oil nanofluids. The analysis of their optical properties, stability, and thermal conductivity provides valuable insights into their potential application in solar radiation absorption and heat transfer systems. Further research can explore the practical implementation of these nanofluids in solar energy conversion and thermal management technologies.

CFD-DEM Evaluation of the Clustering Behavior in a Riser-the Effect of the Drag Force Model.

Riser reactors are frequently applied in catalytic processes involving rapid catalyst deactivation. Typically heterogeneous flow structures prevail because of the clustering of particles, which impacts the quality of the gas-solid contact. This phenomenon results as a competition between fluid-particle interaction (i.e., drag) and particle-particle interaction (i.e., collisions). In this study, five drag force correlations were used in a combined computational fluid dynamics-discrete element method Immersed Boundary Model to predict the clustering. The simulation results were compared with experimental data obtained from a pseudo-2D riser in the fast fluidization regime. The clusters were detected on the basis of a core-wake approach using constant thresholds. Although good predictions for the global (solids volume fraction and mass flux) variables and cluster (spatial distribution, size, and number of clusters) variables were obtained with two of the approaches in most of the simulations, all the correlations show significant deviations in the onset of a pneumatic transport regime. However, the correlations of Felice (Int. J. Multiphase Flow1994, 20, 153-159) and Tang et al. [AIChE J.2015, 61 ( (2), ), 688-698] show the closest correspondence for the time-averaged quantities and the clustering behavior in the fast fluidization regime.

Non-Convex Particle-in-Cell Model for the Mathematical Study of the Microscopic Blood Flow

The field of fluid mechanics was further explored through the use of a particle-in-cell model for the mathematical study of the Stokes axisymmetric flow through a swarm of erythrocytes in a small vessel. The erythrocytes were modeled as inverted prolate spheroids encompassed by a fluid fictitious envelope. The fourth order partial differential equation governing the flow was completed with Happel-type boundary conditions which dictate no fluid slip on the inverted spheroid and a shear stress free non-permeable fictitious boundary. Through innovative means, such as the Kelvin inversion method and the R-semiseparation technique, a stream function was obtained as series expansion of Gegenbauer functions of the first and the second kinds of even order. Based on this, analytical expressions of meaningful hydrodynamic quantities, such as the velocity and the pressure field, were calculated and depicted in informative graphs. Using the first term of the stream function, the drag force exerted on the erythrocyte and the drag coefficient were calculated relative to the solid volume fraction of the cell. The results of the present research can be used for the further investigation of particle–fluid interactions.

A particle-resolved heat-particle-fluid coupling model by DEM-IMB-LBM

Multifield coupling is frequently encountered and also an active area of research in geotechnical engineering. In this work, a particle-resolved direct numerical simulation (PR-DNS) technique is extended to simulate particle-fluid interaction problems involving heat transfer at the grain level. In this extended technique, an immersed moving boundary (IMB) scheme is used to couple the discrete element method (DEM) and lattice Boltzmann method (LBM), while a recently proposed Dirichlet-type thermal boundary condition is also adapted to account for heat transfer between fluid phase and solid particles. The resulting DEM-IBM-LBM model is robust to simulate moving curved boundaries with constant temperature in thermal flows. To facilitate the understanding and implementation of this coupled model for non-isothermal problems, a complete list is given for the conversion of relevant physical variables to lattice units. Then, benchmark tests, including a single-particle sedimentation and a two-particle drafting-kissing-tumbling (DKT) simulation with heat transfer, are carried out to validate the accuracy of our coupled technique. To further investigate the role of heat transfer in particle-laden flows, two multiple-particle problems with heat transfer are performed. Numerical examples demonstrate that the proposed coupling model is a promising high-resolution approach for simulating the heat-particle-fluid coupling at the grain level.

Effect of Non-Linear Radiation and Non-Uniform Heat Source/Sink on Flow over a Linear Stretching Sheet with Fluid Particle Suspension

In this paper, the flow and heat transfer of dusty flow over a linear stretchable sheet has been analyzed. The effects of non-linear radiation and non-uniform heat source or sink have been studied. Considering the mentioned terms and conditions, the problem has been formulated. The formulation consists of systems of nonlinear PDEs which has been converted to a system of ODEs by taking suitable similarity transformations. Then the systems of ODEs have been solved by using Rungakutta method of 4th order. The computations work has been carried out by using BVP4C tool of MATLAB. The impact of flow parameters like radiation parameter, prandtl number, eckret number, fluid Particle interaction parameter β and diffusivity parameter on flow and heat transfer are investigated. The results are presented through graphs and tables. From the graphs, it is concluded that the presence of particles in fluid has some impact of different parameters in flow and heat transfer. The parameters like Eckert number, Fluid- particle interaction parameter, diffusivity parameter have remarkable effects of presence of particles

Investigating the inflow into a granular bed using a locally resolved method

Discrete Element Method – Computational Fluid Dynamics (DEM/CFD) simulations of industrial-scale granular systems employ spatial averaging (porous media approach) for the fluid-particle interaction in the whole domain, which can lead to poor accuracy, for instance at flow inlets, as local particle bulk morphology is not resolved. This paper presents an approach where the interstitial flow in crucial areas with large gradients can be resolved locally in an otherwise unresolved domain, so that a mixed resolved-unresolved method is realized.As a generic example to show the feasibility and performance of the new approach, the inflow of ambient air into a flat-bottom hopper through a narrow orifice is investigated. In an experimental setup, the vertical profile of the pressure decay through the inlet and across the packing is chosen for comparison with respective simulations. Results obtained with the conventional porous media method and the locally resolved approach are compared to these experiments for varying volume flow rates and for two different particle shapes. Spheres of different size as well as dodecahedrons are examined.It is found that although averaging methods already provide good approximations, the locally resolved method can improve the result especially when conventional drag laws are not applicable due to wall effects or if large velocity gradients exist.

A critical assessment of the immersed boundary method for modeling flow around fixed and moving bodies

This paper reports the verification and validation of the immersed boundary methods implemented in the open-source toolbox FOAM-Extend version 4.0 and 4.1. Five test cases with increasing complexity are studied and the simulation results are compared with the experimental and numerical data found in the literature. A solver is developed based on the Immersed Boundary Surface Method (IBS) which is an advanced cut-cell finite volume immersed boundary technique. By integrating the Lagrangian equations of motion with the IBS, this solver is capable of simulating fluid–particle interactions. The benchmark studies indicated that the IBS yields excellent results for stationary immersed bodies. Even though the flow field is captured satisfactorily for moving bodies, spurious force oscillations are present in these cases. Simulating different size particles in a channel flow shows that the spurious force oscillations can negatively affect the prediction of particle dynamic motion, although the severity depends on the value of the particle Stokes number.