Modeling particle-fluid interaction in a coupled CFD-DEM framework

In this work, we present an alternative methodology to solve the particle-fluid interaction in the resolved CFDEM coupling framework. This numerical approach consists of coupling a Discrete Element Method (DEM) with a Computational Fluid Dynamics (CFD) scheme, solving the motion of immersed particles in a fluid phase. As a novelty, our approach explicitly accounts for the body force acting on the fluid phase when computing the local momentum balance equations. Accordingly, we implement a fluid-particle interaction computing the buoyant and drag forces as a function of local shear strain and pressure gradient. As a benchmark, we study the Stokesian limit of a single particle. The validation is performed comparing our outcomes with the ones provided by a previous resolved methodology and the analytical prediction. In general, we find that the new implementation reproduces with very good accuracy the Stokesian dynamics. Complementarily, we study the settling terminal velocity of a sphere under confined conditions

Brittle to ductile transition in a fiber bundle with strong heterogeneity

We analyze the failure process of a two-component system with widely different fracture strength in the framework of a fiber bundle model with localized load sharing. A fraction 0≤α≤1 of the bundle is strong and it is represented by unbreakable fibers, while fibers of the weak component have randomly distributed failure strength. Computer simulations revealed that there exists a critical composition αc which separates two qualitatively different behaviors: Below the critical point, the failure of the bundle is brittle, characterized by an abrupt damage growth within the breakable part of the system. Above αc, however, the macroscopic response becomes ductile, providing stability during the entire breaking process. The transition occurs at an astonishingly low fraction of strong fibers which can have importance for applications. We show that in the ductile phase, the size distribution of breaking bursts has a power law functional form with an exponent μ=2 followed by an exponential cutoff. In the brittle phase, the power law also prevails but with a higher exponent μ=92. The transition between the two phases shows analogies to continuous phase transitions. Analyzing the microstructure of the damage, it was found that at the beginning of the fracture process cracks nucleate randomly, while later on growth and coalescence of cracks dominate, which give rise to power law distributed crack sizes

Discharge of elongated grains in silos under rotational shear

The discharge of elongated particles from a silo with rotating bottom is investigated numerically. The introduction of a slight transverse shear reduces the flow rate Q by up to 70% compared with stationary bottom, but the flow rate shows a modest increase by further increasing the external shear. Focusing on the dependency of flow rate Q on orifice diameter D, the spheres and rods show two distinct trends. For rods, in the small-aperture limit Q seems to follow an exponential trend, deviating from the classical power-law dependence. These macroscopic observations are in good agreement with our earlier experimental findings [Phys. Rev. E 103, 062905 (2021)]. With the help of the coarse-graining methodology we obtain the spatial distribution of the macroscopic density, velocity, kinetic pressure, and orientation fields. This allows us detecting a transition from funnel to mass flow pattern caused by the external shear. Additionally, averaging these fields in the region of the orifice reveals that the strong initial decrease in Q is mostly attributed to changes in the flow velocity, while the weakly increasing trend at higher rotation rates is related to increasing packing fraction. Similar analysis of the grain orientation at the orifice suggests a correlation of the flow rate magnitude with the vertical orientation and the packing fraction at the orifice with the order of the grains. Lastly, the vertical profile of mean acceleration at the center of the silo denotes that the region where the acceleration is not negligible shrinks significantly due to the strong perturbation induced by the moving wall

Diagrama fundamental del movimiento de peatones: Efecto de la competitividad

En este trabajo se ha estudiado numéricamente el proceso de evacuación de un conjunto de personas a través de una salida estrecha. Con este objetivo, se implementaron modelos de elementos discretos (DEM), de sistemas compuestos por partículas no esféricas autopropulsadas (peatones). Así, se desarrollaron nuevas metodologías de interacción entre peatones teniendo en cuenta la forma de la partícula y su correlación con la dirección deseada de movimiento. Para ello, se hizo uso de la estructura paralela de las unidades de procesamiento gráfico (GPU). Se ejecutó un estudio sistemático, ajustando varios parámetros del modelo, con el objetivo de reproducir condiciones experimentales específicas. Así, se varió la agitación de los individuos y la tendencia a mantener una dirección de movimiento deseada, manteniendo constante la magnitud de la velocidad deseada. Como punto de partida, hemos utilizado valores de flujo obtenidos experimentalmente, para validar si el modelo presenta una respuesta coherente. Además, se exploraron propiedades micromecánicas del sistema, como son: orientación de los peatones en posiciones cercanas a la puerta de salida y la proyección de estas sobre la dirección deseada. Finalmente, y motivados por los resultados experimentales muy recientes, hemos añadido un grado de complejidad al sistema, introduciendo un obstáculo. Las conclusiones que hemos obtenido han resultado del todo interesantes, en general obtenemos que introducir un obstáculo no resulta beneficioso para mejorar el flujo de personas. Así, nuestros resultados parecen contradecir opiniones preliminares de que la presencia de un obstáculo favorecería el proceso de evacuación. Sin embargo, reproducimos de manera fidedigna resultados experimentales, obtenidos muy recientemente, en el departamento de Física y Matemáticas de la Universidad de Navarra

Discrete Fracture Model with Anisotropic Load Sharing

A two-dimensional fracture model where the interaction among elements is modeled by an anisotropic stress-transfer function is presented. The influence of anisotropy on the macroscopic properties of the samples is clarified, by interpolating between several limiting cases of load sharing. Furthermore, the critical stress and the distribution of failure avalanches are obtained numerically for different values of the anisotropy parameter $\alpha$ and as a function of the interaction exponent $\gamma$. From numerical results, one can certainly conclude that the anisotropy does not change the crossover point $\gamma_c=2$ in 2D. Hence, in the limit of infinite system size, the crossover value $\gamma_c=2$ between local and global load sharing is the same as the one obtained in the isotropic case. In the case of finite systems, however, for $\gamma\le2$, the global load sharing behavior is approached very slowly

Active particles with desired orientation fowing through a bottleneck

We report extensive numerical simulations of the fow of anisotropic self-propelled particles through a constriction. In particular, we explore the role of the particles’ desired orientation with respect to the moving direction on the system fowability. We observe that when particles propel along the direction of their long axis (longitudinal orientation) the fow-rate notably reduces compared with the case of propulsion along the short axis (transversal orientation). And this is so even when the efective section (measured as the number of particles that are necessary to span the whole outlet) is larger for the case of longitudinal propulsion. This counterintuitive result is explained in terms of the formation of clogging structures at the outlet, which are revealed to have higher stability when the particles align along the long axis. This generic result might be applied to many diferent systems fowing through bottlenecks such as microbial populations or diferent kind of cells. Indeed, it has already a straightforward connection with recent results of pedestrian (which self-propel transversally oriented) and mice or sheep (which self-propel longitudinally oriented)

Particle flow rate in silos under rotational shear

Very recently, To et al. have experimentally explored granular flow in a cylindrical silo, with a bottom wall that rotates horizontally with respect to the lateral wall [Phys. Rev. E 100, 012906 (2019)]. Here we numerically reproduce their experimental findings, in particular, the peculiar behavior of the mass flow rate Q as a function of the frequency of rotation f . Namely, we find that for small outlet diameters D the flow rate increased with f , while for larger D a nonmonotonic behavior is confirmed. Furthermore, using a coarse-graining technique, we compute the macroscopic density, momentum, and the stress tensor fields. These results show conclusively that changes in the discharge process are directly related to changes in the flow pattern from funnel flow to mass flow. Moreover, by decomposing the mass flux (linear momentum field) at the orifice into two main factors, macroscopic velocity and density fields, we obtain that the nonmonotonic behavior of the linear momentum is caused by density changes rather than by changes in the macroscopic velocity. In addition, by analyzing the spatial distribution of the kinetic stress, we find that for small orifices increasing rotational shear enhances the mean kinetic pressure (pk) and the system dilatancy. This reduces the stability of the arches, and, consequently, the volumetric flow rate increases monotonically. For large orifices, however, we detected that (pk) changes nonmonotonically, which might explain the nonmonotonic behavior of Q when varying the rotational shear

Movimiento de un intruso dentro de un medio granular denso: una propuesta didáctica para Bachillerato Internacional

Este proyecto consiste en el análisis de la dinámica de un proyectil en un medio granular denso para su puesta en práctica en un contexto de Bachillerato Internacional (BI). El punto de partida del proyecto son unos datos que se obtienen mediante simulación software facilitados por el Departamento de Física y Matemática Aplicada de la Universidad de Navarra y que otorgan información sobre el movimiento de un cuerpo esférico en un medio granular denso a lo largo de la dirección de la gravedad. En este Trabajo Final de Máster se analizan estos datos para el caso de gravedad cero y se lleva a cabo un estudio crítico apoyándonos en la ecuación de movimiento que define la dinámica del proyectil en el medio dado. También se desarrolla el código que mejor defina el comportamiento de un proyectil a lo largo del medio granular, el cual se valida con los datos proporcionados. Al mismo tiempo, se va a desarrollar una propuesta didáctica a través del Proyecto del Grupo 4 enfocándonos exclusivamente en la disciplina de la Física, en donde el alumnado es capaz de analizar esos datos haciendo uso del código para llegar a unas conclusiones pertinentes. Este tipo de proyectos es idóneo para que los alumnos tomen conciencia de la forma en que los científicos profesionales trabajan y se comunican entre ellos a nivel internacional.This project consists of the analysis of the dynamics of a projectile in a dense granular medium for its implementation in an International Baccalaureate (IB) context. The starting point of the project is data obtained through software simulation, which gives information about the movement of a spherical body in a dense granular medium along the direction of gravity. It is provided by the Department of Physics and Applied Mathematics of the University of Navarra. In this master’s thesis, these data are analysed for the case of zero gravity and a critical study is conducted, which is based on the equation of motion that defines the dynamics of the projectile in the given medium. We develop the code that best defines the behaviour of a projectile along the granular medium, which is validated by the data provided. In parallel with that, a didactic proposal is developed through the Group 4 Project focusing exclusively on the discipline of Physics, where students can analyse these data using the code to reach pertinent conclusions. This type of project is ideal for students to become aware of the way in which professional scientists work and communicate with each other on an international level

Role of particle shape on the stress propagation in granular packings

We present an experimental and numerical study on the influence that particle aspect ratio has on the mechanical and structural properties of granular packings. For grains with maximal symmetry (squares), the stress propagation in the packing localizes forming chainlike forces analogous to the ones observed for spherical grains. This scenario can be understood in terms of stochastic models of aggregation and random multiplicative processes. As the grains elongate, the stress propagation is strongly affected. The interparticle normal force distribution tends toward a Gaussian, and, correspondingly, the force chains spread leading to a more uniform stress distribution reminiscent of the hydrostatic profiles known for standard liquids

The role of initial speed in projectile impacts into light granular media

Projectile impact into a light granular material composed of expanded polypropylene (EPP) particles is investigated systematically with various impact velocities. Experimentally, the trajectory of an intruder moving inside the granular material is monitored with a recently developed non-invasive microwave radar system. Numerically, discrete element simulations together with coarse-graining techniques are employed to address both dynamics of the intruder and response of the granular bed. Our experimental and numerical results of the intruder dynamics agree with each other quantitatively and are in congruent with existing phenomenological model on granular drag. Stepping further, we explore the 'microscopic' origin of granular drag through characterizing the response of granular bed, including density, velocity and kinetic stress fields at the mean-field level. In addition, we find that the dynamics of cavity collapse behind the intruder changes significantly when increasing the initial speed . Moreover, the kinetic pressure ahead of the intruder decays exponentially in the co-moving system of the intruder. Its scaling gives rise to a characteristic length scale, which is in the order of intruder size. This finding is in perfect agreement with the long-scale inertial dissipation type that we find in all cases