Particle Capture and Pattern Evolution on Big Drops in Three-phase Turbulence

The process of particle capture and trapping by large deformable drops in turbulent channel flow are investigated in this thesis using an Eulerian-Lagrangian approach specifically developed for this three-phase flow. The flow field in the carrier fluid and inside the droplets is obtained from Direct Numerical Simulation of the Navier-Stokes equations; the drop interface dynamics are provided by a Phase Field Model; and par- ticle trajectories are calculated via Lagrangian tracking. Drops have the same density and viscosity of the carrier fluid in order to mimic a liquid-liquid dispersion of water and low-viscosity oil. Particles are modelled as neutrally- buoyant, sub-Kolmogorov spheres that interact with each other through collisions (excluded-volume interaction). Simulation results allow a detailed characterization of the particle dynamics during the interface capture and trapping stages. Particle capture is driven by the capillary forces of the interface in combination with near-interface turbulent motions: Particles are transported towards the interface by jet-like turbulent motions and, once close enough, are captured by interfacial forces in regions of positive surface velocity diver- gence. These regions appear to be well correlated with high-enstrophy flow topologies that contribute to enstrophy production via vortex compression or stretching. Upon capture, particles sample preferentially regions of positive surface velocity divergence, which correlate with jet-like fluid motions directed towards the interface. At later times, however, particles are observed to move from these regions under the action of the tangential stresses to the areas where the surface divergence vanishes and form the two-dimensional cluster. long- term trapping regions correlate well with the surface area characterized by higher-than-mean curvature. This finding is important since the presence of tiny particles at the interface is known to affect locally the surface tension, particularly in the presence of concentration gradi- ents: present results suggest that particle-induced modifications of the surface tension should be stronger where the curvature of the interface is higher

A Comparison of Data-Driven Reduced Order Models for the Simulation of Mesoscale Atmospheric Flow

The simulation of atmospheric flows by means of traditional discretization methods remains computationally intensive, hindering the achievement of high forecasting accuracy in short time frames. In this paper, we apply three reduced order models that have successfully reduced the computational time for different applications in computational fluid dynamics while preserving accuracy: Dynamic Mode Decomposition (DMD), Hankel Dynamic Mode Decomposition (HDMD), and Proper Orthogonal Decomposition with Interpolation (PODI). The three methods are compared in terms of computational time and accuracy in the simulation of two well-known benchmarks for mesoscale flow. The accuracy of the DMD and HDMD solutions deteriorates rather quickly as the forecast time window expands, although these methods are designed to predict the dynamics of a system. The reason is likely the strong nonlinearity in the benchmark flows. The PODI solution is accurate for the entire duration of the time interval of interest thanks to the use of interpolation with radial basis functions. This holds true also when the model features a physical parameter expected to vary in a given range, as is typically the case in weather prediction

A Non-Intrusive Data-Driven Reduced Order Model for Parametrized CFD-DEM Numerical Simulations

The investigation of fluid-solid systems is very important in a lot of industrial processes. From a computational point of view, the simulation of such systems is very expensive, especially when a huge number of parametric configurations needs to be studied. In this context, we develop a non-intrusive data-driven reduced order model (ROM) built using the proper orthogonal decomposition with interpolation (PODI) method for Computational Fluid Dynamics (CFD) -- Discrete Element Method (DEM) simulations. The main novelties of the proposed approach rely in (i) the combination of ROM and FV methods, (ii) a numerical sensitivity analysis of the ROM accuracy with respect to the number of POD modes and to the cardinality of the training set and (iii) a parametric study with respect to the Stokes number. We test our ROM on the fluidized bed benchmark problem. The accuracy of the ROM is assessed against results obtained with the FOM both for Eulerian (the fluid volume fraction) and Lagrangian (position and velocity of the particles) quantities. We also discuss the efficiency of our ROM approach

Influence of the coiling porosity on the risk reduction of the cerebral aneurysm rupture: computational study

The formation and progress of cerebral aneurysm is highly associated with hemodynamic factors and blood flow feature. In this study, comprehensive efforts are done to investigate the blood hemodynamic effects on the creation and growth of the Internal Carotid Artery. The computational fluid dynamic method is used for the visualization of the bloodstream inside the aneurysm. Transitional, non-Newtonian and incompressible conditions are considered for solving the Navier-Stokes equation to achieve the high-risk region on the aneurysm wall. OSI and WSS of the aneurysm wall are compared within different blood flow stages. The effects of blood viscosity and coiling treatment on these factors are presented in this work. Our study shows that in male patients (HCT = 0.45), changing the porosity of coiling from 0.89 with 0.79 would decreases maximum OSI up to 75% (in maximum acceleration). However, this effect is limited to about 45% for female patients (HCT = 0.35)

Modal Analysis of the Wake Shed Behind a Horizontal Axis Wind Turbine with Flexible Blades

The proper orthogonal decomposition (POD) has been applied on a full-scale horizontal-axis wind turbine (HAWT) to shed light on the wake characteristics behind the wind turbine. In reality, the blade tip experiences high deflections even at the rated conditions which definitely alter the wake flow field, and in the case of a wind farm, may complicate the inlet conditions of the downstream wind turbine. The turbine under consideration is the full-scale model of the NREL 5MW onshore wind turbine which is accompanied by several simulation complexities including turbulence, mesh motion and fluid-structure interaction (FSI). Results indicated an almost similar modal behaviour for the rigid and flexible turbines at the wake region. In addition, more flow structures in terms of local vortices and fluctuating velocity fields take place at the far wake region. The flow structures due to the wake shed from the tower tend to move towards the center and merge with that of the nacelle leading to an integral vortical structure 2.5D away from the rotor. Also, it is concluded that the exclusion of the tower leads to missing a major part of the wake structures, especially at far-wake positions

Topology of Three-Dimensional Steady Cellular Flow in a Partially Liquid-Filled Rotating Drum

The topology of the steady three-dimensional flow is investigated in a partially liquid-filled horizontal rotating cylindrical drum. The drum is infinity long in its axis direction and it is rotating about its axis. A steady cellular flow arise due to introducing an infinitesimal perturbation and Lagrangian chaos is demonstrated for supercritical Reynolds numbers. The topological analysis is done in order to see the Kolmogorov-Arnold-Moser tori for three Reynolds numbers. This work is conducted by simulating numerically the incompressible Navier-Stokes equations through OpenFOAM

Interface topology and evolution of particle patterns on deformable drops in turbulence

The capture of neutrally buoyant, sub-Kolmogorov particles at the interface of deformable drops in turbulent flow and the subsequent evolution of particle surface distribution are investigated. Direct numerical simulation of turbulence, phase-field modelling of the drop interface dynamics and Lagrangian particle tracking are used. Particle distribution is obtained considering excluded-volume interactions, i.e. by enforcing particle collisions. Particles are initially dispersed in the carrier flow and are driven in time towards the surface of the drops by jet-like turbulent fluid motions. Once captured by the interfacial forces, particles disperse on the surface. Excluded-volume interactions bring particles into long-term trapping regions where the average surface velocity divergence sampled by the particles is zero. These regions correlate well with portions of the interface characterized by higher-than-mean curvature, indicating that modifications of the surface tension induced by the presence of very small particles will be stronger in the highly convex regions of the interface

Effect of particle aspect ratio in targeted drug delivery in abdominal aortic aneurysm

Aneurysm is a permanent irreversible bulge in the artery that can occur with higher prevalence among elderly individuals. Although invasive surgical procedures can prevent their development, they come with considerable side effects. Recently, treatments based on targeted drug delivery have gained a lot of attention to suppress aneurysm growth. Numerical simulations have been shown to be of great role in the prediction of blood hemodynamics and vascular wall behaviour in the case of an aneurysm. Moreover, the utilization of high-fidelity approaches such as the Lagrangian frame of reference can address the motion characteristics of microbubble (MB) contrast agents in particulate flows. This study aims to investigate the effect of particle aspect ratio on the adhesion of oblate spheroid particles to the vascular wall. Accordingly, a two-way fluid–structure interaction (FSI) method consisting of a hyperelastic material model for the vessel along with a non-Newtonian, compressible model for blood was employed to simulate an abdominal aortic aneurysm (AAA). Moreover, the ligand–receptor binding concept has been utilized to address the quantification of MBs adhesion. Five sets of aspect ratios ranging from 1 to 9 have been investigated and results indicated that with the increase of the aspect ratio the rate of adhesion decreases. Two drastic changes in the particle number occurred due to the diastolic peak and negative velocity profile, respectively. However, it was concluded that the hydrodynamic of the MBs in terms of velocity and wall distance is rather insensible to the particle shape.</p