A multi-physics peridynamics-DEM-IB-CLBM framework for the prediction of erosive impact of solid particles in viscous fluids

In this paper, a new fully-resolved framework capable of capturing the fundamental physics of particle–fluid interactions, the collision of particles with solid surfaces, and the resulting damage is proposed. A coupled DEM-IB-CLBM, consisting of a discrete element method (DEM), an immersed boundary (IB) method, and a cascaded lattice Boltzmann method (CLBM), is used to fully resolve the interaction of the particles with the surrounding viscous fluid. The peridynamics theory is then implemented and used to predict the impact damage to the target material. This framework is validated by comparing the trajectory of a particle–wall collision event in a viscous fluid with the previous results in the literature. Furthermore, the variation of the restitution coefficient with the impact velocity is in a good agreement with the available experimental results. The influence of multiple impacts and the resulting surface damage on the fluid dynamics of the system is investigated. It is demonstrated that the method correctly predicts the expected effects of multiple collisions and impact angle variations on the surface damage

Simulation of fracture slip and propagation in hydraulic stimulation of geothermal reservoirs

Rollen til hydraulisk stimulering i å øke produksjonen fra geotermiske reservoarer, og muliggjøre kommersiell utnyttelse av et større spekter av geotermiske ressurser, har fått økt oppmerksomhet de siste tiårene. Under stimulering kan eksisterende sprekker sideforskyves, forplante seg og koble seg til andre sprekker og der igjennom øke permeabiliteten i reservoaret. Prosessene er preget av sterke hydromekaniske interaksjoner, som vi har begrensede muligheter til å overvåke. Numeriske simuleringer er derfor et viktig verktøy for å hjelpe oss til å bedre forstå mekanismene som er i spill. Avhandlingen tar sikte på å utvikle en omfattende matematisk modell og en numerisk tilnærming for å analysere bruddmekanismer og undersøke koblede hydromekaniske prosesser som forekommer i oppsprukne porøse medier. Den foreslåtte modellen benytter en blandet-dimensjonal konseptuell modell, som inkluderer porelastisitet i det porøse mediet og kontaktmekanikk for sprekkene. Modellen tillater også forplantning og koalescens av eksisterende sprekker. Et nytt diskretiseringsskjema for å løse den foreslåtte matematiske modellen presenteres. Den foreslåtte metoden bruker en to-nivå simuleringstilnærming, kategorisert i grove og fine nivåer, for å redusere beregningskostnader og sikre nøyaktighet. En endelig volummetode kombineres med en aktiv-sett løsningsstrategi for å diskretisere porelastisitet og bruddkontaktmekanikk på det grove nivået. Sprekkeforplantning betraktes på et fint nivå, der en endelig elementmetode kombineres med kollapsede kvartpunktselementer for å approksimere singulariteten i spenningen ved enden av sprekkene. Adaptiv gitring basert på en feilestimator og Laplace-glatting av gitteret introduseres på begge nivåer for effektivt å håndtere sprekkepropagering og koalescens. Simuleringene utført i denne avhandlingen forbedrer vår forståelse av hydraulisk stimulering og dens effekt på forbedring av sprekkepermeabilitet og konnektivitet i geotermiske reservoarer.The role of hydraulic stimulation in enhancing geothermal reservoir production and allowing for commercial exploitation of a larger range of geothermal resources has attracted attention from researchers in recent decades. During stimulation, preexisting fractures may slip, propagate, and connect to other fractures to enhance permeability. The processes are characterized by strong hydromechanical interactions, which have limited monitoring opportunities. Therefore, numerical simulations provide a powerful tool to help us better understand the mechanisms. This thesis aims to develop a comprehensive mathematical model and a numerical approach to analyze fracture mechanisms, and to investigate the coupled hydromechanical processes occurring in fractured porous media. The proposed model will employ a mixed-dimensional conceptual model, incorporating the concepts of poroelasticity and fracture contact mechanics. The model will also allow for the growth and coalescence of preexisting fractures. A novel discretization scheme for solving the proposed mathematical model is presented. The proposed scheme employs a two-level simulation approach, categorized into coarse and fine levels, to reduce the computational costs and ensure accuracy. A finite volume method is combined with an active set strategy to discretize poroelasticity and fracture contact mechanics on the coarse level. Fracture propagation is considered on a fine level, in which a finite element method is combined with collapsed quarter-point elements to capture the stress singularity at the fracture tips. Adaptive remeshing based on an error estimator and Laplacian smoothing is introduced on both levels to effectively capture fracture propagation and coalescence in the computational grid. The simulations conducted in this thesis improve our understanding of hydraulic stimulation and its effect on enhancing fracture permeability and connectivity in geothermal reservoirs.Doktorgradsavhandlin

Experimental and numerical investigation of abrasive waterjet polishing technology

The objective of this investigation is the development of the abrasive waterjet (AWJ) based polishing technology. The result of the investigation will assist the implementation of AWJ polishing for manufacturing processes and procedures. Experimental exploration of AWl polishing involving processing of difficultmachine materials such as Alumina ceramic and stainless steel. Surface improvement due to this processing is evaluated by measuring the roughness of the generated surfaces and examining the microhardness and micro-topography of the surfaces using Scanning Electronic Microscopy (SEM). The surface roughness of 0.3 micron was obtained at samples of ceramic and metal alloys at a reasonable rate using 500-mesh garnet. No surface defects are induced. The effect of various process variables on the topography of surfaces generated during AWJ polishing was evaluated, It is shown that the particles dimension and jet impact angle are two critical parameters controlling the process. The former determines the feasibility of AWJ polishing, and the later limits the extent of improvement in the surface topography. The force exerted on the sample surface is measured at various impingement angles. And, the effects of the tangential and normal component of the force on the surface topography is evaluated. The abrasive particles which constitute a machining tool in the AWJ polishing are collected after mixing and after impact, and analyzed using Laser Scanning Sizer and SEM. The acquired data reveal the details of the mechanism of AWJ polishing processes. Numerical simulation of the motion of particles prior and after the impingement are conducted. Numerical solutions of the differential equations as applied to the two-phase turbulent jet flow are obtained using FIDAP package. The numerical prediction of jet velocity and force exerted on the target surface comply with the experimental results. The simulation of particles trajectories reveals existence of five distinctive patterns of particles motion which determine the surface topography. This work pioneers the use of AWJ as a polishing tool. and identifies the principal features of AWJ polishing and its use of computational packages for evaluation of the behavior of ultrahigh speed two-phase flows

Numerical simulation of fluid flow, proppant transport and fracture propagation in hydraulic fractures for unconventional reservoirs.

The distribution of proppant injected in hydraulic fractures significantly affects fracture-conductivity and well-performance. The proppant transport and suspension in thin fracturing fluid used in unconventional reservoirs are considerably different from those of fracturing fluids in conventional reservoirs, due to the very low viscosity of fracturing fluids used in the unconventional reservoirs, poor ability to suspend proppants and hence quick deposition of the proppants. This research presents the development of a three-dimensional computational fluid dynamics (CFD) modelling technique for the prediction of proppant-fluid multiphase flow in hydraulic fractures for unconventional reservoirs. The Eulerian-Lagrangian multiphase modelling approach has been applied to model the fluid flow and proppant transport, and the kinetic theory of granular flow is used to model the inter-proppant, fluid-proppant and proppantwall interactions. The existing proppant transport models ignore the fluid leak-off effect from the fracture side wall and the effect of fracture roughness. Thus, at the interface between the fracture and surrounding porous medium, the mass flow rate from the fracture to porous rock is calculated based on the permeability and porosity of the rock. The leakage mass flow rate is then used to define the mass and momentum source term at the fracture wall as a user-defined function, to investigate the proppant transport in hydraulic fractures with fluid leak-off effect. Furthermore, the hydrodynamic and mechanical behaviour of proppant transport on fracture roughness was studied in detail using different rough fracture profiles, and a relationship between the fracture roughness and proppant transport velocity is proposed. Lastly, an integrated model is developed, which simulates the proppant transport in dynamically propagating hydraulic fractures. The existing models either model the proppant transport physics in static predefined fracture geometry or account for the analytical models for defining the fracture propagation using linear elastic fracture mechanics. This limits the fracture propagation model to brittle rocks and neglect plastic deformations. Thus, in the present study, the fracture propagation was modelled using the extended finite element method (XFEM) and cohesive zone model (CZM), which can model the plastic deformations in the ductile rock. The fracture propagation was coupled with the CFD based proppant transport model, to model the fluid flow and proppant transport. The parametric study was then performed to investigate the effect of variation in proppant properties, fracturing fluid properties and geomechanical properties on the proppant transport. This study has enhanced the understanding of the flow and interaction phenomenon between proppant and fracturing fluid, and provides a technique with potential application in fracturing design for increasing well-productivity. The model can accurately simulate the proppant transport dynamics in hydraulic fracture and the present study proposes a solution to a frequent fracture tip screen out challenge faced in the petroleum industry. Thus, the developed modelling techniques provide petroleum engineers with a more suitable option for designing hydraulic fracturing operations, simultaneously modelling fracture propagation and fluid flow with proppant transport, and improves confidence by accurately tracking the distribution of proppants inside the fracture

Isogeometric Analysis for BIM-Based Design and Simulation of Sub-Rectangular Tunnel

The design and analysis of segmental tunnel lining is today often based on empirical solutions with simplified assumptions. This work showcases the application of Isogeometric Analysis (IGA) for computationally efficient simulations of tunnel linings [1, 2]. In our past research, we developed a design-through-analysis procedure that consists of i) parametric modeling of the segmented tunnel lining; ii) development of an IGA computational framework, iii) reconstruction of the BIM lining model for IGA analysis, and iv) simulation model for lining including a reconstructed IGA model, contact interfaces between the joints, and a non-linear soil-structure interaction model based on the Variational Hyperstatic Reaction Method (VHRM) [3].In this paper, we extend our method for the analysis of subrectangular tunnel linings and demonstrate its efficiency using the example of the Shanghai express tunnel. The advantage of our novel method is the flexibility in adapting the tunnel alignment with the help of NURBS/CAD technology. Employing the high-order geometry definition, convergence of the mesh refinement procedure can be obtained with much faster rate. As a result, the modelling effort and computational time are reduced significantly. Moreover, this approach allows to capture the bending moment with better regularity. The combination with an existing BIM modelling approach via geometryreconstruction leads to a very efficient framework for tunnel lining analysis and design

A mechanism-based approach to modeling ductile fracture.

Ductile fracture in metals has been observed to result from the nucleation, growth, and coalescence of voids. The evolution of this damage is inherently history dependent, affected by how time-varying stresses drive the formation of defect structures in the material. At some critically damaged state, the softening response of the material leads to strain localization across a surface that, under continued loading, becomes the faces of a crack in the material. Modeling localization of strain requires introduction of a length scale to make the energy dissipated in the localized zone well-defined. In this work, a cohesive zone approach is used to describe the post-bifurcation evolution of material within the localized zone. The relations are developed within a thermodynamically consistent framework that incorporates temperature and rate-dependent evolution relationships motivated by dislocation mechanics. As such, we do not prescribe the evolution of tractions with opening displacements across the localized zone a priori. The evolution of tractions is itself an outcome of the solution of particular, initial boundary value problems. The stress and internal state of the material at the point of bifurcation provides the initial conditions for the subsequent evolution of the cohesive zone. The models we develop are motivated by in-situ scanning electron microscopy of three-point bending experiments using 6061-T6 aluminum and 304L stainless steel, The in situ observations of the initiation and evolution of fracture zones reveal the scale over which the failure mechanisms act. In addition, these observations are essential for motivating the micromechanically-based models of the decohesion process that incorporate the effects of loading mode mixity, temperature, and loading rate. The response of these new cohesive zone relations is demonstrated by modeling the three-point bending configuration used for the experiments. In addition, we survey other methods with the potential to provide more detailed information about the near tip deformation fields

Asymptotic Fields for Cracks Terminating at Bi-Material Interface with Arbitrary Angles

The bi-material crack problem is an interesting and important topic in the field of fracture mechanics. The existing mainstream solutions, either analytical or computational, are commonly focused on some specific cases, e.g., a crack lying on exactly the bonded border of dissimilar materials, or a crack impinging upon a bi-material interface at a right angle. However, little attention is paid to the general cases, i.e., cracks approaching or attacking the material divided border arbitrarily, which is more likely to happen in the engineering products. With any possibility of the crack\u27s incidence angle, the asymmetric nature of the geometry and the materials property induces more difficulties in the mathematical formulation of the crack-tip stress field. The conventional analytical methods may not be a convenient way for the derivation, especially of the fracture parameters. For this end, in this study, the Williams\u27 expansion method is exploited to investigate the two-dimensional/three-dimensional fracture problem in which the crack terminates at a biomaterial interface with an arbitrary angle of incidence. The characteristic equation is obtained and solved to investigate the distribution of dominant roots. Mathematically, a matrix-based system is developed, which can be easily used to formulate the general asymptotic solution of the singular stress and displacement fields surrounding the crack-tip. The theory of singularities is introduced to represent the mixed-mode nature of the solution for the arbitrarily-oriented crack. This concept is further employed for the cases with complex singularities. After that, the relationship of the asymptotic field and the linear elastic fracture parameters is established directly through a linear system. In addition, taking advantage of the enriched element approach, the derived formulation in this study is programmed and implemented in a finite element analysis. This provides an efficient and effective method for simulating and solving different types of crack problems, especially with complicated geometries, loading patterns and material combinations. Then different mixed-mode fracture criteria for predicting the direction of crack growth are introduced. With the method discussed in this study, the maximum circumferential stress criterion is considered to be the most appropriate one, but needs to be slightly modified for multiple material problems. Finally, some examples of numerical solution of the asymptotic fields are demonstrated using the computed stress intensity factors and the developed matrix system for the general crack cases with an arbitrary impinging angle with respect to an interface. The numerical results for specific cases are compared with the existing references