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Dynamic fracture evolution.



At Los Alamos National Laboratory (LANL) researchers have developed a hybrid multi-physics software package called HOSS - the Hybrid Optimization Software Suite. Integrating computational fluid dynamics (CFD), with state-of-the-art combined finite-discrete element methodologies (FDEM) - consisting of finite element analysis (FEA) and discrete element methods (DEM) - into a single simulation platform, HOSS is capable of solving complex problems for a myriad of engineering disciplines, industrial applications, and scientific research. Whether on a personal desktop or high-performance computing clusters, HOSS’ parallelization allows it to efficiently handle millions of interacting and fracturing solids and/or particle systems. HOSS has been recognized as a 2016 R&D top 100 finalist.

Functional Capabilities

In the last decade, scientific application-driven developments of FDEM have taken place at Los Alamos National Laboratory, enabling researchers to shift the boundaries of FDEM solutions towards next-generation algorithms.
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When using lower order 2D or 3D finite element formulations, such as constant strain triangles (CSTRI) and tetrahedrons (CSTET), an issue that often appears is that these types of elements will numerically lock. This is due to their full integration scheme and is often recognized by a checkerboard pattern in the stress distributions. To alleviate this deficiency composite triangular (COMPTRI) and tetrahedral (COMPTET) finite elements were developed. Both COMPTRI and COMPTET formulations use selective integration of stresses in order to avoid this artificial stiff response or locking. The developed finite elements utilize a unified hypo-/hyper-elastic approach that allows linkage to user-defined (isotropic or anisotropic) material models. A demonstration of HOSS’ generalized anisotropic deformation kinematics is shown below.
a) Model of a representative geologic structure (all dimensions in meters). Four different types of materials are included in the model. b-e) Comparison of the wave propagation through the geologic medium: anisotropic material (b, c) and isotropic material (d, e). Units for speed are meters per second.

Lei, Z., Rougier, E., Knight, E.E., Munjiza, A., & Viswanathan, H. 2016. A generalized anisotropic deformation formulation for geomaterials. Comp. Part. Mech 3, 215-228.

Lei, Z., Rougier, E., Knight, E.E., Frash, L., Carey, J.W., & Viswanathan, H. 2016. A nonlocking composite tetrahedron element for the combined finite discrete element method. Engineering Computations. 33(7), 1929-1956.

The problem with existing distributed potential contact force algorithms is that the potential field introduces artificial numerical non-smoothness in the contact force, i.e. the contact forces calculated experience a jump (in amplitude and/or direction) when the contact points move from one finite element to another finite element. To overcome this issue, a new solution, which is named the Smooth Contact Algorithm (SCA) was developed in HOSS.

In the SCA, a smooth potential field is introduced according to the global geometry information of each discrete element. In particularly, the SCA calculates contact potential at nodes of the finite element mesh by taking into account nodal connectivity and existing discrete element boundaries. The contact force is then calculated as a function of the gradient of the potential field. Thus, a smooth contact evolution for a smooth surface is recovered.

Both the jump in the amplitude and direction of the contact force calculated in (a) can be observed when the contact point moves from one finite element to another finite element. The contact force calculated in (b) is smooth both in terms of amplitude and direction as the contact point moves from one finite element to another.

Lei, Z., Rougier, E., Bryan, E., Munjiza, A. A smooth contact algorithm for the combined finite discrete element method. Computational Particle Mechanics, 2020.

Cohesive Zone Models (CZMs) usually are used to simulate the fracture and fragmentation of solids. Existing CZMs have different disadvantages, such as artificial compliance and time-discontinuity. In order to avoid these drawbacks, the Unified Cohesize Zone Model (UCZM) was implemented into HOSS. In UCZM, damage surfaces are dynamically inserted into the model according to the local stress state, much like the extrinsic CZM approach. However, in order to avoid sudden jumps in the simulations, the state variables are smoothly transitioned from continua to discontinua through an algorithm that properly balances the nodal forces during the process, while maintaining highly efficient simulations.


Lei Z, Rougier E, Knight EE, Munjiza A. A libraries-based multidimensional fracture workbench. US Provisional Patent Application No. 62906674, filed 9/26/2019

In 2013 LANL unveiled its Integrated Solid-Fluid (ISF) Interaction Solver (US Patent #US10275551B2) to accurately resolve the interaction between fluid and solid domains under an enhanced fluid pressure boundary condition. The HOSS-ISF accounts for fluid flow through fracturing porous solids, fluid flow through crack manifolds, pressure wave propagation through fluid and fluid-solid interaction and is applicable to hydraulic-fracture problems, enhanced geothermal systems, or carbon capture related systems. One of the main advantages of the HOSS-ISF is that the fluid phase is described using the same grid as the solid phase via a modified Eulerian formulation. This eliminates the need of continuously mapping variables between the fluid and solid domains. Most importantly, the HOSS ISF features an explicit time integration solver with an aperture-independent critical time step size.
HOSS-ISF utilizes a pressure node at the end of each individual fracture and introducing a “fluid velocity node” at the middle of each individual fracture. The pressure nodes are used to calculate the fluid pressure according to an appropriate material law while the velocity node resolves the dynamic equilibrium of the flow system.

Lei, Z., Rougier, E., Munjiza, A., Viswanathan, H., & Knight, E.E. Simulation of discrete cracks driven by nearly incompressible fluid via 2D combined finite‐discrete element method. International Journal for Numerical and Analytical Methods in Geomechanics, 43:1724-1743, 2019.

Rougier, E., Knight, E.E., & Munjiza, A. Integrated solver for fluid driven fracture and fragmentation. US Patent US10275551B2, granted 30 April 2019.

A different type of fluid-solid interaction solver is needed to simulate the effects of an explosively-generated shock wave propagating through air and impinging on building structures. For this purpose the FSIS (Fluid-Solid Interaction Solver) was developed and implemented into HOSS. Some of the main design parameters for the FSIS were: a) to be explicit in terms of time integration and maintain a time step similar to the solid phase time step; b) to allow for several fluid grids that could move with the solid; and c) to allow for the resolution of multi-phase flow problems.
The FSIS combines a traditional FDEM Lagrangian framework (i.e., the solid solver) with fluid domains represented on Eulerian domains. The solver accounts for fluid flow through fracturing porous solid, fluid flow through crack manifolds, and pressure wave propagation through fluid. The interaction between the solid and fluid domains is resolved via a modified version of the immerse boundary method.
FSIS Example: A solid disk is placed inside the fluid domain, and a high-energy source is placed inside the fluid at the center of the disk. The high-energy source is initiated at the center, which triggers the progressive initiation of the rest of the charge. A snapshot shows the high temperature/high pressure fluid jetting from the cavity through the fractured solid.

Munjiza, A., Rougier, E., Lei, Z. & Knight, E.E. 2020. FSIS – A novel Fluid-Solid Interaction Solver for Fracturing and Fragmenting Solids. Comp. Part. Mech.

Coupled Thermo-Hydro-Mechanical (THM) simulators are necessary to explore the physical processes involved in complex subsurface operations such as unconventional fossil energy production and underground nuclear test detection. In HOSS pre-existing discrete fractures in the rock media can be explicitly modeled and their influences are sensed by both the solid and fluid domains. In terms of the linking of the two codes, HOSS provides information such as porosity and permeability to the fluid solver, while the fluid solver passes fluid state variables such as pressure and temperature back to HOSS.
Thermal-hydro stress mapping in a rock matrix after 20 years injection. On the left a constant pressure of 10 MPa was applied, while the right side is fixed to 0 MPa. On the top and bottom a no-flow Neumann boundary was also applied. The domain is initially at 180 degrees Celsius. The inflow temperature at the left side of the domain was set to 50 degrees Celsius.

Jansen, G., Valley, B., Miller, S.A. THERMAID-A matlab package for thermo-hydraulic modeling and fracture stability analysis in fractured reservoirs. arXiv: 1806.10942.

Knight, E.E., Rougier, E., Lei, z., Euser, B., Chau, V., Boyce, S., Gao, K., Okubo, K., Froment, M. 2020. HOSS: An Implementation of the Combined Finite-Discrete Element Method. Comp. Part. Mech.

Utilizing a generalized massively parallel solution based on a virtual parallel machine for FDEM, it has been demonstrated that large increases to the number of processors only results in marginal increases in the specific time. This solution is problem-specific (as opposed to computer architecture specific) thus, a parallel FDEM code can be adapted to different hardware platforms ranging from desktops to 10,000+ node high-performance-computing (HPC) clusters.

Speed up as a function of number of processors: more than 900 times speedup was obtained for a typic FDEM problem using 1024 processors.

Lei, Z., Rougier, E., Knight, E.E., & Munjiza, A. 2014. A Framework for Grand Scale Parallelization of the Combined Finite Discrete Element Method in 2D. Comp. Part. Mech. 1, 307-319.