Charles Charman

The numerical simulation of strongly unsteady flow with hundreds of moving bodies

A methodology for the simulation of strongly unsteady flows with hundreds of moving bodies has been developed. An unstructured grid, high-order, monotonicity preserving, ALE solver with automatic refinement and remeshing capabilities was enhanced by adding equations of state for high explosives, deactivation techniques and optimal data structures to minimize CPU overheads, automatic recovery of CAD data from discrete data, two new remeshing options, and a number of visualization tools for the preprocessing phase of large runs. The combination of these improvements has enabled the simulation of strongly unsteady flows with hundreds of moving bodies. Several examples demonstrate the effectiveness of the proposed methodology

Large-scale fluid-structure interaction simulations

Combining computational-science disciplines, such as in fluid-structure interaction simulations, introduces a number of problems. The authors offer a convenient and cost-effective approach for coupling computational fluid dynamics (CFD) and computational structural dynamics (CSD) codes without rewriting them. With the advancement of numerical techniques and the advent, first, of affordable 3D graphics workstations and scalable compute servers, and, more recently, PCs with sufficiently large memory and 3D graphics cards, public-domain and commercial software for each of the computational core disciplines has matured rapidly and received wide acceptance in the design and analysis process. Most of these packages are now at the threshold mesh generation pre-processor. This has prompted the development of the next logical step: multidisciplinary links of codes, a trend that is clearly documented by the growing number of publications and software releases in this area. In this paper, we concentrate on fluid-structure and fluid-structure-thermal interaction, in which changes of geometry due to fluid pressure, shear, and heat loads considerably affect the flowfield, changing die loads in turn. Problems in this category include: steady-state aerodynamics of wings under cruise conditions; aeroelasticity of vibrating - that is, elastic - structures such as flutter and buzz (aeroplanes and turbines), galloping (cables and bridges), and maneuvering and control (missiles and drones); weak and nonlinear structures, such as wetted membranes (parachutes and tents) and biological tissues (hearts and blood vessels); and strong and nonlinear structures, such as shock-structure interaction (command and control centers, military vehicles) and hypersonic flight vehicles.

Recent Developments of a Coupled CFD/CSD Methodology

A recently developed loose-coupling algorithm that combines state-of-the-art Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) methodologies has been applied to the simulations of weapon-structure interactions. The coupled methodology enables cost-effective simulation of fluid-structure interactions with a particular emphasis on detonation and shock interaction. The coupling incorporates two codes representing the state-of-the-art in their respective areas: FEFLO98 for the Computational Fluid Dynamics and DYNA3D for the Computational Structural Dynamics simulation. An application of the methodology to a case of weapon detonation and fragmentation is presented, as well as fragment and airblast interaction with a steel wall. Finally, we present results of simulating airblast interaction with a reinforced concrete wall, in which concrete and steel rebar failure and concrete break-up to thousands of chunks and dust particles are demonstrated.

Convergence study for the discrete particle method

Publisher Summary The Discrete Particle Method (DPM) is a numerical technique in the class of the discrete element methods for the modeling of cementitious material in the pre- and post-failure regimes. Because the DPM is not based on continuum mechanics, the conventional convergence properties of Galerkin based methods, such as the finite element method, are not expected to apply. This chapter presents the results of a study to assess the convergence properties of the DPM for elastic problems. The DPM belongs to the general class of methods denoted as discrete element methods. The benchmark problem is based on the vibrations of a concrete beam free in space. Axial oscillations are used to adjust DPM model parameters, bending oscillations are used to test convergence characteristics. Results show that the DPM solution converges to the equivalent converged finite element solution.

Adaptive Embedded Unstructured Grid Methods

A simple embedded domain method for node-based unstructured grid solvers is presented. The key modification of the original, edge-based solver is to remove all geometryparameters (essentially the normals) belonging to edges cut by embedded surface faces. Several techniques to improve the treatment of boundary points close to the immersed surfaces are explored. Alternatively, higher-order boundary conditions are achieved by duplicating crossed edges and their endpoints. Adaptive mesh refinement based on proximity to or the curvature of the embedded CSD surfaces is used to enhance the accuracy of the solution. User-defined or automatic deactivation for the regions inside immersed solid bodies is employed to avoid unnecessary work. Recent work has led to a notable improvement in speed via suitable data structures, the option to treat dispersed particles in the context of embedded surfaces, a direct link to Discrete Particle Methods (DPM), a volume to surface meshing technique that obtains bodyfitted grids by post-processing adaptive embedded grids; and links to simplified CSD models. Several examples are included that show the viability of this approach for inviscid and viscous, compressible and incompressible, steady and unsteady flows, as well as coupled fluid-structure problems.

Fluid-structure interaction simulations using parallel computers

A methodology to simulate large-scale fluid-structure interaction problems on parallel machines has been developed. Particular emphasis was placed on shock-structure interaction problems. For the fluid, a high-resolution FEM-FCT solver based on unstructured grids is used. The surface motion is handled either by moving, body fitted grids, or via surface embedding. For the structure, a Lagrangean large-deformation finite element code is employed. The coupled system is solved using a loose coupling algorithm, with position and velocity interpolation and force projection. Several examples, run on parallel machines, demonstrate the range of applicability of the proposed methodology.

Recent development of a coupled CFD/CSD methodology using an embedded approach

A new algorithm for modeling the response of structures to severe airblast and fragment loading, including the modeling of large plastic deformations, structural failure and break-up, is described in this paper. The coupled Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) methodologies required to describe these phenomena include the FEFL098 flow solver and DYNA3D structural solver. The original coupling between the two domains was based on the so-called “glued-mesh” approach, where the CFD and CSD interfaces match. Recent failure of this approach to model severe structural deformation, as well as crack propagation in steel and concrete, led us to the development and use of the “embedded-mesh” approach. Here, the CSD objects float through the CFD domain. While each approach has its own advantages, limitations and deficiencies, the embedded approach was proven to be superior for the problems modeled here. Critical applications of both approaches are described, including weapon detonation and fragmentation, airblast interaction with a reinforced concrete wall, and fragment/airblast interaction with a steel wall. The final applications model the interaction of an external airblast with a generic steel ship hull and a generic multi-chamber steel tower.

Development and Applications of a Coupled CFD/CSD Methodology Using an Embedded CSD Approach

This paper describes recent algorithm developments and select applications of a program that couples parallel Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) methodologies. FEFL098 is the CFD code used while DYNA3D handles the CSD portion. FEFL098 solves the time-dependent, compressible Euler and Reynolds-Averaged Navier-Stokes equations on an unstructured mesh of tetrahedral elements. DYNA3D solves explicitly the large deformation, large strain formulation equations on an unstructured grid composed of bricks and hexahedral elements.

A Coupled CFD/CSD Methodology for Simulating Structural Response to Airblast and Fragment

Several classes of important engineering problems require the concurrent application of CFD and CSD techniques. Currently, attempts to model these problems are solved either iteratively, requiring several cycles of CFD run followed by CSD run, or by assuming that the CFD and CSD solutions can be decoupled. The various efforts to develop a fluid/structure coupling can be classified according to the complexity level of the approximations used for each of the domains. These range from simple 6 DOF integration to finite elements with complex models for elasto-plastic materials with rupture laws and contact. Similarly, the fluid dynamics approximations range from the potential flow (irotational, inviscid, isentropic flows) to the full Navier-Stokes set of equations. The present research interests focus on non-linear applications, in particular, structures that experience severe deformations due to blast loads. Hence, the fluid applies either the Euler or Reynolds-Averaged Navier-Stokes equations, while elasto-plastic materials with rupture criteria are used for the structural modeling.