Rainald Lohner

Numerical simulation of shock interaction with complex geometry canisters

The objective of this research is to investigate shock interaction with complex geometry canisters suspended above a rigid elevated surface. Several geometries were examined in an effort to reduce the loads on the canisters during the shock diffraction phase. A new transient, two‐dimensional, finite‐element shock capturing scheme was utilized. Excellent shock resolution was demonstrated, as well as the efficiency of the newly developed adaptive refinement/coarsening algorithm. In addition to interesting shock wave propagation and interaction processes, the results demonstrated the capability of the new code to capture, and define in great detail, vortices shed from the downstream side of the canister.

Numerical Modeling of Multi-Phase, Multi-Material Blast- Structure Interactions

This paper describes results of a combined experimental and computational effort intended to validate predictions of a coupled CFD/CSD methodology of a multi-plate steel structure response to blast loading. To improve our understanding of the complex controlling physical mechanisms we formulated a simplified, multi-step approach. First, we investigated a precision test of a single event, the response of a single steel plate to a close-in bare charge. Next, we added a second plate to examine the response of the second plate to blast and flyer plate loading. Finally, we placed water-filled tube s under the first plate, to investigate the feasibility of using water tubes to disperse and di ssipate flyer-plate kinetic energy. The modeling of blast and structure (flyer plate) inter action with water required the development of a new numerical algorithm that combines flow solvers for both the gas and the liquid via an immersed body approach. Both solvers run concurrently. In the gas phase region (i.e. compressible flow), the velocities of the liquid were imposed wherever liquid is present. For the liquid region (incompressible (+VO F)), the pressures of the gas region were imposed wherever gas was present. This multiphase flow solver is then coupled to our structural mechanics solver to calculate structural response to blast and the feasibility of using fluid dampers. The results demonstrate that t he approach taken here is capable of efficiently modeling complex multiphase problems.

Inter-Element Stabilization for Linear Large-Deformation Elements to Solve Coupled CFD/CSD Blast and Impact problems

In this work a stabilized large deformation element suitable for real coupled fluid/solid simulations is presented. The element uses a mixed interpolation (Q1/P0): Standard continuous tri-linear finite element (FE) functions for the kinematic variables (displacements, velocities and accelerations), and a constant pressure per element (piecewise discontinuous pressures). It is well known that this type of element may show spurious pressure modes (chessboard mode) when is used to approximate incompressible fields (i.e. plastic flow, incompressible fluids, etc,). The mathematical explanation for such a behavior is the element inability of fulfilling the BB condition (the element is not div-stable). However, in Codina et al., the P1/P0 element is stabilized by means of a variational multiscale method (VMS), and it is used to solve the Stokes problem (incompressible flow equations at very low Reynolds number). Following the ideas of the cited reference, the authors of this work added to the standard large-deformation Lagrangian FE (Galerkin) formulation, a stabilization contribution which is only evaluated over the inter-element boundaries. Such a term enforces in a weak manner the pressure continuity and, in that way, it adds control over the inter-element pressure jumps (in general this procedure may be used to stabilize elements with discontinuous pressures). The method is clearly consistent: At the continuous level the pressures are continuous and the new term enforces such continuity at the discrete level. The stabilized IEOSS-Q1/P0 solid element (Inter-Element Orthogonal Subgrid-Scale Stabilized Q1/P0 element) was embedded into an efficient FE scheme to deal with large deformation problems. Others main ingredients of the formulation are: Some phenomenological material models (concrete, steel, sand, rock, etc,) to deal with damage and fracture of structures, a general contact algorithm which uses bin technology to perform the nodeface searching operations in a very efficient manner, and a cracking procedure to deal with the topology changes due to crack propagation and fragment formation. All the schemes, contact included, have been fully parallelized and coupled using a loose-embedded procedure with the well-established CFD (computational fluid dynamics) code FEFLO. Several real 3D coupled CFD/CSD cases, two of them with experimental comparison, are presented to validate the scheme.

Numerical Simulation Modeling of Aluminum Burning for Heavily Aluminized HE

The objective of this investigation is to model the burning of aluminum particles. An aluminum evaporation/reaction model was incorporated within a multi-phase flow model. The new scheme was applied to the simulation of blast wave evolution, where the HE model includes a significant percentage of aluminum particles, whose long-time burning and energy release must be considered. The evaporation of small aluminum particles with diameters from 5 to 500micron, reacting with oxygen, water, and carbon dioxide was initially tested in a 1-D code. The pressure profiles were significantly different than those obtained an inert aluminum model and from a single-reaction model which only considered a reaction with oxygen. Once validated, the models from 1-D code were incorporated into a 3-D production code. The newly developed 3D flow that includes the aluminum burning model was compared with experimental data. Results with the new model showed very good agreement in terms of the blast evolution, pressure, impulse, and energy behind the wave.

Numerical Simulation of Long-Duration Blast Wave Evolution in Confined Facilities

The objective of this research effort is to investigate the quasi-steady flow field produced by explosives in confined facilities. In this effort we modeled tests in which HE cylindrical charge were hung in the center of a room and detonated. The HE used for the test were C-4 and AFX 757. While C-4 is just slightly underoxidized and is typically modeled as an ideal explosive, AFX 757 includes a significant percentage of aluminum particles, whole long-time afterburning and energy release must be considered. The LLNL-produced thermo-chemical equilibrium algorithm, “Cheetah”, was used to estimate the remaining burnable detonation products. From these remaining species, the afterburning energy was computed and added to the flow field. Computations of the detonation and afterburn of two HE in the confined multi-room facility were performed. The results demonstrate excellent agreement with available experimental data in terms of blast wave time of arrival, peak shock amplitude, reverberation and total impulse (and hence, total energy release, via either the detonation or afterburn processes.

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.