Numerical Studies of Wave Generation Using Spiral Detonating Cord

Abstract. The control of underwater explosions is an industrial concern. In this paper, a comparison of experimental and numerical results of high-pressure generation using underwater explosion of spiral detonating cord is presented. To demonstrate that the converging process of underwater shock wave yields high pressure near the spiral center, the experimental investigation aims to compare underwater shock wave pressures obtained with several detonating cord geometrical configurations and study the wave converging process for a spiral cord. Because the experimental approach of these fast transient events is expensive and time-consuming, numerical simulations of experimental cases by using multi-material Eulerian formulation are carried out. The multi-material Eulerian, which is a particular multi-material ALE (Arbitrary Lagrangian Eulerian) formulation was successfully used in many industrial applications involving computational fluid dynamic problems. By using an explicit finite element method, a good agreement between numerical and experimental results will valid multi-material Eulerian formulation abilities to solve accurately underwater shock wave problems for spiral detonating cord in various shapes

EULER-LAGRANGE COUPLING WITH DEFORMABLE POROUS SHELLS

A newly developed approach for tridimensional fluidstructure interaction with a deformable thin porous media is presented under the framework of the LS-DYNA software. The method presented couples a Arbitrary Lagrange Euler formulation for the fluid dynamics and a updated Lagrangian finite element formulation for the thin porous medium dynamics. The interaction between the fluid and porous medium are handled by a Euler-Lagrange coupling, for which the fluid and structure meshes are superimposed without matching. The coupling force is computed with an anisotropic Ergun porous flow model. As test case, the method is applied to an anchored porous MIL-c-7020 type III fabric placed in an air stream

Application of the penalty coupling method for the analysis of blood vessels

Due to the significant health and economic impact of blood vessel diseases on modern society, its analysis is becoming of increasing importance for the medical sciences. The complexity of the vascular system, its dynamics and material characteristics all make it an ideal candidate for analysis through fluid structure interaction (FSI) simulations. FSI is a relatively new approach in numerical analysis and enables the multi-physical analysis of problems, yielding a higher accuracy of results than could be possible when using a single physics code to analyse the same category of problems. This paper introduces the concepts behind the Arbitrary Lagrangian Eulerian (ALE) formulation using the penalty coupling method. It moves on to present a validation case and compares it to available simulation results from the literature using a different FSI method. Results were found to correspond well to the comparison case as well as basic theory

Observations from Preliminary Experiments on Spatial and Temporal Pressure Measurements from Near-Field Free Air Explosions

It is self-evident that a crucial step in analysing the performance of protective structures is to be able to accurately quantify the blast load arising from a high explosive detonation. For structures located near to the source of a high explosive detonation, the resulting pressure is extremely high in magnitude and highly non-uniform over the face of the target. There exists very little direct measurement of blast parameters in the nearfield, mainly attributed to the lack of instrumentation sufficiently robust to survive extreme loading events yet sensitive enough to capture salient features of the blast. Instead literature guidance is informed largely by early numerical analyses and parametric studies. Furthermore, the lack of an accurate, reliable data set has prevented subsequent numerical analyses from being validated against experimental trials. This paper presents an experimental methodology that has been developed in part to enable such experimental data to be gathered. The experimental apparatus comprises an array of Hopkinson pressure bars, fitted through holes in a target, with the loaded faces of the bars flush with the target face. Thus, the bars are exposed to the normally or obliquely reflected shocks from the impingement of the blast wave with the target. Pressure-time recordings are presented along with associated Arbitary-Langrangian-Eulerian modelling using the LS-DYNA explicit numerical code. Experimental results are corrected for the effects of dispersion of the propagating waves in the pressure bars, enabling accurate characterisation of the peak pressures and impulses from these loadings. The combined results are used to make comments on the mechanism of the pressure load for very near-field blast events

A Numerical Investigation of Blast Loading and Clearing on Small Targets

When a blast wave strikes a finite target, diffraction of the blast wave around the free edge causes a rarefaction clearing wave to propagate along the loaded face and relieve the pressure acting at any point it passes over. For small targets, the time taken for this clearing wave to traverse the loaded face will be small in relation to the duration of loading. Previous studies have not shown what happens in the late-time stages of clearing relief, nor the mechanism by which the cleared reflected pressure decays to approach the incident pressure. Current design guidance assumes a series of interacting clearing waves propagate over the target face - this assumption is tested in this article by using numerical analysis to evaluate the blast pressure acting on small targets subjected to blast loads. It is shown that repeat propagations of the rarefaction waves do not occur and new model is proposed, based on an over-expanded region of air in front of the loaded face of the target

New Mesh Relaxation Technique P V P 2 0 0 4 -2 8 6 3 in Multi-Material ALE Applications

dissipation errors generated when treating the advective terms in the governing equations. The Arbitrary Lagrangian-Eulerian (ALE) method is a method that contains both pure Lagrangian and pure Eulerian formulations. It is assumed to be capable to control mesh geometry independently from material geometry. However for transient problems involving pressure wave, this method will not allow to maintain a fine mesh in the vicinity of the shock wave for accurate solution. A new mesh relaxation method for explicit multi-material arbitrary Lagrangian Eulerian f'mite element simulations has been developed to keep an as "Lagrange like" fluid mesh as possible as in the vicinity of shock fronts, while at the same time keeping the mesh distortions on an acceptable level. However, the relaxation parameter must be defined for general applications of high pressures, it is the objective of this work. In this paper we present numerical results of three shock waves problems. For every application, numerical results will be compared with the experimental results in order to improve to understanding how the relaxation parameter is chosen

Transient Response of a Projectile in Gun Launch Simulation Using Lagrangian and Ale Methods

This paper describes the usefulness of Lagrangian and arbitrary Lagrangian/Eulerian (ALE) methods in simulating the gun launch dynamics of a generic artillery component subjected to launch simulation in an air gun test. Lagrangian and ALE methods are used to simulate the impact mitigation environment in which the kinetic energy of a projectile is absorbed by the crushing of aluminum honeycomb mitigator. In order to solve the problem due to high impact penetration, a new fluid structure coupling algorithm is developed and implemented in LS-DYNA, a three dimensional FEM code. The fluid structure coupling algorithm used in this paper combined with ALE formulation for the aluminum honeycomb mitigator allows to solve problems for which the contact algorithm in the Lagrangian calculation fails due to high mesh distortion. The numerical method used for the fluid and fluid structure coupling is discussed. A new coupling method is used in order to prevent mesh distortion. Issues related to the effectiveness of these methods in simulating a high degree of distortion of Aluminum honeycomb mitigator with the commonly used material models (metallic honeycomb and crushable foam) are discussed. Both computational methods lead to the same prediction for the deceleration of the test projectile and are able to simulate the behavior of the projectile. Good agreement between the test results and the predicted projectile response is achieved via the presented models and the methods employed

Initialisation of volume fraction in fluid/structure interaction problem

In this paper, an algorithm that generates volume fraction initialisation for fluid-structure interaction problems is presented and implemented in an Arbitrary Lagrangian Eulerian (ALE) code. This algorithm improves the flexibility and efficiency of multi-material ALE formulations, enabling fluid-structure coupling problems to be dealt with, in which complex-shaped structures are embedded in a Cartesian fluid mesh, and where several fluid materials are involved. When implemented in a finite element code the algorithm will be extremely useful in the modelling of highly transient problems involving fluid-structure interactions and complex structural topologies. In such problems the volume fraction of each cell must be accurately initialised in order for the fluid-structure coupling algorithm to be effective. In many fluid-structure coupling problems if the volume fraction initialisation is not processed properly, the coupling algorithm leads to numerical fluid leakage through the structure involving an erroneous solution. In this paper the volume fraction initialisation method is described in detail and applied to the modelling of a cylindrical shock tube problem and to a crash problem involving a fuel tank of complex geometry under braking conditions

Numerical and experimental investigations of water hammers in nuclear industry

In nuclear and petroleum industries, supply pipes are often exposed to high pressure loading which can cause to the structure high strains, plasticity and even, in the worst scenario, failure. Fast Hydraulic Transient phenomena such as Water Hammers (WHs) are of this type. It generates a pressure wave that propagates in the pipe causing high stress. Such phenomena are of the order of few msecs and numerical simulation can offer a better understanding and an accurate evaluation of the dynamic complex phenomenon including fluid-structure interaction, multi-phase flow, cavitation … For the last decades, the modeling of phase change taking into account the cavitation effects has been at the centre of many industrial applications (chemical engineering, mechanical engineering, …) and has a direct impact on the industry as it might cause damages to the installation (pumps, propellers, control valves, …). In this paper, numerical simulation using FSI algorithm and One-Fluid Cavitation models ("Cut-Off" and "HEM (Homogeneous Equilibrium Model) Phase-Change" introduced by Saurel et al. [1]) of WHs including cavitation effects is presented