Mhamed Souli

ALE and Fluid/Structure Interaction in LS-DYNA

Fluid-structure interactions play an important role in many different types of real-world situations and industrial applications involving large structural deformation and material or geometric nonlinearities. Numerical problems due to element distortions limit the applicability of a Lagrangian description of motion when modeling large deformation processes. An alternative technique is the multi-material Eulerian formulation for which the material flows through a mesh, fixed in space and each element is allowed to contain a mixture of different materials. The method completely avoids element distortions and it can, through an Eulerian-Lagrangian coupling algorithm, be combined with a Lagrangian description of motion for parts of the model. The Eulerian formulation is not free from numerical problems. There are dissipation and dispersion problems associated with the flux of mass between elements. In addition, many elements might be needed for the Eulerian mesh to enclose the whole space where the material will be located during the simulated event. This is where the multi-material arbitrary Lagrangian-Eulerian (ALE) formulation has its advantages. By translating, rotating and deforming the multi-material mesh in a controlled way, the mass flux between elements can be minimized and the mesh size can be kept smaller than in an Eulerian model. A new Fluid Structure coupling algorithms based on the penalty method is presented in this paper. The coupling algorithm and improved multi-material ALE-capabilities have made LS-DYNA an efficient tool for analyzing large deformation processes, such as bird strike events, forging operations and penetration problems and airbag simulations. This paper contains five example problems that illustrate the current features of the code.Copyright

Metaheuristic algorithms for optimisation of infrared heating in thermoforming process

Thanks to the low cost and good formability of thermoplastic materials, thermoforming process is widely used in plastic processing. However, some problems remain to be solved, notably the non-uniform thickness distribution caused by uneven energy distribution in the plastic sheet during infrared heating. To overcome this problem, the energy intercepted and absorbed by the plastic sheet must be carefully optimised. In this study, two metaheuristic approaches [simulated annealing (SA) and genetic algorithms (GAs) ] are used and compared for optimising the energy intercepted by the material sheet. Optimisation results obtained for three examples (solution quality, computing time, and convergence) were analysed.

Fatigue life estimate of landing Gear's leg using modal analysis

The present work concerns solving Noise, Vibration and Harshness (NVH) and fatigue based on Power Spectrum Density (PSD) analysis of a landing gears leg for an Un-Manned Aerial Vehicle (UAV). This analysis includes random vibration and high-cycle fatigue analysis in a random vibration environment. In this analysis, the cumulative damage ratio is computed using material S-N (Stress-Number of cycles) fatigue curve. Dirlik method is used for the analysis of lifetime as it is proven to provide accurate results for large number of applications, both in automotive and aerospace industry. It is also compared to other methods that have been developed in LS-DYNA® as well. The input acceleration PSD data are provided through measurements. The obtained analysis results shows that although the landing gear design is safe according to dynamic and static load, its service life is about 3037 hours due to random vibration effect.

Fluid-structure interaction for water hammers effects in petroleum and nuclear plants

Fluid-Structure Interaction (FSI) becomes more and more the focus of computational engineering in Petroleum and Nuclear Industry in the last years. These problems are computer time consuming and require new stable and accurate coupling algorithms to be solved. For the last decades, the new development of coupling algorithms, and the increasing of computer performance have allowed to solve some of these problems and some more physical applications that has not been accessible in the past; in the future this trend is supposed to continue to take into account more realistic problem.In this presentation, numerical simulation using FSI capabilities in LS-DYNA, of hydrodynamic ram pressure effect occurring in nuclear industry is presented.

Deployment of a Neo-Hookean membrane: experimental and numerical analysis

The aim of this research is to assess the response of a thin membrane subjected to high-pressure gas loading for inflation. This procedure is applied during the design process of the membrane structure to allow the product to resist high-pressure loading and to further characterize the hyper-elastic material. The simulation in this work considers the standard procedures used in the LS-DYNA software, which applies such assumptions as a uniform airbag pressure and temperature in addition to a more recently developed procedure that takes into account the fluid-structure interaction between the inflation gas source and the hyper-elastic membrane; this approach is referred to as the Arbitrary Lagrangian Eulerian (ALE) formulation. Until recently, to simulate the inflation of the hyperelastic membrane, a uniform pressure based on a thermodynamic model or experimental test has been applied to the structure as the boundary conditions. To conserve CPU time, this work combines both methods; the fluid structure coupling method is used at an earlier stage in which the fluid is modeled using full hydrodynamic equations, and at the later stage, the uniform pressure procedure is applied, and the fluid mesh and analysis are removed from the computation. Both simulations were compared to test data, indicating satisfactory correlation with the more recently developed procedure, the ALE theory, which showed the greatest accuracy both in terms of graphical and schematic comparison, particularly in the early stages of the inflation process. As a result, the new simulation procedure model can be applied to research on the effects of design changes in the new membrane.

Numerical investigation of vibration and dynamic pressure of a vertical axis wind turbine

In the environmental field, the problems of noise reduction have become a major preoccupation, particularly on the noise generated by the acoustic radiation pressure produced by wind turbines. This paper is aimed at presenting the investigation on the application of variational indirect boundary element method for study the acoustic radiation pressure produced by vertical-axis wind turbine. For this initiative, we considered Neumann boundary condition. The formulation has two advantages: the first one is to avoid the meshing of the fluid domain; the second advantage is to treat the singular integral of the Greens function, solution of fundamental solution of the wave equation in frequency domain.

A delayed remap technique in multi-material ALE methods

A new mesh refinement method for multi-material ALE formulations is presented. The computational timestep of this approach is divided in 2 steps: a so-called Lagrangian step, during which the mesh deforms with the update of the solution, and a so-called Eulerian step, during which the mesh is remapped in order to preserve the mesh regularity and refine in the vicinity of the shock front. As test case the method is applied to the propagation of an explosive airblast, for which experimental results are available.

Simulation of Acoustical Response Using Rayleigh Method

The Boundary Element Method is one of the most used techniques for the simulation of acoustic problems especially for external ones. However, it leads to large computational time because of the complex character of the resulting linear system and the calculation of its different terms by surface integration. In this paper, the Rayleigh method is used to calculate the acoustical pressure at any point in the space. This method is very fast since it does not need to construct and to solve a linear system.Copyright

BEM Modeling of Non Linear Systems Noise

A noticeable part of acoustic environment is made of sounds radiated by vibrating structures. Most of these sounds are considered as undesirable noise. Especially, those radiated from complex processes involving several non linear effects. In the present work, the transient response of mechanical system is computed first by using LSDYNA, an explicit finite element code for general fluid, structure and fluid-structure interaction problems. By applying the FFT, it’s transformed into frequency response which allows to use BEM for computing the noise radiated at any point into space. Results are checked for simple acoustic and classical vibroacoustic problems before applying it for a non linear case. The numerical examples show the efficiency of the present method.Copyright