Development of CFD Capability for Airbag Out-of-Position Applications

The need for Out-Of-Position (OOP) simulation capabilities in crash safety software has risen in importance in the automotive industry after the final ruling of FMVSS 208 by the National Highway Transportation Safety Administration (NHTSA) in 2000. However, because of current technical challenges, the thermodynamics airbag models are not capable of either accurately simulating the flow-bag interactions under OOP conditions, or differentiating the effect of some important design changes, such as vent locations, inflator configuration, and flow diverging devices. The development of these capabilities entails overcoming tremendous technical challenges and numerical difficulties in computational fluid dynamics (CFD) and crash simulations due to the complexity and extreme conditions of OOP. This paper summarizes the developments of algorithms used in the context of an Arbitrary Lagrangian-Eulerian (ALE) formulation. The main developments of this study include the gas dynamic model for mixing gases, the special treatment of inflator gas flows, a penalty based fluid-structure coupling algorithm, and a permeability algorithm for porous fabrics. To expedite the developments and resolve the technical difficulties, a set of benchmark problems was used in this study. Each of the benchmark problems addresses specific technical difficulties of airbag OOP simulations by comparing simulation results to analytical solutions, well-known numerical solutions, or test results. The benchmark set was designed to start from simple CFD problems and progress to the most complicated OOP applications such that the weakness and algorithmic errors of the simulation code can be easily identified. Simulation results of the benchmark problems and the issues addressed by them are discussed in this paper. The simulation results have demonstrated that LS-DYNA now has the capabilities to simulate certain OOP problems.