Transient Fluid Structure Simulation of Ground Vehicles

Ground vehicle aerodynamics is an important stage in the design of a car. The field is currently well established, and the final goal is to decrease the lift to drag ratio functional by maintaining certain design constraints. Traditionally the studies are performed in wind tunnels but with the advance of hardware and software in the past two decades most of the design is now performed by simulation using Computational Fluid Dynamics (CFD). The automotive industry has embraced these techniques due to their low cost and high accuracy. But in recent year the tougher environmental regulations together with the natural evolution of technology have pushed the industry into new lighter materials, thinner panels and more compact parts distribution. These changes bring new challenges to the design process. The clay models traditionally used in wind tunnels cannot predict the response of the real structures subject to aerodynamic or thermal loads. Traditional CFD simulations are faced with similar limitations. Furthermore the internal organization of the CAE departments are not catching up fast enough to adapt to this new reality and the result is that last minute unexpected behaviors happen during drive test conditions forcing late modifications and the retooling of parts, greatly increasing the design cost. It is our goal to introduce in the design cycle intermediate steps where coupled Multiphysics simulations will be used to anticipate unexpected behaviors and correct the design before the tooling stage. In this work a real world model of a mid-size sedan is used as a showcase of the different possibilities that are available in LS-DYNA® to perform CFD together with Fluid Structure Interaction (FSI) to study the response of different structural parts of the vehicle subject to aerodynamic loads. One of the main advantages is that complex structural parts are "borrowed" from a LS-DYNA crash model and easily introduced into the CFD model greatly simplifying the process. All the material settings and geometry will be automatically ready for the FSI simulation.