CFD-based Modelling of Flow Conditions Capable of Inducing Hood Flutter

This paper presents a methodology for simulating Fluid Structure Interaction (FSI) for a typical vehicle bonnet (hood) under a range of onset flow conditions. The hood was chosen for this study, as it is one of the panels most prone to vibration; particularly given the trend to make vehicle panels lighter. Among the worst-case scenarios for inducing vibration is a panel being subjected to turbulent flow from vehicle wakes, and the sudden peak loads caused by emerging from a vehicle wake. This last case is typical of a passing manoeuvre, with the vehicle suddenly transitioning from being immersed in the wake of the leading vehicle, to being fully exposed to the free-stream flow. The transient flowfield was simulated for a range of onset flow conditions that could potentially be experienced on the open road, which may cause substantial vibration of susceptible vehicle panels. As these conditions cannot generally be replicated in the wind tunnel, a comprehensive numerical simulation methodology is required if this issue is to be addressed before vehicle prototypes are built. Transient aerodynamic simulations were performed using a Lattice-Boltzmann Method (LBM) and several different approaches were used to create dynamic onset flow conditions approaching the vehicle, such as simulating a: instantaneous transition to a twenty-degree yaw crosswind; twenty- degree harmonic yaw at 10 Hz; flow behind a 'bluff body' turbulence generating grid; 'passing disturbance'; vehicle in the wake of another vehicle, at a range of separation distances. A structural solver was used to predict the amplitude of vibration of the hood under these flow conditions. The amplitude response was largest when the frequency content of the oncoming flow matched the natural frequencies of the hood; the turbulence grid was found to produce the largest vibration for this reason. This study shows that transient aerodynamics simulation coupled with a structural solver can be used to predict the onset of vibration for different flow conditions, which could be encountered on the road but cannot be easily reproduced in the wind tunnel.