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Simulating Bonnet Flutter - Unsteady Aerodynamics and its Structural Response
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on April 06, 2021 by SAE International in United States
Event: SAE WCX Digital Summit
Government regulations and consumer needs are driving automotive manufacturers to reduce vehicle energy consumption. However, this forms part of a complex landscape of regulation and customer needs. For instance, when reducing aerodynamic drag or vehicle weight for efficiency other important factors must be taken into account. This is seen in vehicle bonnet design. The bonnet is a large unsupported structure that is exposed to very high and often fluctuating aerodynamic loads, due to travelling in the wake of other vehicles. When travelling at high speed and in close proximity to other vehicles this unsteady aerodynamic loading can force the bonnet structure to vibrate, so-called bonnet flutter. A bonnet which is stiff enough to not flutter may be either too heavy for efficiency or insufficiently compliant to meet pedestrian safety requirements. On the other hand, a bonnet which flutters may be structurally compromised or undermine customer perceptions of vehicle quality. This work demonstrates a numerical simulation that is capable of predicting bonnet flutter during pre-prototype development, providing an approach which can allow engineers to balance the many demands made on this structure. This is accomplished using a combination of a Lattice-Boltzmann based Very Large Eddy flow simulation tool combined with finite element analysis. Both the unsteady surface pressures and subsequent structural response of the bonnet are compared to measurements made during track testing. This demonstrates that only by reproducing the real world unsteady flow environment we can effectively optimize the structure of vehicle bonnets, so that bonnet flutter can be avoided while at the same time minimizing structural complexity and weight.