In the Wake of Others: Unsteady Bonnet Surface Pressure Predictions and Measurements

In use cars often drive through the wakes of other vehicles. It has long been appreciated that this imposes a fluctuating onset flow which can excite a structural response in vehicle panels, particularly the bonnet. This structure must be designed to be robust to such excitation to guarantee structural integrity and maintain customer expectations of quality. As we move towards autonomous vehicles and exploit platoons for drag reduction, this onset flow condition merits further attention. The work reported here comprises both measurements and simulation capturing the unsteady pressure distribution over the bonnet of an SUV following a similar vehicle at high speed and in relatively close proximity. Measurements were taken during track testing and include 48 static measurement locations distributed over the bonnet where the unsteady static pressures were recorded. This is complemented by computational fluid dynamics simulations using a commercially available Lattice-Boltzmann based Very Large Eddy Simulation approach. Comparisons between experiment and simulation are based on time-averaged static pressure coefficients, pressure time-histories and spectra. This provides a dataset that includes both time-averaged and unsteady correlation, demonstrating that such simulations can provide a good representation of on-track effects.