This study examines the unsteady underbody aerodynamics of a simplified road vehicle using Large Eddy Simulation. Time-resolved pressure and velocity fields are analysed using Proper Orthogonal Decomposition, Spectral Proper Orthogonal Decomposition, Extended Proper Orthogonal Decomposition and spectral analysis to determine spatio-temporal interactions within the underbody flow and base wake. Two-phase dependent front-wheel shedding states are identified; an ‘in-phase’ state that leads to a streamwise pumping motion between the rear wheels, and an ‘anti-phase’ state that leads to a lateral bias across the rear axle. An analysis of the floor pressure to underbody velocity correlation based on Extended Proper Orthogonal Decomposition identifies a coherent floor pressure field containing approximately 55% of the pressure fluctuation energy that is linearly and deterministically correlated with a coherent underbody velocity field accounting for about 20% of the velocity fluctuation kinetic energy. Temporal correlations show strong intra-underbody coupling and weak coupling between underbody and base wake modes at higher frequencies, implying limited direct influence of front wheel shedding on base wake. Links between underbody POD modes and forces are established. High frequency lift and drag variations are associated with ‘in-phase’ shedding and the resulting pumping motion, whereas side force receives low, related to base wake motion, and high, related to wheel shedding, frequency contributions linked to the ‘anti-phase’ state. The study shows correlated unsteadiness across the front wheels, rear wheels, and base wake, with clear implications for aerodynamic performance and highlighting the importance of considering these interactions in future vehicle aerodynamic studies. • Vehicle underbody pressure field reveals dominant ‘symmetric’ and ‘anti-symmetric’ shedding modes from the inside of front wheels. • SPOD reveals each symmetry state exhibits distinct front and rear wheel shedding frequencies. • EPOD extends pressure symmetry states to correlate with ‘in-phase’ or ‘anti-phase’ coupled vortex pairs in the underbody velocity field. • The phase state of the upstream vortex pairs convects to determine rear wheel shedding dynamics. • Rear wheel dynamics couple to the high frequency force fluctuations and base wake dynamics.
Wallace et al. (Wed,) studied this question.