Modal Decomposition for Vehicle Aerodynamics

Iso-surfaces of the streamwise
velocity component of the most
dominant DMD mode at 9Hz

A new modal decomposition approach is employed for analyzing temporally resolved flow field data from Detached Eddy Simulation (DES) using the Open Source CFD environment OpenFOAM®. Velocity components are temporally filtered with moving average filter before being interpolated to a coarser equidistant mapping mesh. These filtering operations reduce the amount of spurious numerical oscillations in the data to be analyzed and cut off high frequency, low energy content. In order to extract the most dominant flow structures for in depth analysis, an incremental variant of Dynamic Mode Decomposition (DMD) was found to be most useful. DMD generates modes of distinct frequency that can be reconstructed in time. Several modifications to an already existing variant are implemented to increase the applicability for large data sets, mainly reducing the required amount of memory, which is the most limiting factor in modal analysis for industrial applications with large data sets. The modes represent flow structures of vortex shedding and stationary recirculation processes. Reconstruction enables tracking of structures to their respective excitation mechanisms and allows for identification of geometrical features that introduce strong perturbations to the flow. Strong perturbations lead to an increase in potential for viscous dissipation in the wake of bluff bodies and thus to generally lower base pressure and increased drag. The DrivAer reference model in notchback configuration with structured underbody, engine bay flow, open wheel houses and open rotating rims is simulated in wind tunnel conditions with rolling road system. The results from CFD are processed using the DMD approach described before and the most dominant flow structures are visualized and discussed. Small scale detachments that are generated far upstream of the mean detachment line of the vehicle’s rear end travel downstream along the surface, triggering large scale structures in the wake. The frequency of those structures is also dominant in the frequency spectra of the integrated force coefficient.

Frequency spectrum of the integrated drag force coefficient