Proper orthogonal decomposition analysis and modelling of the wake deviation behind a squareback Ahmed body

We investigate numerically the three-dimensional (3D) flow around a squareback Ahmed body at Reynolds number Re = 10(4). Proper orthogonal decomposition (POD) is applied to a symmetry-augmented database in order to describe and model the flow dynamics. Comparison with experiments at a higher Reynolds number in a plane section of the near wake at midheight shows that the simulation captures several features of the experimental flow, in particular the antisymmetric quasisteady deviation mode. 3D POD analysis allows us to classify the different physical processes in terms of mode contribution to the kinetic energy over the entire domain. It is found that the dominant fluctuating mode on the entire domain corresponds to the 3D quasisteady wake deviation, and that its amplitude is well estimated from 2D near-wake data. The next most energetic flow fluctuations consist of vortex shedding and bubble pumping mechanisms. It is found that the amplitude of the deviation is negatively correlated with the intensity of the vortex shedding in the spanwise direction and the suction drag coefficient. Finally, we find that despite the slow convergence of the decomposition, a POD-based low-dimensional model reproduces the dynamics of the wake deviation observed experimentally, as well as the main characteristics of the global modes identified in the simulation.