Wake dynamics and pollutant dispersion behind a light-duty lorry

We report a large-eddy simulation of the turbulent pollutant dispersion in the near wake behind a real, light-duty lorry. The time-averaged mean wake and dominant global flow instabilities are compared with the results of simplified square-back vehicle models in the literature, showing good agreement. In addition to the ring-shape mean wake topology, by proper orthogonal decomposition (POD), the near-wake flow instabilities observed in this study include the primary flow instabilities in the vertical and horizontal directions with a (dimensionless) characteristic frequency of 0.13-0.14 and 0.19-0.20, respectively, and the pumping motion at 0.07-0.08. Furthermore, the vertical flow instability shows its maximum impact on the wake flow above the separation point while pumping activity works significantly below. The coherence analysis demonstrates the strong modulation behavior via symmetry preserving dynamics. In addition, the near-ground, lower vortex system evolves spirally and in-phase with a dimensionless characteristic frequency centered at 0.6. The extended POD is employed to build up a two-way velocity-concentration analysis framework with the temporal correlation as the link. The spatial concentration patterns associated with those energetic large-scale motions as well as corresponding extreme events are revealed and reconstructed as a typical example.