Hybrid turbulence modelling of aerodynamics and wake structures in vehicle platoons at close spacing

The aerodynamics of vehicles travelling in close proximity are strongly influenced by complex wake interactions, making accurate turbulence modelling essential for reliable predictions. In this study, four approaches based on the Spalart–Allmaras model are compared for simulating the flow over a two-vehicle platoon at a spacing of x/L = 0.5: steady RANS, Delayed Detached Eddy Simulation (DDES), Improved DDES (IDDES), and a shear-layer adapted DDES (DDES-SLA). The analysis focuses on key flow physics including separation, reattachment, vortex formation, and turbulence intensity, examined through velocity fields, Q-criterion structures, surface pressure distributions, and resolved turbulent kinetic energy. The results demonstrate that RANS underpredicts separation and fails to reproduce the unsteady wake, whereas standard DDES and IDDES suffer from delayed transition to LES in separated regions, leading to unrealistic vortex shedding and weakened turbulence levels. In contrast, the shear-layer adapted formulation enables an earlier RANS-to-LES switch in free shear layers, promoting more accurate vortex development and reattachment, and yielding a compact wake in closer agreement with experimental observations. These findings highlight the importance of properly controlling the RANS–LES transition in platoon aerodynamics and establish DDES-SLA as the most reliable approach among the tested models.