Large-Eddy simulation of the airflow around a truck

Understanding the complex and unsteady flow around commercial vehicles is crucial for improving the aerodynamic performance and safety. Large-Eddy Simulation (LES) is an excellent tool for understanding and visualising the flow structures, which is normally difficult to achieve in experiments. This paper examines a 1:25 scale model truck subjected to a headwind. Both LES and Reynolds-Averaged Navier-Stokes (RANS) techniques were used to acquire the aerodynamic coefficients and surface pressure. Pressure coefficients from LES show good agreement with the full-scale data while discrepancies were found for RANS. The complex flow structures around the truck are presented in detail through streamlines, isosurface contours and surface trace lines, based on the LES results. Both time-averaged and instantaneous vortex structures are identified, highlighting the highly turbulent regions with high energy dissipations, as well as the propagation of small vortices along the truck surface. Spectral analysis is carried out on the time-varying aerodynamic coefficients, showing the dominant frequencies in vortex shedding. Two potential instability modes were identified corresponding to large-scale vortex shedding at low Strouhal number and small-scale vortex shedding due to Kelvin Helmholtz instability. The outcome of the work will help the designers and manufacturers improve the aerodynamic performance and safety of commercial trucks.