Unsteady aerodynamic forces on long lorry platoons

In an earlier paper the authors describe an experimental investigation to measure the forces on 1/20th scale vehicles in long platoons, together with the flow field around the platoons, using the University of Birmingham moving model TRAIN rig facility, that can accommodate 8-vehicle platoons and provide a realistic ground simulation. This paper analyses the experimental results further to look at the unsteady forces and flows. Using conventional Fourier spectral analysis and wavelet analysis, it is found that there are two modes of unsteady flow – one at a low frequency that affects multiple vehicles in the platoon at any one time, and has a wavelength of several vehicle lengths; and one at a high frequency with a much shorter wavelength that is localised on specific vehicles. These flows are very intermittent and unsteady, with switching between modes both spatially and temporally. Associated DDES calculations only capture the low frequency mode. Scaling up the measured values to the full-scale situation gives values of the standard deviation of side force on an individual lorry of around 260N, with the major frequency of oscillation at 1.0Hz.