A considerable body of work exists concerning the aerodynamic optimisation of the vehicle form in isolation. Some valid generalised conclusions have been reached concerning optimal and sub-optimal key vehicle geometries and their relevant flow mechanics; generalised test forms representing various characteristic vehicle geometries - “squareback”, “notchback” and “fastback” - have been developed and extensively studied, with critical geometries highlighted. The study of organised vehicle convoys has similarly been researched since the early 1970’s primarily as a means to increase traffic throughput on existing road arterials, with ultimate “future-generation intelligent transport systems” envisioning scenarios where vehicles on major arterials may travel under fully automatic control, allowing possibilities in vehicle organisation not previously envisioned. Initial research simply considered reducing the spacing between vehicles travelling in localised groups to similar destinations - “platoons” - with traffic throughput scaling positively with platoon length and reduced spacing. The significant majority of research in this field is dedicated to developing concepts that increase traffic throughput; aerodynamic concerns are only recently being explored, however it is clear from relevant research concerning tandem bluff bodies that aerodynamic interaction is heightened with closer proximity. A variety of recent studies examining aerodynamic force effects in platooning confirm advantages for all vehicles in homogenous platoons of squareback and notchback geometries. The case for fastback geometries is unclear, with preliminary studies suggesting that there can be an increase in the drag force of trailing vehicles in the wake of a fastback geometry. The work presented explores the fundamental phenomena underscoring the performance of two fastback vehicles travelling in close proximity. Vehicles are simulated using a well-known reference automotive form. A primary extension to existing works concerns effect of changing the leading vehicles geometry to one of two different (yet practically characteristic) fastback configurations, constituting an important variable known to offer (in isolation) two markedly unique flow structures and drag force coefficients. A series of wind-tunnel experiments were performed where rear slantback angles were varied and measurements of pressures, forces and flow visualisations were made on upstream and downstream models in addition to interrogation of the intervening gap flow field. It is demonstrated that irrespective of the upstream models form (and thus irrespective of dominant flow phenomena for such a model considered in isolation), force characteristics remain broadly similar for leading and trailing models in the platoon, primarily owing to the development of streamwise vortices originating from the C-pillar of the leading model which are shown to entrain a high-momentum flow between them, impinging on the trailing model forebody. A variety of methods - from qualitative flow visualisation to spectral methods applied to dynamic data - are employed to demonstrate that even at the closest spacing examined, salient flow phenomena of the leading and trailing models are broadly retained. A detailed investigation of gap flows and trailing model spectra effects as a function of leading model geometry and model spacing is also presented.
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