Application of upstream wind tunnel model mounting to the investigation of lobe-mixing geometries for road vehicle drag reduction

Abstract

The aerodynamics of road vehicles is an important topic at the centre of scientific effort to reduce the transport’s impact on climate change. Commercial vehicles are among the least efficient, contributing approximately 18% of the transport emissions, while accounting for only 5% of total vehicle miles. In typical operating conditions, the aerodynamic drag generated by these vehicles can be responsible for up to 50% of total fuel consumption. The base region contributes a significant amount of aerodynamic drag and remains particularly difficult to optimise. This area is the focus of this thesis. Wind tunnels have been an important tool used in the aerodynamic design of ground vehicles and remain the most popular choice for this application. The development of low-drag concepts requires highly accurate testing environments. This work utilises a wind tunnel to investigate the effects of moving ground on the unsteady base wake of a commercial vehicle model. The results show that the wake dynamics are markedly affected by the varying condition, making moving ground an important aspect for correct aerodynamic representation. Additionally, a new approach of upstream model mounting is evaluated. This technique is shown to combine the benefits and minimise the deficiencies of the typical supports from the top and sides and is, therefore, proposed to be a suitable alternative for low-interference flow-field characteristics at the model base. The second part of this thesis investigates the use of lobed-mixing geometries for base drag reduction. Lobed mixers are a popular device for mixing enhancement within the aerospace industry. In this work a similar concept is applied to a boat-tailed model, with further investigations including the integration of the geometries directly into the base trailing edges. This concept is demonstrated to produce significant drag reductions of up to 7% at high aspect ratios, highlighting the possibility that such devices may be used to improve the fuel efficiency while minimising the impact on internal trailer space. In this work, the experiments are conducted on two 1/24th-scale models representative of a commercial vehicle, at a width-based Reynolds number of up to 2.8 × 105. Load and base pressure are measured for different configurations, with hot-wire anemometry used to interrogate the flow-field. Results are considered from both time-averaged and time-dependent perspectives.