title: Ground vehicle platoons: aerodynamics and flow control: An experimental and computational investigation
Creators: name: Nolan, Craig
Publisher: name: University of Glasgow
Year: 2023

description: Road transport contributes approximately 20% to the United Kingdom’s greenhouse gas emissions, accelerating the effects of global warming. Since the United Kingdom, like many other countries, has pledged to reach net zero carbon emissions over the next two decades, reducing emissions from road vehicles has become a priority. A further adverse effect of road vehicle emissions is their link to serious health issues such as respiratory and cardiovascular diseases. To achieve the required substantial reduction in emissions, a multi-faceted approach will be required. In this project, one important aspect, the aerodynamics of ground vehicle platoons, is explored with the aim of expanding the understanding of road vehicle aerodynamics and exploring innovative solutions to improve road vehicle efficiency. Vehicle platooning is a form of cooperative travelling in which vehicles drive closely together, with the intention to reduce overall air resistance, fuel consumption and vehicle emissions. Platooning, i.e., the cooperative movement of a group of individuals, is a concept that is not unique to road vehicles, but can be commonly observed in nature (e.g., a school of fish) or in sport (e.g., cyclists riding their bikes in a train). Here the trailing individuals take advantage of the sheltering provided by the leading individuals of the group. As a continuation of this observation, it would be natural to assume that road platooning is always beneficial, and that the trailing vehicles of a platoon reliably experience a reduction in drag. However, there are several examples in the literature that report a rear vehicle in a platoon receiving a drag increase. With the wide range of vehicle geometries on the roads, it is vital that additional research is targeted at understand the fundamental aerodynamic principles that lead to such adverse platooning results and understand the role that geometry plays in influencing the effectiveness of a platoon. In the first stage of this project, the geometry dependence of platooning was explored by systematically altering the shape of a simplified ground vehicle to change its platooning behaviour from the ‘classical’ platooning behaviour, where the rear vehicle experiences a high drag reduction, to ‘inverted’ platooning behaviour, where the rear vehicle suffers an increase in drag. To this end, a large parameter study was completed using unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations. A key outcome of this study was that the combination of a more streamlined rear vehicle, coupled with strong wake-impingement caused by the lead vehicle results in the most adverse platooning outcome. The second stage of the project focused on establishing the potential of using passive flow control to alleviate the adverse platooning effects that were observed in a platoon composed of two Ahmed bodies with 25◦ rear slant angles. First, the potential of plasma actuators as flow control devices was explored by experimentally characterising the performance of a serrated dielectric barrier discharge (DBD) plasma actuator. This was followed by another set of URANS simulations which considered the application of flow control in the context of a platoon of two 25◦ Ahmed vehicles. This covered both plasma-actuator like induced jets as well as flaps as flow control devices. The flow control devices were located at the top of the rear slant of the front vehicle and were designed to induce flow separation to increase the size of the front vehicle’s wake. Using this technique a drag reduction for the rear vehicle of up to 25% compared to the configuration without flow control was achieved. In the final stage, the effectiveness of flow control was tested experimentally in the University of Glasgow’s Handley-Page wind tunnel. First the dependency of the drag coefficient of a platoon composed of two 25◦ Ahmed vehicles on inter-vehicle spacing and Reynolds number was investigated, showing that a significant dependency on both parameters exists. Then, flow control was introduced in the form of a flap, with the previous sets of experiments being repeated for three flap angles and two flap lengths. While the flap was not quite as effective as predicted by the URANS simulations, the flap still induced a significant reduction in drag (ca. 9%) when compared to the rear vehicle of the baseline case that was subject to inverted platooning conditions.