In this paper, the drag reduction benefits associated with 2 and 3 cars in platoon have been investigated. Following a validation of initial CFD simulations against experimental measurements, predictions of surface pressures and wake structure for alternative platoon configurations have been analysed to determine the changes of flow structure that influence the pressure field and drag force on each vehicle. Contrary to several publications it was found that in a platoon of two vehicles, the drag force of the trailing vehicle exceeded that of an isolated vehicle for close spacings. Analysis of this surprising result revealed that design features introduced to optimise the wake of an isolated vehicle can lead to a drag increase on a following vehicle. For three-vehicle platoons, the flow interaction between the leading and middle vehicles remained largely unchanged but the additional effect of the third vehicle resulted in all three vehicles exhibiting lower drag than that of an isolated vehicle. A clear implication of this work is that results from the analysis of vehicle platoons are likely to be sensitive to the geometry and wake structures of the chosen test vehicle which helps to explain why many previous studies have been seemingly contradictory.
Recent developmentsin sensing and communications between vehicles (V2V) and their surroundings have provided the technology to allow cars to operate autonomously or semi-autonomously in closely spaced 'platoon' formation without the risk of collision. This is known to reduce the aerodynamic drag and thus consequently limits the energy consumption and associated emissions. Although wind tunnel investigations have been performed to mimic platoon operations, most experimental evaluations of multiple vehicles in platoon are severely compromised by the restricted length of the wind tunnel test section. Therefore, the model scale must be reduced which decreases the measurement accuracy. The innovative solution presented here is to reproduce the flow structure that is created by a leading road car through the use of a 'bluff-body wake generator' with a much reduced length which eliminates the need to decrease the scale of the following test model. Validated computational fluid dynamics (CFD) data and analysis are presented to evaluate an optimized design of a wake generator based on the Ahmed model [1] and the effect of inter-vehicle spacing on the aerodynamic characteristics of the following vehicle. It is shown that accurate reproduction of the wake is possible at half the characteristic length, thus correctly determining the flow impact on the downstream model. This demonstrates that the bluff body wake generator provides a reliable approach that allows platooning studies to be performed without sacrificing aerodynamic resolution.
The potential aerodynamic benefits of operating full-scale electric vehicles in platoons of 2 and 3 vehicles have been investigated. Since drag reduction has a direct impact on vehicle range, power consumption was measured directly and surface pressure measurements were made to characterise the changes in pressure field that influence the power required to overcome aerodynamic drag. CFD simulations were validated against the track measurements to assess the limitations of using a practical, limited number of pressure tappings to measure drag. The overall power consumption for the whole platoon was found to reduce proportionally with the reduction of vehicle spacing and it was also observed that increasing the number of vehicles in the platoon from 2 to 3 further increased the power savings from 33.4% to 39.1%. These power savings were attributed primarily to changes in surface pressure acting on the base of the leading vehicle and the forebody of the trailing vehicle.
It is well known that platoons of closely spaced passenger cars can reduce their aerodynamic drag yielding substantial savings in energy consumption and reduced emissions as a system. Most published research has focused on platoons of identical vehicles which can arguably be justified by some evidence that geometric variety has little to no effect on the overall flow characteristics in platoons of three vehicles or more. It is known that much of the aerodynamic benefit from platooning is gained by the leading two cars, so operating as vehicle pairs could potentially achieve similar environmental benefits whilst addressing many of the practical challenges associated with the safe operation of long platoons on public roads. However, it has been reported that unlike long platoons, the effect of geometry and arrangement is critical if the drag reduction of a pair is to be optimised. This paper describes a parametric study based on three geometric variants of the popular DrivAer model with different combinations and spacings. It is confirmed that vehicle geometry crucially affects the results with the best combinations matching those of long platoons and others creating a net drag increase.
This paper analyses the effect of façade curvature with varying the surrounding building heights on pedestrian-level wind speeds and comfort for walking, using computational fluid dynamics. The case study focuses on 20 Fenchurch Street site in London as several complaints have risen in relation to high wind speeds, the cause of which is not thoroughly understood. The results of the simulation revealed that although the increase of surrounding building heights reduces overall pedestrian-level wind speeds for both curved existing and cuboid building, façade curvature impacts differently on winds, compared to the cuboid. Isolated curved and cuboid building would perform similarly with the exception of the northwest corner. However, introducing the existing surrounding buildings, the curved façade geometry would create larger area of walking discomfort compared to the cuboid geometry. When the height of the surrounding buildings is increased, both buildings would perform similarly with minor aerodynamic advantage to the curved-façade.
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