Evaluation of vehicle platooning aerodynamics using bluff body wake generators and CFD

Abstract

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.

Aerodynamics of electric cars in platoon SAGE publications

“…The adopted CFD settings were previously validated and proven to correlate with experimental measurements of vehicles in platoon. 4,5 The platoon configurations including the inter-vehicle spacing and Reynolds number were identical for the CFD and track measurements to allow for a direct comparison, although, the vehicle geometry was simplified in CFD by enclosing the front grill, wheelhouse, wheels and smoothing the underbody. Enclosure of the front grill is known to reduce the drag of the vehicle as the internal flows are neglected.…”

“…The intensity of both parameters increase as the inter-vehicle spacing is reduced and therefore to capture the time-dependent flow dynamics accurately the use of unsteady simulation approaches is vital. However, for drag estimations and general surface pressure trends, 4,5 showed that the Reynolds Averaged Navier-Stokes (RANS) is capable of producing reasonable results.…”

“…To visualise the regions of the surface that contribute to pressure drag; the force (using equation (5)) acting on individual cells in the stream-wise component alone was plotted on the surface as depicted in Figure 18. The produced contour map is defined into three distinct components of neutral, positive and negative force.…”

“…4 In general, experimental investigations in wind tunnels have been compromised by the reduced model scale required to accommodate multiple vehicles within the confines of the test section, which limits the aerodynamic resolution and further reduces the Reynolds number. A method to improve the model scale was proposed by Ebrahim et al 5 through the use of bluff body wake generators in an attempt to reduce the Reynolds number deficit and improve the aerodynamic resolution particularly for platoon investigations. Full scale wind tunnels are usually even more restrictive and it is rare to be able to accommodate two passenger vehicles with more than half a vehicle length between them.…”

Wake and surface pressure analysis of vehicles in platoon

“…Both He et al and Bruneau, Khadra, and Mortazavi demonstrated the advantages of using higher order numerical solvers to determine time dependant flow features in platoon that are usually omitted by RANS that contribute to the vehicle's lateral instability. However, in terms of the averaged flow features that influence the mean drag measurements, the RANS solver has been shown to reproduce the wake structures and pressure field reasonably well for various aerodynamic studies including Bordei and Popescu (2011), Axerio et al (2009), Soares, Garry, and Holt (2017), Fu et al (2017), Maleki, Burton, and Thompson (2017) and Ebrahim, Dominy, and Leung (2016).…”