Application of Lobed Mixers to Reduce Drag of Boat-Tailed Ground Vehicles

Abstract

Minimising the aerodynamic drag of commercial vehicles is important economically and ecologically. This work demonstrates the effective use of lobed-mixing geometries, traditionally used to enhance flow mixing, as a viable, passive flow control method for reducing base pressure drag of boat-tailed ground vehicles. Experiments were performed on a 1/24 th-scale Heavy Goods Vehicle representative model at a Reynolds number of 2.3 × 10 5 with force and hot-wire anemometry measurements used to quantify drag and wake characteristics. Tests on a baseline (no boat-tail), an unaltered boat-tail, and lobed-mixing configurations with varying pitch and height were compared. Overall, the baseline and unaltered boat-tail exhibited good correlation to previous results. This provided confidence in the methodology adopted. Results using lobed mixers showed up to a 10.2% drag reduction with the added vorticity produced acting to fundamentally shift the nature of the wake. This is manifested principally through the generation of counter-rotating vortical structures which enhance crosswise flow entrainment into the base wake. This action is observed to limit flow entrainment towards the ground leading to a higher wake and a characteristic 'waist'. Enhanced mixing is also demonstrated. Overall, results suggest the suitability of lobed mixers as an effective means for drag reduction of boat-tailed ground vehicles.

On the drag reduction of road vehicles with trailing edge-integrated lobed mixers

“…The nature of these vortices is such that the pairs are counter-rotating horizontally, but also vertically along each side (all adjacent vortices are counter-rotating), resembling the topology produced by the lobed boat-tails described by Rejniak and Gatto. 49 The vortices originate from the four base corners, with Grandemange et al 71 suggesting the optimum drag reduction is achieved when all adjacent structures are counter-rotating, as is the case here. As described by Rejniak and Gatto, 49 the principal action of these vortices is to contract the wake around the middle ( z * ≈ 0.8) with subsequent stretching occurring in the lower parts ( z * < 0.6).…”

“…The baseline used (Figure 1) is a simplified 1/24 th -scale model (width, W = 110 mm) representative of a HGV. This model is based on the one used by Rejniak and Gatto, [49][50][51] but incorporates a more streamlined tractor profile to minimise frontal flow separation. Constructed from Perspex, the baseline consists of two primary components: a tractor and trailer bottom section, and a trailer.…”

“…Integrating the lobed mixing geometries directly into the vehicle's trailing edges aims to deliver a more practical solution likely to be suitable for real applications. Following from the work described by Rejniak and Gatto 49 on a boat-tailed vehicle, here the lobed profiling is achieved by cutting longitudinal grooves in the model's trailing edges, each sloping inboard and generating a penetration angle (b) relative to the freestream. The profiling is such that the lobes which would normally protrude (outboard penetration angle relative to the axial flow) are reduced to b = 0°, as presented in Figure 3.…”

“…Following from the work of Rejniak and Gatto, 49 this study investigates the effects of integrating the lobed mixing geometries directly into the vehicle's trailing edges. This design aims to provide a solution more readily adaptable to real vehicles.…”

“…Howard and Goodman 47 and Paterson et al 48 suggested similar geometries to have the potential for drag reduction on a range of bluff bodies. More recently, Rejniak and Gatto 49 applied the lobed mixer geometries to the trailing edges of a boat-tailed HGV model. Reductions in drag coefficient of up to 10.2% were found, with the lobes demonstrated to generate pairs of counter-rotating streamwise vortices, as in the other applications.…”