application of lobed mixers to reduce drag of boat-tailed ground vehicles

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 1/24th-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.