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.

New insights into how different ground simulation methods affect road vehicle aerodynamics are presented. Experiments are conducted on a 1/24th-scale model, representative of a Heavy Goods Vehicle, at a Reynolds number, based on width of 2.3 × 10 5. Particular focus lay in characterising differences in unsteady wake development, with mean drag, base pressures, and wake velocities quantified, compared, and evaluated. Distinctly, these tests include the effects of elevated blockage ratio and wheel rotation. Results show moving ground use can have a substantial influence under these conditions, with increases in wake length and average base pressure coefficient of 17% and 9%, respectively. The dominant wake dynamics, characterised by a global streamwise oscillation commonly referenced as the bubble pumping mode, was also found dependent with asymmetric shedding frequencies from both vertical and horizontal base edges higher with static ground use. For these conditions, development of a low-frequency turbulence source, near omni-directional in nature, positioned behind the model, further contaminates the flow-field. This feature disappears with moving ground use. Both the nature and characteristics of the turbulence generated behind the wheels were also found to evolve differently, with a moving ground promoting stronger and more defined oscillatory behaviour up to model mid-height, two-and-a-half widths downstream. Overall, these results highlight that while variations in time-independent quantities to differing ground simulation can often be very subtle, prompting the interpretation of negligible overall effects, in-depth consideration from a time-dependent perspective may lead to a different conclusion.

A new method for supporting ground vehicle wind tunnel models is proposed. The technique employs a centrally mounted sting connecting the front face of the vehicle, adjacent to the floor, to a fixed point further upstream. Experiments were conducted on a 1/24th-scale model, representative of a Heavy Goods Vehicle, at a width-based Reynolds number of 2.3 × 105, with detailed comparisons made to more established support methodologies. Changes to mean drag coefficients, base pressures and wake velocities are all evaluated and assessed from both time-independent and time-dependent perspectives, with a particular focus within the wake region. Results show subtle changes in drag coefficient, together with discrete modifications to the flow-field, dependent on the method adopted. Subtle differences in base pressures and wake formation are also identified, with mounting the model upstream found to demonstrate retention of many of the beneficial effects of other techniques without suffering their deficiencies. Overall, these results identify the upstream mounting methodology as a viable alternative to currently available and more well-established techniques used to facilitate wind tunnel aerodynamic interrogation.

Trailing edge-integrated lobed-mixing geometries are proposed as a viable method for road vehicle aerodynamic drag reduction. Experiments are conducted on a 1/24th-scale model, representative of a Heavy Goods Vehicle, at a width-based Reynolds number of 2.8 × 105. A broad range of pitches and penetration angle values is examined, with detailed comparisons also made to high-aspect-ratio rear tapering. Changes to mean drag coefficients and wake velocities are evaluated and assessed from both the time-independent and time-dependent perspectives. Results show significant drag reductions for lower pitches at higher penetration angles, where the performance of regular tapering is found substantially degraded. The mechanisms responsible for drag reduction are identified to be reductions in the wake size and a shift in the vertical wake balance. The former is shown to be a result of the enhancement in inboard momentum close to the trailing edges through the generation of pairs of counter-rotating streamwise vortices, with the latter attributed to the downstream evolution of the vortices. Overall, these results identify such geometries to be suitable for improving vehicle drag while minimising the losses in internal space.