Control of afterbody vortex flows using sweeping jets

Abstract

Afterbody vortex flows have raised great concern in the daily practice of road and air vehicles due to their strong association with safety issues and aerodynamic drag. A sweeping jet is a promising and robust solution for controlling flows, as it generates temporal and spatial oscillation without needing any moving part. This study explores the use of sweeping jets in controlling afterbody vortices, being the first to do so. The characteristics of sweeping jets were first studied based on actuators with different chamber lengths. The sweeping jets were then applied to a slanted-base cylinder, which is a generic model that has a similar afterbody vortex pair to air and road vehicles, to experimentally investigate the control effects and mechanism in both time-averaged and phase-averaged senses. The drag reduction efficacy of a sweeping jet pair was also examined. The results demonstrate that sweeping jets can keep their oscillation stable when the actuator chamber length has had a limited range of modifications. In terms of afterbody vortex control, the sweeping jet resulted in increasingly diffused and larger afterbody vortices in a time-averaged sense as jet strength (momentum coefficient) increased. A further increase in jet strength from a momentum coefficient of 0.167 introduced additional vorticity into the afterbody vortices, leading to a higher vortex strength. In a phase-averaged sense, the transient flow structures showed that the jet velocity performed a Z-shape distribution on the slanted endplate of the model, where the intervals of the adjacent jet-vortex contact positions were equal along the streamwise direction and remained unchanged over time. The turbulence was injected into the vortex core with the jet sweeping motion, which subsequently caused a diffusion of the velocity gradient in the vortices, thereby weakening their strength. Net energy saving by drag reduction was obtained when a smaller sweeping jet pair was placed at the most upstream location tested. The sweeping jets were not only found capable of altering the surface pressure distributions via directly imposing a footprint of high pressure on the surface but also that of affecting the roll-up of the afterbody vortex and/or reducing its strength via injecting turbulence into the afterbody vortex. This study paves the way for applying sweeping jets to improve aerodynamic performance on transportation vehicles, such as high-speed trains, in the future.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Shan Zhong (Supervisor) & Andrew Weightman (Supervisor)