Salient three-dimensional features of the turbulent wake of a simplified square-back vehicle

Pavia, Giancarlo; Passmore, Martin A.; Varney, Max; Hodgson, Graham

doi:10.1017/jfm.2020.71

Cited by 40 publications

(54 citation statements)

References 41 publications

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“…The scenario partly builds on the mechanism proposed by Gerrard (1966) for the formation of the recirculation region behind two-dimensional (2-D) bluff bodies through flow entrainment by the surrounding shear layers. Although the present flow configuration is highly three-dimensional (3-D) (Pavia et al 2020), which cannot be completely neglected, the interaction of opposite shear layers is an important feature of this flow configuration. Grandemange et al (2013b) have shown the existence of global instabilities related to the interaction of opposite shear layers in the wake of the Ahmed body, similarly to the global instability in 2-D wakes but with the notable difference that these interactions are less strong.…”

Section: Discussion and Concluding Remarksmentioning

confidence: 99%

Mechanics of bluff body drag reduction during transient near-wake reversals

et al. 2020

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Section: Discussion and Concluding Remarksmentioning

confidence: 99%

Mechanics of bluff body drag reduction during transient near-wake reversals

et al. 2020

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“…The purpose of the present work is to elucidate the mechanism of the wake asymmetry behind notchback configurations with a specific focus on the random switch observed in the bi-stable wake. The bi-stable nature observed for the notchback body is fundamentally different from that for the squareback type (Grandemange et al 2012(Grandemange et al , 2013a(Grandemange et al , 2013bPavia et al 2020), due to the distinguishable shedding vortices from the slant and the flow reattachment to the deck. The results of this work help to explore the fluid mechanisms of the wake bi-stability behind a notchback bluff body with the interaction of the wake structures.…”

Section: Introductionmentioning

confidence: 74%

“…Moreover, although switches between two asymmetric mirrored states of wake bi-stability have been experimentally observed for squareback bluff bodies (Grandemange et al 2013a(Grandemange et al , 2013bVolpe et al 2015;Haffner et al 2020;Pavia et al 2020), the random switch in the notchback case has remained unexplained. It is worth noting that random switches between bi-stable wakes behind squareback Ahmed bodies were predicted by 926 A36-3 Östh et al (2014) andDalla Longa et al (2019) with the LES approach.…”

Section: Introductionmentioning

confidence: 99%

“…2020; Pavia et al. 2020), the random switch in the notchback case has remained unexplained. It is worth noting that random switches between bi-stable wakes behind squareback Ahmed bodies were predicted by Östh et al.…”

Section: Introductionmentioning

confidence: 99%

“…2012, 2013 a , 2013 b ; Pavia et al. 2020), due to the distinguishable shedding vortices from the slant and the flow reattachment to the deck. The results of this work help to explore the fluid mechanisms of the wake bi-stability behind a notchback bluff body with the interaction of the wake structures.…”

Section: Introductionmentioning

confidence: 99%

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Numerical investigation of the wake bi-stability behind a notchback Ahmed body

Minelli

Wang

et al. 2021

J. Fluid Mech.

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Large-eddy simulations are used to investigate the origin of the wake asymmetry and symmetry behind notchback Ahmed bodies. Two different effective backlight angles, ${\beta _1} = 17.8\mathrm{^\circ }$ and ${\beta _2} = 21.0\mathrm{^\circ }$ , are simulated resulting in wake asymmetry and symmetry in flows without external perturbations, in agreement with previous experimental observations. In particular, the asymmetric case presents a bi-stable nature showing, in a random fashion, two stable mirrored states characterized by a left or right asymmetry for long periods. A random switch and several attempts to switch between the bi-stability are observed. The asymmetry of the flow is ascribed to the asymmetric separations and reattachments in the wake. The deflection of the near-wall flow structures behind the slant counteracting the asymmetry drives the wake to be temporarily symmetric, triggering the switching process of the bi-stable wake. The consequence of deflection that forces the flow structure to form on the opposite side of the slant is the decisive factor for a successful switch. Modal analysis applying proper orthogonal decomposition is used for the exploration of the wake dynamics of the bi-stable nature observed.

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Drag reduction mechanisms on a generic square-back vehicle using an optimised yaw-insensitive base cavity

et al. 2021

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Regulations on global greenhouse gas emission are driving the development of more energy-efficient passenger vehicles. One of the key factors influencing energy consumption is the aerodynamic drag where a large portion of the drag is associated with the base wake. Environmental conditions such as wind can increase the drag associated with the separated base flow. This paper investigates an optimised yaw-insensitive base cavity on a square-back vehicle in steady crosswind. The test object is a simplified model scale bluff body, the Windsor geometry, with wheels. The model is tested experimentally with a straight cavity and a tapered cavity. The taper angles have been optimised numerically to improve the robustness to side wind in relation to drag. Base pressures and tomographic Particle Image Velocimetry of the full wake were measured in the wind tunnel. The results indicate that a cavity decreases the crossflow within the wake, increasing base pressure, therefore lowering drag. The additional optimised cavity tapering further reduces crossflow and results in a smaller wake with less losses. The overall wake unsteadiness is reduced by the cavity by minimising mixing in the shear layers as well as dampening wake motion. However, the coherent wake motions, indicative of a balanced wake, are increased by the investigated cavities. Graphical abstract

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Salient three-dimensional features of the turbulent wake of a simplified square-back vehicle

Cited by 40 publications

References 41 publications

Mechanics of bluff body drag reduction during transient near-wake reversals

Mechanics of bluff body drag reduction during transient near-wake reversals

Numerical investigation of the wake bi-stability behind a notchback Ahmed body

Drag reduction mechanisms on a generic square-back vehicle using an optimised yaw-insensitive base cavity

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