Abstract
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.
Highlights
The aerodynamic performance of ground vehicles is generally dependent on the dynamics and behaviours of the surrounding flow
In the asymmetric wake observed by Grandemange et al (2012), the flow status was found to be sensitive to the Reynolds number in the laminar regime (310 < Re < 415), and a steady asymmetric state was possible to be maintained for a certain period for Re = 315
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
Summary
The aerodynamic performance of ground vehicles is generally dependent on the dynamics and behaviours of the surrounding flow. The time-averaged flow around a symmetric bluff body placed at zero yaw angle normally forms a wake symmetry This assumption is rooted among researchers performing experimental and numerical results and often used as a criterion for judging the quality of simulations. The flow asymmetry was found in a turbulent regime and to switch between two asymmetric states (Grandemange, Gohlke & Cadot 2013a), showing a wake bi-stability This phenomenon has experimentally proven to be relevant to several factors that can cause changes in the flow, such as the aspect ratio and ground clearance of the model (Grandemange, Gohlke & Cadot 2013b), yaw angles (Volpe, Devinant & Kourta 2015), the depth of a cavity attached to the rear body (Evrard et al 2016), upstream perturbations in the near-wall region (Barros et al 2017), rotating wheels (Pavia & Passmore 2017) and others. Symmetry breaking has been observed in both the laminar and the turbulent regimes (Östh et al 2014; Pasquetti & Peres 2015; Evstafyeva, Morgans & Dalla Longa 2017; Lucas et al 2017; Dalla Longa, Evstafyeva & Morgans 2019) in large-eddy simulations (LES)
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