Abstract
The bi-modal behaviour of the turbulent flow past three-dimensional blunt bluff bodies is simulated using wall-resolved large eddy simulations. Bi-modality (also called bi-stability) is a phenomenon that occurs in the wakes of three-dimensional bluff bodies. It manifests as a random displacement of the wake between preferred off-centre locations. Two bluff bodies are considered in this work: a conventional square-back Ahmed body representative of road cars, and a simplified lorry, which is taller than it is wide, with its aspect ratio corresponding to a 15 % European lorry scale model. To our knowledge, this is the first time that the asymmetric bi-modal switching behaviour of the wake, observed experimentally, has been captured in simulations. The resulting unsteady flow fields are then analysed, revealing instantaneous topological changes in the wake experiencing bi-modal switching. The best-resolved case, the simplified lorry geometry, is then studied in greater detail using modal decomposition to gain insights into the energy content and the dominant frequencies of the wake flow structures associated with the asymmetric states. High-frequency snapshots of the switching sequence allow us to propose that large hairpin vortices are responsible for the triggering of the switching.
Highlights
The flow around a road vehicle determines its aerodynamic drag
The wake flow is composed of detached interacting shear layers, which form the envelope of a large low-pressure recirculation area and is responsible for most of the aerodynamic drag
The flow separates over the blunt end of the body and forms a large recirculation area, with the low-pressure bubble extending up to x = 1.17H downstream
Summary
The flow around a road vehicle determines its aerodynamic drag. Most vehicles are blunt bluff bodies, meaning that the flow separates over the blunt back end of the vehicle (Hucho 1998). A very recent finding is that this symmetry-breaking behaviour persists to high, turbulent Reynolds numbers (Grandemange, Gohlke & Cadot 2013b, 2014; Rigas et al 2014), but with the flow exhibiting switching between different asymmetric states. This switching occurs over slow, random time scales, typically three orders of magnitude slower than the vortex-shedding mode associated with separation at the blunt end of the body. The present study achieves what we believe are the first simulations of wake bi-modality, capturing both asymmetric states, for a blunt bluff body. This article presents the simulation set-ups (§ 2), the wake flow results and in particular the bi-modal switching sequence (§ 3), a modal analysis of the wake (§ 4) and finishes with concluding remarks
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