On multistabilities of real car's wake

Bonnavion, Guillaume; Cadot, Olivier; Evrard, Antoine; Herbert, Vincent; Parpais, S.; Vigneron, Rémi; Délery, Jean

doi:10.1016/j.jweia.2017.02.004

Cited by 46 publications

(22 citation statements)

References 19 publications

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“…The same dependency between and Ψ was found by Bonnavion et al [4] while assessing the presence of bi-stability on the wake generated by a commercially available light van. They reported the existence of a bi-stable regime between two different positions of the reverse flow impinging on the base when the model was at 4° yaw.…”

Section: Introductionsupporting

confidence: 77%

Characterisation of Wake Bi-stability for a Square-Back Geometry with Rotating Wheels

Pavia

Passmore

2017

Progress in Vehicle Aerodynamics and Thermal Management

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In this paper the effects produced by the wheels on the bi-stable reflectional symmetry breaking (RSB) mode seen for the wake of a square-back geometry (Grandemange et al. [11]) are investigated considering a modified version of the Windsor body already studied in Perry et al. [18]. The contribution of the wheels and their rotation to the changes in the base pressure distribution and the wake topology is characterised by means of pressure tappings and 2D-3C particle image velocimetry. Balance measurements are used to further characterise the changes in the strength of the RSB mode. For the pure square-back configuration, the results show a general increase of the base drag as a consequence of the strengthening of the suction over the lower portion of base, due to the formation of a pair of counter rotating vortices acting close to the bottom trailing edge. At the same time, the RSB mode is weakened, leading to a reduction in the fluctuations recorded for the lateral component of the aerodynamic force. The sensitivity of the RSB mode to small changes in the shape of the model's trailing edges is characterised by looking at the effects produced by short tapers, with a slant angle of 12° and a chord equal to 4% of the model length, applied to either the horizontal or the vertical trailing edges. The results show that the RSB mode disappears when the effect of the wheels is paired to the upwash generated by the slanted surface (when applied to the bottom trailing edge), although it is still clearly visible when the tapers are applied to the side edges of the base, in contrast with the results reported by Pavia et al. [16] for the same geometry without wheels.

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Section: Introductionsupporting

confidence: 77%

Characterisation of Wake Bi-stability for a Square-Back Geometry with Rotating Wheels

Pavia

Passmore

2017

Progress in Vehicle Aerodynamics and Thermal Management

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“…whether it occurs with top-bottom asymmetry or side-side asymmetry) was found to be sensitive to the bluff body aspect ratio, ground proximity, pitch/yaw angle or any flow perturbations (Grandemange et al 2014;Barros et al 2017;Gentile et al 2017). Bi-modal switching has not only been observed in the case of simplified square-back geometries such as the Ahmed body (Grandemange et al 2014) and the Windsor model (Pavia, Passmore & Sardu 2018) but also for real full-sized road vehicles (Bonnavion et al 2017). Its presence affects the forces and moments on the body, including the aerodynamic drag.…”

mentioning

confidence: 92%

Simulations of the bi-modal wake past three-dimensional blunt bluff bodies

2019

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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.

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“…Existing literature shows studies covering yaw and pitch effects on generalized car shapes, such as the Ahmed et al 2 body and passenger cars, both experimentally [3][4][5][6][7] and computationally. [8][9][10][11][12][13] However, similar efforts with a direct motorsports application are almost non-existent, be it numerical or experimental.…”

Section: Introductionmentioning

confidence: 99%

Turbulence modeling effects on the aerodynamic characterizations of a NASCAR Generation 6 racecar subject to yaw and pitch changes

Chen

Bounds

Selent

et al. 2019

Proceedings of the Institution of Mechanical Engineers, Part D:

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The characterization of a racecar’s aerodynamic behavior at various yaw and pitch configurations has always been an integral part of its on-track performance evaluation in terms of lap time predictions. Although computational fluid dynamics has emerged as the ubiquitous tool in motorsports industry, a clarity is still lacking about the prediction veracity dependence on the choice of turbulence models, which is central to the prediction variability and unreliability for the Reynolds Averaged Navier–Stokes simulations, which is by far the most widely used computational fluid dynamics methodology in this industry. Subsequently, this paper presents a comprehensive assessment of three commonly used eddy viscosity turbulence models, namely, the realizable [Formula: see text] (RKE), Abe–Kondoh–Nagano [Formula: see text], and shear stress transport [Formula: see text], in predicting the aerodynamic characteristics of a full-scale NASCAR Monster Energy Cup racecar under various yaw and pitch configurations, which was never been explored before. The simulations are conducted using the steady Reynolds Averaged Navier–Stokes approach with unstructured trimmer cells. The tested yaw and pitch configurations were chosen in consultation with the race teams such that they reflect true representations of the racecar orientations during cornering, braking, and accelerating scenarios. The study reiterated that the prediction discrepancies between the turbulence models are mainly due to the differences in the predictions of flow recirculation and separation, caused by the individual model’s effectiveness in capturing the evolution of adverse pressure gradient flows, and predicting the onset of separation and subsequent reattachment (if there be any). This paper showed that the prediction discrepancies are linked to the computation of the turbulent eddy viscosity in the separated flow region, and using flow-visualizations identified the areas on the car body which are critical to this analysis. In terms of racecar aerodynamic performance parameter predictions, it can be reasonably argued that, excluding the prediction of the %Front prediction, shear stress transport is the best choice between the three tested models for stock-car type racecar Reynolds Averaged Navier–Stokes computational fluid dynamics simulations as it is the only model that predicted directionally correct changes of all aerodynamic parameters as the racecar is either yawed from the 0° to 3° or pitched from a high splitter-ground clearance to a low one. Furthermore, the magnitude of the shear stress transport predicted delta force coefficients also agreed reasonably well with test results.

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On multistabilities of real car's wake

Cited by 46 publications

References 19 publications

Characterisation of Wake Bi-stability for a Square-Back Geometry with Rotating Wheels

Characterisation of Wake Bi-stability for a Square-Back Geometry with Rotating Wheels

Simulations of the bi-modal wake past three-dimensional blunt bluff bodies

Turbulence modeling effects on the aerodynamic characterizations of a NASCAR Generation 6 racecar subject to yaw and pitch changes

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