Near wake interactions and drag increase regimes for a square-back bluff body

Bao, Di; Borée, Jacques; Haffner, Yann; Sicot, Christophe

doi:10.1017/jfm.2022.28

Cited by 10 publications

(8 citation statements)

References 46 publications

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“…However, here, it seems possible that the experimental results were overcorrected by blockage, e.g., the C b of the wW case was corrected from 0.291 (Urquhart et al, 2020) to 0.184 (the reference area here is the projected frontal area including the wheels). The contribution of the base suction to the total drag for the nW model in this study is about 84%, which is higher than the value reported by Varney (2020), but is consistent with that achieved from the experiment of Bao et al (2022). More convincing evidence is that the center of the vortices in the wake fits well in the match between current simulations and the PIV data, as shown in Fig.…”

Section: Resupporting

confidence: 91%

“…Note that less switching occurs in case wW, suggesting a more stable asymmetry state than in case nW. The same trend has also been identified by Bao et al (2022) that the wheels (or the obstacles) have the ability to promote the appearance of bimodal dynamics in some cases. This will be explained in Sec.…”

Section: A Detection Of Wake Bi-stability and Corresponding Wake Topo...supporting

confidence: 75%

“…Note the underbody flow rate in case wW2 is only 8% less than in case wW, but the inclination angle of the bottom flow (measured by the tangent to the bold streamline at x Ã ¼ 1) increases from 19 to 26 . This emphasizes the high sensitivity of bi-stability to upstream perturbations (Grandemange et al, 2013b;Plumejeau et al, 2020;and Bao et al, 2022).…”

Section: B Reasons For the Altered Behavior Behind The Wakementioning

confidence: 90%

“…As a result of the randomness of the long-time dynamics, the symmetry breaking position of case nW and wW happens to predominate on the right side (state R) in the current time window. Such statistical asymmetry induced by insufficient acquisition time is very common in the literature(Grandemange et al, 2013a;Cadot et al, 2015;Dalla Longa et al, 2019;Pavia et al, 2020;Plumejeau et al, 2020;Kang et al, 2021;Burton et al, 2021;and Bao et al, 2022) when bi-stability is involved. This is because achieving perfect statistical symmetry requires a recording time of over 60 times the long timescale T l $ 10 3 H=U 1(Volpe et al, 2015), which is time costly even for experiments.…”

mentioning

confidence: 99%

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Comparison of flow characteristics behind squareback bluff-bodies with and without wheels

et al. 2023

Physics of Fluids

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The wake dynamics of two referenced variations of the squareback Windsor model with and without wheels are numerically studied by performing Improved Delayed Detached Eddy Simulation (IDDES). Numerical assessments are validated against publicly available experimental data. The focus of this study is on the wake states influenced by the wheels and the thick oncoming floor boundary layer. Results show that the addition of the wheels significantly changes the aerodynamic forces, the underbody flow, and the wake topology. The wake bi-stability is also enhanced with wheels in place due to the increased curvature of lateral shear layers in the near wake. However, the bi-stable behavior is largely suppressed when immersed in a thick boundary layer. These alterations depend on the degree of interaction between the wake recirculation and the bottom flow, and such degree is strongly affected by the underbody flow momentum. The evolution of low-order flow organizations and complementary spectral analysis highlight the differences in the coherent dynamics of the wake. The finding of the present work suggests that the wake bi-stability behind the squareback body can exist not only for a simplified geometry but also for a more realistic car with wheels in real-world upstream conditions.

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Section: Resupporting

confidence: 91%

Section: A Detection Of Wake Bi-stability and Corresponding Wake Topo...supporting

confidence: 75%

Section: B Reasons For the Altered Behavior Behind The Wakementioning

confidence: 90%

mentioning

confidence: 99%

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Comparison of flow characteristics behind squareback bluff-bodies with and without wheels

et al. 2023

Physics of Fluids

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“…Previous studies have shown that the wake switching is sensitive to the underbody flow. Experimental studies have triggered wake switching by disturbing the underbody flow, for example using a passive device in the clearance between the body and the ground (Barros et al 2017;Bao et al 2022). Here, we consider that the disturbances in the underbody flow may in part be caused by the interaction of smaller vortices from other surfaces due to the boundary layer dynamics.…”

Section: Unforced Flowmentioning

confidence: 99%

Wake bi-modality: the effect of upstream boundary layer dynamics

Ahmed,

Morgans

2023

J. Fluid Mech.

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The turbulent wake past a square-back Ahmed body in close proximity to the ground experiences random side-to-side switching between two asymmetric positions, a phenomenon known as bi-modality. It has been observed to be sensitive to the dynamics of the upstream boundary layers formed along the body surfaces. Close to the body fore end, these separate and reattach, with hairpin vortices emanating from the reattachment points and growing along the surfaces before breaking down upstream of the base. This study uses wall-resolved large eddy simulations to investigate the effect of using suction to suppress these upstream boundary layer separations on the wake bi-modality. It is seen that, in the unforced flow (in the absence of suction), the smaller top and side surface vortices resulting from breakdown interact as they convect downstream. Steady suction is confirmed to suppress the boundary layer separations on the different body surfaces. When the boundary layer separations on the two side (vertical) surfaces are suppressed, it is found that horizontal bi-modality is completely inhibited with weak vertical asymmetry preserved. The interaction of the small top/side surface vortices is interrupted, damping boundary layer fluctuations just upstream of the base. Applying suction on different combinations of side/top/bottom boundary layer separations is found to have different effects on the underbody flow and the wake vertical balance, with bi-modality suppression dependent on side surface suction. This confirms that bi-modality is triggered, at least in part, by boundary layer disturbances on the surfaces perpendicular to the switching direction.

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Drag reduction using longitudinal vortices on a flat-back Ahmed body

et al. 2023

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The paper presents different cavity wall designs at the rear of a flat-back Ahmed body to achieve drag reduction. The wake balance is assessed using the variance of the base pressure gradient and correlated to the obtained drag reduction. Without the rear cavity, the flat-back Ahmed body is subject to a steady instability producing a strong wake imbalance in the horizontal direction. The low drag rear design consists of a $$5^\circ$$ 5 ∘ inclined spoiler at the bottom (refered to as a diffuser) that first provokes a vertical imbalance and a non-inclined top spoiler with central imprints that re-balance the wake. The drag reduction is 11.5%, the wake structure is observed to respect the symmetry of the body base in both lateral and vertical directions suggesting a full stabilization of the initial steady instability. Both the wake balance and the stabilization are associated with the formation of a longitudinal vortex pair initiated at the imprint edges and penetrating the recirculating area.

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Near wake interactions and drag increase regimes for a square-back bluff body

Cited by 10 publications

References 46 publications

Comparison of flow characteristics behind squareback bluff-bodies with and without wheels

Comparison of flow characteristics behind squareback bluff-bodies with and without wheels

Wake bi-modality: the effect of upstream boundary layer dynamics

Drag reduction using longitudinal vortices on a flat-back Ahmed body

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