The influence of background turbulence on Ahmed-body wake bistability

Burton, David; Wang, S.; Smith, Daniel Tudball; Scott, Harry; Crouch, Timothy; Thompson, M. C.

doi:10.1017/jfm.2021.706

Cited by 20 publications

(8 citation statements)

References 28 publications

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“…(2020) and Burton et al. (2021) for an Ahmed body subjected to the -instability. It is found that the turbulent intensity magnitude does not affect the steady instability strength, and increases the switching rate of the mirror states in the bi-stable dynamics for values larger than 5 %.…”

Section: Introductionmentioning

confidence: 97%

Wake transitions and steady -instability of an Ahmed body in varying flow conditions

2022

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The paper investigates experimentally the flow past a flat-back, taller than wide Ahmed body having rectangular base aspect ratio $H/W=1.11$ in the context of ground vehicle aerodynamics. Parametric studies at Reynolds number $2.1\times 10^5$ explore the sensitivity of the aerodynamic force and body pressure distribution with respect to varying flow conditions defined from variable ground clearance $C$ (taken at mid-distance from the front and rear axles of the body), pitch angle $\alpha$ , and yaw angle $\beta$ (equivalent to a crosswind). Two-dimensional parametric maps in the parametric spaces $(\beta,C)$ and $(\beta,\alpha )$ are obtained for parameter ranges covering real road vehicle driving conditions. The study of the base pressure gradient reveals non-trivial and sharp transitions identified as significant changes of near wake orientation in both parametric spaces. All unsteady transitions correspond to fluctuation crises of the vertical gradient component only. These transitions are summarized in phase diagram representations. Both phase diagrams in $(\beta,C)$ and $(\beta,\alpha )$ parametric spaces can be unified at large yaw in the $(\beta,C_r)$ space, where the rear clearance $C_r$ separating the lower edge of the base from the ground is a simple function of the pitch $\alpha$ and the clearance $C$ . The impacts of the wake transitions are clearly identified in the base drag, drag, lift and side force coefficients. The contribution of the steady near wake $z$ -instability (Grandemange et al., Phys. Fluids, vol. 25, 2013a, pp. 95–103) is assessed by repeating the sensitivity analysis with a rear cavity. As reported previously, the rear cavity suppresses the steady instability by symmetrizing the wake. A domain for the existence of the instability is finally proposed in the attitudes map $(\beta,\alpha )$ defined from regions where the mean lateral force coefficients are significantly decreased by the presence of the rear cavity. In addition, it is found that the steady instability forces the wake to be less unsteady for all attitudes that do not correspond to unsteady transitions.

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Section: Introductionmentioning

confidence: 97%

Wake transitions and steady -instability of an Ahmed body in varying flow conditions

2022

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“…Therefore, the nearwake flow with a higher blockage ratio is more turbulent. For the wake bi-stability behind the squareback Ahmed body, Burton et al (2021) have proved that a high turbulence level of the background flow leads to highly frequent switches. Moreover, for the squareback Ahmed body, He et al (2022) found that a higher TKE in the wake produced by a higher Reynolds number results in highfrequency wake switches, presenting a symmetric phenomenon.…”

Section: Physics Of Fluidsmentioning

confidence: 99%

Blockage influence on bi-stable flows of a notchback bluff body

Minelli

et al. 2021

Physics of Fluids

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The expected flow asymmetry behind a three-dimensional notchback Ahmed body is numerically investigated using large-eddy simulations with different blockage ratios of 0%, 5%, 10%, and 20%. The focus of the study is on the natural bi-stable flow influenced by the blockage ratio. Although the wake asymmetry can be observed under the blockage ratio within 0% À 10%, the significant blockage influence on the sensitive bi-stability is indicated by the wake structures, pressure gradients, and wake dynamics achieved with the proper orthogonal decomposition. The higher blockage ratio increases the turbulence kinetic energy, velocity, and negative pressure in the near-wake region, resulting in the decrease in the asymmetry degree. The consistency of the asymmetric wake is found with blockage ratios between 0% and 5%. However, a 20% blockage ratio symmetrizes the bi-stable wake. Several existing drag corrections for the blockage influence are discussed. The wind tunnel experiment verifies the bi-stable flow with low blockage ratios and the wake symmetrization with a 20% blockage ratio.

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“…The occurrence of such rapid switches reveals a high sensitivity of the wake (Haffner et al, 2020), as the asymmetric state stability is maintained by a weak interaction between underbody flow and the wake (Plumejeau et al, 2020). In many previous experimental studies (Grandemange et al, 2013a;Cadot et al, 2015;Varney, 2020;Burton et al, 2021;and He et al, 2022), limited by high sampling intervals or the use of filtering to remove highfrequency sampling noise, certain short-time switches were usually omitted. As identified by Volpe et al (2015) in their experiments on squareback Ahmed body, the calculated mean duration dT Ã of the two stable positions decreased from 1092 to 421 when the resampling interval Dt Ã s was reduced from 92.7 to 4.6.…”

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

confidence: 99%

“…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.…”

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confidence: 99%

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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The influence of background turbulence on Ahmed-body wake bistability

Cited by 20 publications

References 28 publications

Wake transitions and steady -instability of an Ahmed body in varying flow conditions

Wake transitions and steady -instability of an Ahmed body in varying flow conditions

Blockage influence on bi-stable flows of a notchback bluff body

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

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