Experimental analysis of the effect of local base blowing on three-dimensional wake modes

Lorite-Díez, M.; Jiménez-González, J.I.; Pastur, Luc; Martı́nez-Bazán, C.; Cadot, Olivier

doi:10.1017/jfm.2019.917

Cited by 45 publications

(18 citation statements)

References 51 publications

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“…Thus the feedback control reduces the length of the recirculation bubble while also reducing the drag. This differs to findings in the absence of unsteady actuation or feedback control, where shorter recirculation bubbles imply increased drag [52,53], but is consistent with other feedback control studies, where the change in the recirculation length was more related to actuation location than drag reduction [54]. Figure 12 shows the streamlines on a horizontal plane at the mid-height of the body.…”

Section: B Mean Flow Properties and Wake Topologysupporting

confidence: 85%

Nonlinear feedback control of bimodality in the wake of a three-dimensional bluff body

Morgans¹

2022

Phys. Rev. Fluids

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The turbulent wake behind a square-back Ahmed body in close proximity to the ground exhibits bi-modal switching. This manifests as the centre of the wake switching between one of two asymmetric positions, either horizontally or vertically. Switches occur over random timescales, with the wake recovering symmetry in the long time-average. The present work employs wall-resolved large eddy simulations to investigate feedback control for suppressing horizontal (lateral) wake bi-modality of a square-back Ahmed body at ReH ∼ 3.3 × 10 4 based on the body height. Base-mounted pressure sensors are used to estimate the position of the wake as an input signal for the controller, while actuation targets the near-wake region via synthetic jets emanating from a gap around the perimeter of the Ahmed body base. A nonlinear feedback controller based on a Langevin model of the wake dynamics is synthesized. This successfully suppresses the wake lateral bi-modal switching. However, this switching is replaced by a time-periodic streamwise motion of the large coherent structure occupying the near-wake region, leading to amplification of the higher frequency dynamical wake modes. The action of feedback control also leads to base pressure recovery and a reduction in pressure drag. Upon varying the controller parameters, a trade-off between the degree of bi-modality suppression and drag reduction is observed. A maximum drag reduction of 7.4% is achieved for a semi-symmetrised wake, with a fully symmetrised wake achieving 2.5% reduction. Bi-modality suppression is proposed to have an indirect link to drag reduction through the effect of the wake state on the separated free shear layers and the upstream boundary layers.

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Section: B Mean Flow Properties and Wake Topologysupporting

confidence: 85%

Nonlinear feedback control of bimodality in the wake of a three-dimensional bluff body

Morgans¹

2022

Phys. Rev. Fluids

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“…Volpe et al (2015) showed that angular changes of 1 degree yaw can almost entirely fix one of the two lateral wake states. More recently, attempts have been made to reduce the drag by suppressing bistability through active or passive flow control (Brackston et al 2016;Evstafyeva, Morgans & Dalla Longa 2017;Li 2017;Lorite-Díez et al 2020). Barros et al (2017) were able to control the wake flows through applying grids and cylinders to an underbody, while Bonnavion & Cadot (2018) examined the effects of ground proximity and inclination.…”

Section: Previous Studiesmentioning

confidence: 99%

The influence of background turbulence on Ahmed-body wake bistability

et al. 2021

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The discovery of wake bistability has generated an upsurge in experimental investigations into the wakes of simplified vehicle geometries. Particular focus has centred on the probabilistic switching between two asymmetrical bistable wake states of a square-back Ahmed body; however, the majority of this research has been undertaken in wind tunnels with turbulence intensities of less than $1\,\%$ , considerably lower than typical atmospheric levels. To better simulate bistability under on-road conditions, in which turbulence intensities can easily reach levels of $10\,\%$ or more, this experimental study investigates the effects of free-stream turbulence on the bistability characteristics of the square-back Ahmed body. Through passive generation and quantification of the free-stream turbulent conditions, a monotonic correlation was found between the switching rate and free-stream turbulence intensity.

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“…Such a thinner wake is in accordance with the theories of Roshko (1993) and Sychev et al (1998), resulting in a lower momentum deficit and thus increased base pressure as measured. The fact that this shorter wake is associated with base pressure recovery is rather opposite to the conventional relation between increased recirculation length and base pressure recovery found throughout the literature for square-back geometries (Grandemange, Gohlke & Cadot 2013a;Mariotti, Buresti & Salvetti 2015;Lorite-Díez et al 2020). In this case, there exists a relatively simple relation between the base pressure and the recirculation length L r since the flow curvature can be estimated by the ratio L r /H.…”

Section: Mechanisms Of Pressure Drag Decrease: a Matter Of Flow Curvamentioning

confidence: 68%

Unsteady Coanda effect and drag reduction for a turbulent wake

et al. 2020

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Experimental analysis of the effect of local base blowing on three-dimensional wake modes

Cited by 45 publications

References 51 publications

Nonlinear feedback control of bimodality in the wake of a three-dimensional bluff body

Nonlinear feedback control of bimodality in the wake of a three-dimensional bluff body

The influence of background turbulence on Ahmed-body wake bistability

Unsteady Coanda effect and drag reduction for a turbulent wake

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