Numerical investigation of the wake bi-stability behind a notchback Ahmed body

He, Kan; Minelli, Guglielmo; Wang, Jiabin; Dong, Tianyun; Gao, Guangjun; Krajnović, Siniša

doi:10.1017/jfm.2021.748

Cited by 30 publications

(9 citation statements)

References 37 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This geometry originates from the hatchback Ahmed body (Ahmed et al, 1984) by adding a trunk attached to the rear vertical base. The wake bi-stability of the notchback Ahmed body has been previously confirmed in largeeddy simulation (LES) by He et al (2021b).…”

Section: Introductionmentioning

confidence: 63%

“…, where D i are the computational cell sizes in three coordinate directions. The wall-adapting local eddy-viscosity (WALE) model proposed by Nicoud and Ducros (1999), which has been successfully applied to predict wakes of the hatchback Ahmed body (Krajnovi c and Davidson, 2005;Aljure et al, 2014), and symmetrybreaking modes behind the notchback Ahmed body (He et al, 2021b;2021c), is used in this work. This SGS model is based on the square of the velocity gradient tensor and accounts for the effects of the strain and rotation rates.…”

Section: B Numerical Methodsmentioning

confidence: 99%

“…For this reason, the random switch of the bi-stable wake is not captured in the present work due to the limitation of the physical time. However, the readers can refer to He et al (2021b) for details of the wake switching process and the mirrored asymmetric states of the studied flow.…”

Section: A Flow Statesmentioning

confidence: 99%

“…Considering the wake behind the notchback configuration involving the interaction of separations and reattachment, the wake asymmetry formed behind the notchback Ahmed body (Sims-Williams et al, 2011) is selected as the studied flow. Performing LES to predict the bistable wake behind the squareback ( € Osth et al, 2014;Evstafyeva et al, 2017;Lucas et al, 2017;Dalla Longa et al, 2019) and the notchback (He et al, 2021a;2021b;2021c). Ahmed bodies has proved to be successful due to its possibility to resolve the highly turbulent flow.…”

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

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

Minelli

et al. 2021

Physics of Fluids

Self Cite

View full text Add to dashboard Cite

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.

show abstract

Section: Introductionmentioning

confidence: 63%

Section: B Numerical Methodsmentioning

confidence: 99%

Section: A Flow Statesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

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

Minelli

et al. 2021

Physics of Fluids

Self Cite

View full text Add to dashboard Cite

show abstract

“…Bi-stability has also been observed in the flow past an Ahmed body (Grandemange, Gohlke & Cadot 2013 a , b ; He et al. 2021), with Dalla Longa, Evstafyeva & Morgans (2019) proposing the switch is caused by the formation of sufficiently large hairpin vortices. In the present case, the wake demonstrates multi-stable behaviour, in which the wake locally jumps between von Kármán shedding and suppression states, which may feature bias.…”

Section: Resultsmentioning

confidence: 86%

Asymmetric wakes in flows past circular cylinders confined in channels

Aljubaili

Zahtila

et al. 2023

J. Fluid Mech.

View full text Add to dashboard Cite

In this paper, we investigate the flow past a circular cylinder confined in a channel at a blockage ratio of $\beta =0.7$ (the ratio of the cylinder diameter and the channel height) for Reynolds numbers between ${\textit {Re}}=300$ and $3900$ using direct numerical simulation (DNS). We show for varying Reynolds numbers a wide range of wake dynamics occur as the spanwise domain length is changed. At a lower Reynolds number of ${\textit {Re}}=300$ , a reverse von Kármán wake alongside either a top- or bottom-biased asymmetry was observed at different spanwise locations. The asymmetry was structurally similar the two-dimensional asymmetry studied by prior investigators, and was found to be a result of the confinement effect. Further, wake-jumping between the two intermittent states was present. For larger Reynolds numbers, ${\textit {Re}}=1000$ and $3900$ , these asymmetric structures were found to become dominant. We also examine the dependence of the asymmetries on the spanwise domain. For small spanwise domains the asymmetries were uniformly orientated across the span. In contrast, for sufficiently large spanwise domains, the asymmetry flips its orientation at different spanwise locations. Comparisons of flow statistics demonstrate good agreement between the different spanwise domains, which suggests the same mechanism maintains the asymmetry in both cases. Further analysis at ${\textit {Re}}=1000$ found the number of times the wake flips is dependent on the initial conditions, with a wake that flips zero (purely asymmetric), two and four times being observed. These structures were also determined to remain stable over time scales of $1000D/U$ .

show abstract

Flow Topology of the Bi-Stable Wake States for the DrivAer Fastback Model

Aultman,

Duan

2024

Flow Turbulence Combust

View full text Add to dashboard Cite

For this work, conditional averaging and Proper Orthogonal Decomposition (POD) were used to analyze the salient three-dimensional structures in the wake of a DrivAer fastback model with smooth underbody. Conditional averaging revealed that the bi-stable structure of the wake consists of a ring-like structure with three vortex legs, which includes a vortex pair on the side associated with the bi-stability and one on the opposite side associated with the wheel vortex. POD revealed the entrainment of low-momentum fluid from the wheel wake into the vortex pair leads to an induced spanwise crossflow which drives a feedback loop for the bi-stability. The resultant bi-stable structure was dependent on the state of the wheels. With stationary wheels, the feedback mechanism is enhanced, leading to higher spanwise crossflow that breaks the ring-like vortex. A different structure was observed when the wheels rotate, wherein the ring-like structure is unbroken and pierced by the vortex pair. The feedback mechanism and resultant vortex structure are similar to those found in simplified square-back models. Given the similarity in bi-stability between realistic and simplified vehicles, the suppression of the bi-stability in realistic vehicles could initially be based on the same mechanism as that for simplified square-back models.

show abstract

Numerical investigation of the wake bi-stability behind a notchback Ahmed body

Cited by 30 publications

References 37 publications

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

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

Asymmetric wakes in flows past circular cylinders confined in channels

Flow Topology of the Bi-Stable Wake States for the DrivAer Fastback Model

Contact Info

Product

Resources

About