Wake transitions behind a cube at low and moderate Reynolds numbers

Meng, Qingrui; An, Hongwei; Cheng, Liang; Kimiaei, Mehrdad

doi:10.1017/jfm.2021.406

Cited by 13 publications

(11 citation statements)

References 28 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Notably, similar phase diagram trajectories have been reported for the initiation of unsteady motions due to the vortex shedding phenomenon in the wake of a cube by Meng et al. (2021). Further increasing the Reynolds number to led to the formation of multiple phase trajectories, which did not collapse on each other.…”

Section: Resultssupporting

confidence: 84%

The unsteady wake transition behind a wall-mounted large-depth-ratio prism

2022

View full text Add to dashboard Cite

The wake of a long rectangular wall-mounted prism is investigated at Reynolds numbers of $Re=250\unicode{x2013}1200$ by directly solving the Navier–Stokes equations. The aim of this study was to examine the unsteady transition mechanism in the wake of a large-depth-ratio (streamwise length to width) prism as well as to characterize the unsteady wake evolution at low Reynolds numbers. The results highlighted that increasing Reynolds number significantly altered the dominance of upwash and downwash flows in the time-averaged flow and changed the characteristics of coherent structures, including their size, dominant frequency and interaction with other structures in the flow. The wake is, therefore, categorized into three regimes within the transition process: steady regime at $Re \leq 625$ , regular unsteady regime at $625 < Re < 750$ and irregular unsteady regime at $Re \geq 750$ . Particularly, the wake started to exhibit unsteady features at $Re=625\unicode{x2013}650$ , which transitioned to an early irregular unsteady wake topology at $Re=750$ . At $Re \geq 675$ , horseshoe vortices transformed to vortex loops. There were hairpin-like structures formed on the upper and side faces of the long prism. The results further indicated that the transition to unsteadiness is attributed to separated leading-edge shear-layer instabilities and interactions of the shear layer with the horseshoe structures. The wake was more complex due to the interactions of multiple coherent structures in the flow, which resulted in a multiple-periodicity wake system. A skeleton model is proposed for a large-depth-ratio prism, to incorporate details of the unsteady flow features and specify various flow coherent structures at low Reynolds numbers.

show abstract

Section: Resultssupporting

confidence: 84%

The unsteady wake transition behind a wall-mounted large-depth-ratio prism

2022

View full text Add to dashboard Cite

show abstract

“…(2020 a , b ) and Meng et al. (2021) scrutinize the mechanism of shedding and various wake regimes for isolated cubes. They note a reduction of the shedding frequency to at a Reynolds number of and at , relative to suspended thin flat plates.…”

Section: Resultsmentioning

confidence: 99%

“…2020 a ; Meng et al. 2021) as well as wall-mounted prisms (Saha 2013; Diaz-Daniel et al. 2017; Rastan et al.…”

Section: Resultsmentioning

confidence: 99%

“…aspect ratio and depth ratio. Past literature has focused on wake transition mechanisms in the case of suspended cubes (Saha 2004;Khan et al 2020a;Meng et al 2021) as well as wall-mounted prisms (Saha 2013;Diaz-Daniel et al 2017;Rastan et al 2017;Zhang et al 2017). For the case of a suspended cube, Hopf bifurcation (Saha 2004;Khan et al 2020a) results in transition to unsteady flow, mainly at Reynolds numbers of 250-300.…”

Section: Instantaneous Wake Characteristicsmentioning

confidence: 99%

“…The unsteady flow loses planar symmetry, and the wake is characterized by shedding of hairpin-like vortices in the wake. Further, Khan et al (2020a,b) and Meng et al (2021) scrutinize the mechanism of shedding and various wake regimes for isolated cubes. They note a reduction of the shedding frequency to ≈ 0.09 at a Reynolds number of 270 and ∼0.13 at Re = 400, relative to suspended thin flat plates.…”

Section: Upwash and Downwash Motionmentioning

confidence: 99%

See 2 more Smart Citations

Mechanisms of wake asymmetry and secondary structures behind low aspect-ratio wall-mounted prisms

Goswami

Hemmati²

2022

J. Fluid Mech.

View full text Add to dashboard Cite

The wake of a wall-mounted finite prism is numerically studied and characterized with an aspect ratio (height-to-width) of $1$ and varying depth ratios (length to width) of between $0.016$ and $4$ at Reynolds numbers of 50–500. The prism is immersed in a laminar boundary layer. The minimum depth ratio considered here accounts for the special case of a wall-mounted very thin prism (similar to a flat plate), which is used to establish the mechanism and evolution of the wake associated with free-end effects and the shear-layer dynamics in small aspect-ratio prisms. The onset of an unsteady wake behind a very thin prism at a Reynolds number of $200$ is characterized by symmetric shedding of hairpin-like vortices. A unique asymmetric wake pattern appears at lower depth ratios starting at a Reynolds number of 250, which transitions to an symmetric wake with increasing depth ratio. The threshold depth ratio for this symmetric transition increases with Reynolds number. The asymmetric wake results from alternate shear-layer peel-off from either side of the prism, which itself is attributed to the out-of-phase shedding of tip vortices at a lower Strouhal number ( $St_{sh}/2$ ) that interact with the detaching side shear layers. Alternate shedding of tip vortices form secondary vortex structures that are fed by the excess vorticity resulting from shear-layer detachment from either side of the prism. Increasing the depth ratio leads to simultaneous shedding of the tip vortices, which restores the commonly observed wake symmetric patterns. Thus, we identify and characterize the formation and interaction mechanisms of symmetric and asymmetric wakes during the transition process with increasing Reynolds number for different depth-ratio prisms.

show abstract