Three-dimensional structural characteristics of flow separation induced by a forward-facing step in a turbulent channel flow

Fang, Xingjun; Tachie, Mark F.; Bergstrom, Donald J.; Yang, Zixuan; Wang, Bing-Chen

doi:10.1017/jfm.2021.395

Cited by 6 publications

(18 citation statements)

References 54 publications

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“…This confinement of low-frequency flapping motion to the first half of the mean separation bubble marks a significant difference between the flapping motions of separation bubbles induced by bluff bodies in uniform flow and those induced by surface-mounted bluff bodies subjected to incoming TBL investigated by Pearson et al (2013), Fang & Tachie (2019b, 2020 and Fang et al (2021). For instance, Fang et al (2021) reported that the reverse flow area of the entire separation bubble over a forward-facing step (FFS) in a turbulent channel flow experiences quasi-periodic expansion/contraction at multiple frequencies. These frequencies coincide with the fundamental and harmonic frequencies of incoming hairpin structures, but are significantly different from the typical frequencies of flapping motion due to development of separation shear layers (Eaton & Johnston 1982;Kiya & Sasaki 1983;Cherry et al 1984;Tafti & Vanka 1991;Cimarelli et al 2018).…”

Section: Unsteadiness Of Flow Separationmentioning

confidence: 89%

“…Cimarelli et al (2018) also argued that the low-frequency flapping motion is 'strictly related to the behaviour of the secondary vortex', which occurs in the first half of the primary mean separation bubble. This confinement of low-frequency flapping motion to the first half of the mean separation bubble marks a significant difference between the flapping motions of separation bubbles induced by bluff bodies in uniform flow and those induced by surface-mounted bluff bodies subjected to incoming TBL investigated by Pearson et al (2013), Fang & Tachie (2019b, 2020 and Fang et al (2021). For instance, Fang et al (2021) reported that the reverse flow area of the entire separation bubble over a forward-facing step (FFS) in a turbulent channel flow experiences quasi-periodic expansion/contraction at multiple frequencies.…”

Section: Unsteadiness Of Flow Separationmentioning

confidence: 89%

“…2018; Moore, Letchford & Amitay 2019; Fang & Tachie 2020; Fang et al. 2021). For surface-mounted bluff bodies, the role of upstream wall roughness on the mean separation bubble, turbulent statistics and unsteady features of the separated shear layers has been examined critically in the recent past (Essel et al.…”

Section: Introductionmentioning

confidence: 99%

“…Flow separations induced by two-dimensional sharp-edged bluff bodies in uniform flow as well as surface-mounted bluff bodies in the form of forward-facing or backward-facing steps immersed in a boundary layer have been studied extensively (Eaton & Johnston 1981;Hillier & Cherry 1981;Kiya & Sasaki 1983;Tafti & Vanka 1991;Pearson, Goulart & Ganapathisubramani 2013;Lander et al 2016;Graziani et al 2018;Moore, Letchford & Amitay 2019;Fang & Tachie 2020;Fang et al 2021). For surface-mounted bluff bodies, the role of upstream wall roughness on the mean separation bubble, turbulent statistics and unsteady features of the separated shear layers has been examined critically in the recent past Nematollahi & Tachie 2018;Fang & Tachie 2020).…”

Section: Introductionmentioning

confidence: 99%

“…It has been demonstrated that wall roughness upstream of a forward-facing step reduces the mean reattachment length over the step by 20-50 %. The interactions between the coherent structures embedded in the approaching wall boundary layers and flow separations have been investigated recently (Pearson et al 2013;Fang & Tachie 2019b, 2020Fang et al 2021). The results of these studies demonstrated that the large-scale motion (Adrian, Meinhart & Tomkins 2000) and hairpin structures (Zhou et al 1999) residing in wall boundary layers impose a quasi-periodic flapping motion at the same characteristic frequencies.…”

Section: Introductionmentioning

confidence: 99%

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Turbulent separations beneath semi-submerged bluff bodies with smooth and rough undersurfaces

2022

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The spatio-temporal characteristics of turbulent separations beneath semi-submerged bluff bodies with different undersurface roughness conditions are studied using a time-resolved particle image velocimetry. The Reynolds number based on the free-stream velocity and submergence depth was fixed to 14 400. Three different undersurface conditions – smooth, sandpaper roughness and cube roughness – were examined. The results showed that wall roughness reduces the mean reattachment length, and suppresses the Reynolds stresses in the second half of the mean separation bubble. The Kelvin–Helmholtz instability is observed at the leading edge of the smooth bluff body, but is bypassed in the rough cases. In the first half of the mean separation bubble, the frequencies in the separated shear layer migrate to lower values in a discrete manner through the vortex pairing mechanism. Consequently, multiple vortex shedding motions at different frequencies are nested in the separated shear layer, and the cores of shed vortices are aligned near the isopleth of free-stream velocity. The shed vortex is accompanied with multiple vortices along the edge of mean flow reversal in the upstream locations. These vortices are influenced significantly by wall roughness. A low-frequency flapping motion manifests as enlargement/shrinkage of reverse flow areas in the first half of the mean separation bubble. The frequencies of flapping motion in the smooth and sandpaper cases are similar, but are relatively lower than that in the cube roughness case. This flapping motion is associated with an extremely large vortex shed from the mean reattachment point to the free-stream region.

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Section: Unsteadiness Of Flow Separationmentioning

confidence: 89%

Section: Unsteadiness Of Flow Separationmentioning

confidence: 89%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Turbulent separations beneath semi-submerged bluff bodies with smooth and rough undersurfaces

2022

Self Cite

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On the low-frequency flapping motion in flow separation

Fang,

Wang

2024

J. Fluid Mech.

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Transitional separating flow induced by a rectangular plate subjected to uniform incoming flow at Reynolds number (based on the incoming velocity and half plate height) 2000 is investigated using direct numerical simulation. The objective is to unveil the long-lasting mystery of low-frequency flapping motion (FM) in flow separation. At a fixed streamwise-vertical plane or from the perspective of previous experimental studies using pointwise or planar measurements, FM manifests as a low-frequency periodic switching between low and high velocities covering the entire separation bubble. The results indicate that in three-dimensional space, FM reflects an intricate evolution of streamwise elongated streaky structures under the influence of separated shear layer and mean flow reversal. The FM is an absolute instability, and is initiated through a lift-up mechanism boosted by mean flow deceleration near the crest of the separating streamline. At this particular location, the shear bends the vortex filament abruptly, so that one end is vertically struck into the first half of the separation bubble, whereas the other end is extended in the streamwise direction in the second half of the separation bubble. These two ends of vortex filament are mutually sustained and also stretched by the vertical acceleration and streamwise acceleration in the first and second halves of the separation bubble, respectively. This process periodically switches the low-velocity (or high-velocity) streaky structure to a high-velocity (or low-velocity) streaky structure encompassing the entire separation bubble, and thus flaps the separated shear layer up and down in the vertical direction. A ‘large vortex’ shedding manifests when the streaky structure switches signs. This large vortex is fundamentally different from the spanwise vortex shedding residing in the shear layer originated from the Kelvin–Helmholtz instability and successive vortex amalgamation. It is also believed that the three-dimensional evolution of streaky structures in the form of FM is applicable for both geometry- and pressure-induced separating flows.

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The vitality of very-large-scale motions upstream of an overflow structure

Yan

Zhu

2023

AIP Advances

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The flows upstream of a run-of-river dam, commonly utilized as an overflow structure on rivers, are complex due to heterogeneities in both streamwise and spanwise directions. In particular, very-large-scale motions (VLSMs) are greatly influenced by the overflow structure, yet relevant understandings remain limited. Reported as novel coherent structures in turbulent flows, VLSMs are recognized with the scale up to several and tens of the outer-scaled unit, and they contribute significantly to turbulent transport and mixing. To fill the gap, experiments with particle image velocimetry were conducted to investigate the vitality of VLSMs upstream of a model dam. Measurements were designed to cover broad hydraulic scope with flow heterogeneities. The results reveal that VLSMs in the present flow scenario show noticeable characteristics in both streamwise and spanwise directions. Compared to those in uniform flows, the VLSMs in present flows are found to be more energetic and stress-active.

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Three-dimensional structural characteristics of flow separation induced by a forward-facing step in a turbulent channel flow

Abstract: Abstract

Cited by 6 publications

References 54 publications

Turbulent separations beneath semi-submerged bluff bodies with smooth and rough undersurfaces

Turbulent separations beneath semi-submerged bluff bodies with smooth and rough undersurfaces

On the low-frequency flapping motion in flow separation

The vitality of very-large-scale motions upstream of an overflow structure

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