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

The near wake of a hemisphere immersed in a laminar boundary layer is studied utilizing time-resolved tomographic particle image velocimetry (TPIV). Focus is placed on the three-dimensional vortical structures and the formation details of hairpin vortices before the onset of transition. The three-dimensional instantaneous pressure field of the hemisphere wake is reconstructed for better understanding the flow mechanism. Experiments are carried out with Reynolds number $Re_{r}=1370$ , based on the hemisphere radius $R$ . Features of periodicity of the near wake are analysed using proper orthogonal decomposition and Fourier transformation. The velocity fluctuation in the wall-normal direction is shown to be crucial to the formation of hairpin vortices in the near wake. By investigating the transport of mass and vorticity, and the correlation between pressure and hairpin vortex strength, the formation mechanism is revealed clearly. Specifically, the main hairpin vortices (MHVs) are formed within the reaction of outer high-speed flow and near-wall flow. The formation of the head portion is followed by the leg portion formation. The shedding of the MHVs is highly correlated with the pressure, as well as the pressure gradient in the wall-normal direction. For the side hairpin vortices (SHVs), the leg portion is formed first, followed by the generation of the head portion thanks to induction of the re-oriented standing vortices. The generation of the SHVs can be regarded as the downstream bridging of the standing vortices, similar to the generation of hairpin vortices due to the connection of streamwise vortices in turbulent boundary layers.

Section: Coherent Structures and Formation Of Hairpin Vortexsupporting

confidence: 65%

Tomographic PIV investigation on near-wake structures of a hemisphere immersed in a laminar boundary layer

Tu,

Wang,

Gao

et al. 2023

“…This, however, cannot explain the FM observed by Fang et al. (2022) beneath a semi-submerged rectangular bluff body, where the shear layer was one-sided. The aforementioned mechanisms of FM all relied on (or were limited to) the dynamics of separated shear layers in a two-dimensional (2-D) plane.…”

Section: Introductionmentioning

confidence: 70%

“…This numerical set-up mimics the experimental condition of Fang et al. (2022), and avoids any potential smearing by the unsteadiness originated from the wake flow or shear layer on the opposite side of the plate.…”

Section: Introductionmentioning

confidence: 95%

“…2021) and rectangular plate (Kiya & Sasaki 1983; Cherry, Hillier & Latour 1984; Tafti & Vanka 1991; Cimarelli et al. 2018; Fang, Tachie & Dow 2022). As summarized in table 1, in spite of the differences in flow configurations and Reynolds numbers, the frequency of such a large-scale unsteadiness is, surprisingly, consistent with (where and are the incoming velocity and mean reattachment length, respectively).…”

Section: Introductionmentioning

confidence: 99%

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On the low-frequency flapping motion in flow separation

Fang,

Wang

2024

Self Cite

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.

“…The region occupied by each cylinder was blanked out during the analysis. For each selected dominant frequency, the snapshot POD method (Sirovich 1987;Sen, Bhaganagar & Juttijudata 2007) was performed on the Fourier-transformed data using singular value decomposition (Fang, Tachie & Dow 2022;Chalmers, Fang & Tachie 2023;Essel et al 2023). The singular value decomposition computes the modes and eigenvalues (λ), which represent the energy content of each mode.…”

Section: Spectral Proper Orthogonal Decompositionmentioning

confidence: 99%

Unsteady wake interference of unequal-height tandem cylinders mounted in a turbulent boundary layer

Ouedraogo,

Essel

2023

The unsteady wake interference of unequal-height tandem finite wall-mounted cylinders (FWMCs) fully submerged in a turbulent boundary layer (TBL) was investigated using time-resolved particle image velocimetry. The aspect ratios of the cylinders were fixed at $h/d = 5.3$ for the upstream cylinder (UC) and $H/d = 7.0$ for the downstream cylinder (DC) to achieve a height ratio of $h/H = 0.75$ , where d is the diameter of the cylinders. The Reynolds number based on the cylinder diameter was $Re = 5540$ and the submergence ratio was $\delta /H = 1.2$ , where $\delta $ is the TBL thickness. Three main flow regimes of tandem FWMCs were examined by varying the centre-to-centre spacing ( $s$ ) between the cylinders: extended-body ( $s/d = 2$ ), reattachment ( $s/d = 4$ ) and co-shedding ( $s/d = 6$ ) regimes. These test cases denoted as SR2, SR4 and SR6, respectively, were compared with a reference isolated cylinder (SC) with an aspect ratio similar to that of the DC. Spatio-temporal analysis of the flow field showed that the gap region of SR2 is characterized by a strong downwash of alternating low- and high-momentum fluid induced by the approach flow that is deflected from the unsheltered portion of the DC. In contrast, the gap region of SR4 and SR6 exhibited both downwash and upwash flow with a saddle point that moves closer to the mid-height of the UC as the spacing ratio increases. The upwash and downwash shear layers were associated with small-scale vortices with Strouhal numbers larger than that of the Kármán vortex shedding in the spanwise shear layers. The wake structure behind the DC was significantly altered compared with the SC due to sheltering effects, and the spacing ratio had a significant impact on the spatio-temporal evolution of the vortices.