Effect of free stream turbulence on the topology of laminar separation bubble on a sphere

Desai, Aditya; Mittal, Sanjay

doi:10.1017/jfm.2022.696

Cited by 3 publications

(1 citation statement)

References 36 publications

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“…It separates on encountering the seam and reattaches in a laminar state for to finally separate in the shoulder region. A SV, similar to the findings of Desai & Mittal (2022) for a sphere and Chopra & Mittal (2022 b ) for a cylinder, forms in the wake bubble whose azimuthal extent decreases with an increase in . For , the separated shear at the seam undergoes transition and reattaches as a turbulent boundary layer.…”

Section: Discussionsupporting

confidence: 77%

Swing and reverse swing of a cricket ball: laminar separation bubble, secondary vortex and wing-tip-like vortices

Parekh,

Chaplot,

Mittal

2024

J. Fluid Mech.

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Large eddy simulation of flow past a cricket ball with its seam at $30^\circ$ to the free stream is carried out for $5 \times 10^4 \le Re \le 4.5 \times 10^5$ . Three regimes of flow are identified on the basis of the time-averaged swing force coefficient ( $\bar {C}_Z$ ) – no swing (NS), conventional swing (CS, $\bar {C}_Z>0$ ) and reverse swing (RS, $\bar {C}_Z<0$ ). The effect of seam on the boundary layer is investigated. Contrary to the popular belief, the boundary layer does not transition to a turbulent state in the initial stages of CS. The seam energizes the laminar boundary layer and delays its separation. The delay is significantly larger in a region near the poles, whose extent increases with an increase in $Re$ causing $\bar {C}_Z$ to increase. Here $\bar {C}_Z$ assumes a near constant value in the later stage of CS. The boundary layer transitions to a turbulent state via formation of a laminar separation bubble (LSB) in the equatorial region and directly, without a LSB, in the polar region. The extent of the LSB shrinks while the region of direct transition near the poles increases with an increase in $Re$ . A LSB forms on the non-seam side of the ball in the RS regime. A secondary vortex is observed in the wake bubble. While it exists on the non-seam side for the entire range of $Re$ considered, the mixing in the flow introduced by the seam causes it to disappear beyond a certain $Re$ on the seam side. The pressure difference between the seam and non-seam sides sets up wing-tip-like vortices. Their polarity reverses with the switch from the CS to RS regime.

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Section: Discussionsupporting

confidence: 77%

Swing and reverse swing of a cricket ball: laminar separation bubble, secondary vortex and wing-tip-like vortices

Parekh,

Chaplot,

Mittal

2024

J. Fluid Mech.

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Influence of turbulent incoming flow on aerodynamic behaviors of train at 90° yaw angle

Xue

Xiong

et al. 2023

Physics of Fluids

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Turbulent incoming flow conditions are closely matched to the crosswinds experienced by trains in windy areas. Therefore, it is important to investigate how the turbulent inflow affects the flow dynamics around a train. The aerodynamic characteristics of a 1:8-scaled high-speed train at a 90{degree sign} yaw angle was studied based on the Improved Delayed Detached Eddy Simulation (IDDES) turbulence model. Four incoming flow conditions were set using a Synthetic Eddy Method (SEM) turbulent generator, including uniform, Lu = 0.5H, Lu = 1H, and Lu = 2H inflow ( Lu is turbulence integral length scale and H is reference height). The aerodynamic loads, surface pressure, mean vorticity, vortex structure, velocity deficit, turbulence characteristics, Reynold stresses, turbulence production term, and anisotropy of turbulence were thoroughly analyzed. Turbulent inflow and increasing inflow Lu increased the standard deviation of the aerodynamic loads on the train. A crisis of inflow Lu appeared around 0.5H, meaning the rolling moment and overturning moment were largest under this crisis condition. Turbulent inflow caused vortices on the train's leeward side to come closer to the train, increasing the vorticity thickness and shortening the back flow region. The Reynolds stresses on the train's leeward side under turbulent inflow conditions were strengthened. The spectrum proper orthogonal decomposition method was used to analyze the dominant mode within the train's leeward region and the corresponding energy distribution in the frequency domain. The aerodynamic admittance function was used to investigate the frequency characteristics of the aerodynamic loads on the train.

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Evolution of clusters of turbulent reattachment due to shear layer instability in flow past a circular cylinder

Chopra,

Mittal,

Sujith

2024

Physics of Fluids

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We perform large eddy simulations of flow past a circular cylinder for the Reynolds number (Re) range, 2×103≤Re≤4×105, spanning subcritical, critical, and supercritical regimes. We investigate the spanwise coherence of the flow in the critical and supercritical regimes using complex networks. In these regimes, the separated flow reattaches to the surface in a turbulent state due to the turbulence generated by the shear layer instability. In the early critical regime, the turbulent reattachment does not occur simultaneously at all span locations. It occurs incoherently along the span in clusters. We treat strong surface pressure fluctuations due to the shear layer instability as extreme events and construct time-varying spatial proximity networks where links are based on synchronization between events. This analysis unravels the underlying complex spatiotemporal dynamics by enabling the estimation of characteristics of clusters of turbulent reattachment via the concept of connected components. In the critical regime, the number and size of the clusters increase with the increase in Re. At higher Re in the supercritical regime, they coalesce to form bigger clusters, resulting in increase in spanwise coherence of turbulent reattachment. We find that the size and number of clusters govern the variation of the time-averaged coefficient of drag (C¯D) in the critical and supercritical regimes. C¯D exhibits power-law distribution with the largest cluster size (C¯D∝S¯CL−25) and the most probable cluster size [C¯D∝E(SC)−25].

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Effect of free stream turbulence on the topology of laminar separation bubble on a sphere

Cited by 3 publications

References 36 publications

Swing and reverse swing of a cricket ball: laminar separation bubble, secondary vortex and wing-tip-like vortices

Swing and reverse swing of a cricket ball: laminar separation bubble, secondary vortex and wing-tip-like vortices

Influence of turbulent incoming flow on aerodynamic behaviors of train at 90° yaw angle

Evolution of clusters of turbulent reattachment due to shear layer instability in flow past a circular cylinder

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