Onset of vortex shedding around a short cylinder

Yang, Yongliang; Feng, Zhe; Zhang, Mengqi

doi:10.1017/jfm.2021.1034

Cited by 5 publications

(27 citation statements)

References 53 publications

(132 reference statements)

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“…In addition, the very short length of the N-cell also makes it difficult to form a closed N–N loop. The C-shaped N-loop vortices in figure 12 resemble the hairpin vortices in the wake of a very short cylinder (Yang, Feng & Zhang 2022) or a sphere (Nagata et al. 2018).…”

Section: Numerical Resultsmentioning

confidence: 99%

“…In addition, the very short length of the N-cell also makes it difficult to form a closed N-N loop. The C-shaped N-loop vortices in figure 12 resemble the hairpin vortices in the wake of a very short cylinder (Yang, Feng & Zhang 2022) or a sphere (Nagata et al 2018). When a rotating sphere is placed in a flow a single-sided harpin vortex street was found (Giacobello, Ooi & Balachandar 2009;Rajamuni, Thompson & Hourigan 2018).…”

Section: Numerical Resultsmentioning

confidence: 99%

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Three-dimensional numerical simulation of flow past a rotating step cylinder

Zhao

Zhang

2023

J. Fluid Mech.

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Flow past a rotating step cylinder is investigated through three-dimensional numerical simulations for a diameter ratio of 0.5 and a Reynolds number of 150. The step cylinder comprises two cylinders with different diameters arranged coaxially with a step between them. The rotation rate α is defined as the ratio of the rotation speed of the larger cylinder surface to the free-stream velocity. Vortex shedding happens for both cylinders at α = 0, 0.5 and 1, and is suppressed only for the larger cylinder at α = 2 and 3 and fully suppressed for both cylinders at α = 4. The vortex shedding suppression for the larger cylinder or for both cylinders has significant effects on the wake. The S-, N- and L-cells at α = 0 are in good agreement with those reported in previous studies and still exist at α = 1. The N-cell disappears at α = 0.5, and as a result, the L- and S-cells interact with each other directly at the step position. An additional cellular zone is found at α = 0.5 and 1 and this zone has multiple cells with vortex dislocation between them. At α = 2 and 3, there is a strong hub vortex in the streamwise direction behind the step and the vortices in the wake of the smaller cylinder form helical vortices after they roll around this hub vortex. The hub vortex is generated by the difference between the Magnus effect between the smaller and larger cylinders. At α = 4, the hub vortex still exists but the helical vortices disappear because vortex shedding is suppressed for both cylinders. At this rotation rate, the pressure on the cylinder surface oscillates with a frequency much higher than the vortex shedding frequency. The oscillation of the pressure is caused by the combination of periodic generation of ring vortices and their motion along the span of the larger cylinder.

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Section: Numerical Resultsmentioning

confidence: 99%

Section: Numerical Resultsmentioning

confidence: 99%

Three-dimensional numerical simulation of flow past a rotating step cylinder

Zhao

Zhang

2023

J. Fluid Mech.

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“…The time step Δt is determined by the Courant-Friedrichs-Lewy condition with the target Courant number 1.0. Following the practice in our previous work Yang et al (2022), the boundary layer elements in the vicinity of the rotating cylinder have been refined by the O-type mesh (see figure 1b). The only difference lies in the enlarged computational domain and increased number of elements to accommodate the rotational effects.…”

Section: Sensitivity Analysismentioning

confidence: 99%

“…The separation bubble is always confined between angles θ s = 0 and θ s = π, as shown in figure 3(a). Note that in the case of a non-rotation cylinder (Yang et al 2022), the separation angles may be varying depending on Re and the aspect ratio. We define the separation bubble length as x s = max{x − D/2 | U x (x, y, z = 0) ≤ 0}; see figure 3(a).…”

Section: Separation Bubblementioning

confidence: 99%

“…Now, we turn to figure 4 to discuss the drag and lift coefficients in the short rotating cylinder flow. Starting from the fixed finite-length cylinder case (Yang et al 2022), figure 4(a) shows that the drag coefficient increases as the rotation rate increases, which is caused by the fact that the rotation makes the negative pressure in the leeward area smaller, but has little effect on the maximum positive pressure at the stagnation point on the windward side. These can be seen from the pressure contour in panels (a,b) and (c,d) of figure 2, and more quantitative results on the lift and drag coefficients varying with α are shown in figure 28(a,b) in Appendix C. A comparative analysis reveals that the augmentation of α increases the lift coefficient of both the 2-D infinite cylinder and present 3-D finite-length cylinder.…”

Section: Drag and Lift Coefficientsmentioning

confidence: 99%

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Numerical analyses of the flow past a short rotating cylinder

Yang,

Wang,

Guo

et al. 2023

J. Fluid Mech.

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This work studies the three-dimensional flow dynamics around a rotating circular cylinder of finite length, whose axis is positioned perpendicular to the streamwise direction. Direct numerical simulations and global stability analyses are performed within a parameter range of Reynolds number $Re=DU_\infty /\nu <500$ (based on cylinder diameter $D$ , uniform incoming flow velocity $U_\infty$ ), length-to-diameter ratio ${\small \text{AR}}=L/D\leq 2$ and dimensionless rotation rate $\alpha =D\varOmega /2U_\infty \leq 2$ (where $\varOmega$ is rotation rate). By solving Navier–Stokes equations, we investigated the wake patterns and explored the phase diagrams of the lift and drag coefficients. For a cylinder with ${\small \text{AR}}=1$ , we found that when the rotation effect is weak ( $0\leq \alpha \lesssim 0.3$ ), the wake pattern is similar to the unsteady wake past the non-rotating finite-length cylinder, but with a new linear unstable mode competing to dominate the saturation state of the wake. The flow becomes stable for $0.3\lesssim \alpha \lesssim 0.9$ when $Re<360$ . When the rotation effect is strong ( $\alpha \gtrsim 0.9$ ), new low-frequency wake patterns with stronger oscillations emerge. Generally, the rotation effect first slightly decreases and then sharply increases the $Re$ threshold of the flow instability when $\alpha$ is relatively small, but significantly decreases the threshold at high $\alpha$ ( $0.9<\alpha \leq 2$ ). Furthermore, the stability analyses based on the time-averaged flows and on the steady solutions demonstrate the existence of multiple unstable modes undergoing Hopf bifurcation, greatly influenced by the rotation effect. The shapes of these global eigenmodes are presented and compared, as well as their structural sensitivity, visualising the flow region important for the disturbance development with rotation. This research contributes to our understanding of the complex bluff-body wake dynamics past this critical configuration.

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