Dynamics of tandem cylinders in the vicinity of a plane moving wall (original) (raw)
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Case Studies in Thermal Engineering , 2021
Flow across tandem square cylinders placed close to a plane moving wall, has been studied extensively for various cylinder inter-spacing ratio (0.5 ≤ S/D ≤ 8) and cylinder-to-wall gap ratio values (0.1 ≤ G/D ≤ 4) at a fixed value of Reynolds number, Re = 100. Numerical experiments are performed using the computational package ANSYS FLUENT®. Results indicate that unsteady flow past tandem cylinders relies primarily on the presence of a moving wall. Exhaustive details on the effects of varying S/D and G/D values on the onset and suppression of vortex shedding behind the tandem cylinders are given. A detailed description of the underlying flow physics is provided through instantaneous vorticity and streamline contours. Forces such as the lift and drag acting on the cylinders are reasoned qualitatively through time-averaged pressure, lift, and drag coefficient plots. A remark on existence and suppression of flow unsteadiness is given through Strouhal number variation. A rigorous comparison of the present flow field with flow past an isolated square cylinder, square cylinder near a moving wall, and unbounded tandem cylinders is given. Overall, unlike the lift, drag coefficient values happen to be highest for single cylinder case, followed by the upstream cylinder, and they are least for the downstream cylinder.
Dynamics and stability of the wake behind tandem cylinders sliding along a wall
Journal of Fluid Mechanics, 2013
The dynamics and stability of the flow past two cylinders sliding along a wall in a tandem configuration is studied numerically for Reynolds numbers ($\mathit{Re}$) between 20 and 200, and streamwise separation distances between 0.1 and 10 cylinder diameters. For cylinders at close separations, the onset of unsteady two-dimensional flow is delayed to higher mathitRe\mathit{Re}mathitRe compared with the case of a single sliding cylinder, while at larger separations, this transition occurs earlier. For Reynolds numbers above the threshold, shedding from both cylinders is periodic and locked. At intermediate separation distances, the wake frequency shifts to the subharmonic of the leading-cylinder shedding frequency, which appears to be due to a feedback cycle, whereby shed leading-cylinder vortices interact strongly with the downstream cylinder to influence subsequent leading-cylinder shedding two cycles later. In addition to the shedding frequency, the drag coefficients for the two cylinders are d...
Global Aerodynamic Instability of Two Cylinders Subjected to Cross Flow.
This paper provides an in-depth physical discussion on flow-induced vibration of two circular cylinders in light of time-mean lift force on stationary cylinders and interaction mechanisms. While time-mean and fluctuating lift forces are measured for stationary cylinders using a sectional load cell, flow-induced vibration results are integrated from literatures. The gap-spacing ratio T/D is varied from 0.1 to 5 and the attack angle α from 0 to 180 where T is the gap width between the cylinders, and D is the diameter of a cylinder. Six different interaction mechanisms are observed, namely interaction between boundary layer and cylinder, shear layer/wake and cylinder, shear layer and shear layer, vortex and cylinder, vortex and shear layer, and vortex and vortex. The interactions between vortex and cylinder results, and vortex and shear layer result in a high fluctuating lift. On the other hand, the interaction between shear layer/wake and cylinder suppresses fluctuating lift as well as weakens flow unsteadiness for stationary cylinders but may cause violent galloping vibration when the cylinders are elastic. The interaction between boundary layer and cylinder also may generate galloping vibrations. Introduction While much is known of the flow physics around a single isolated cylinder, not much is known at that around a cylinder neighbored by another. There is no doubt that flow physics around two cylinders is much more complex and complicated than that around a single cylinder, because of interference between the cylinders. Mutual flow interaction between two structures makes the wake very excited or tranquil depending on the spacing between the structures. The excited wake-enhancing forces in some cases cause a catastrophic failure of the structures. The study of the aerodynamics of two closely separated structures is thus of both fundamental and practical significance. Zdravkovich (1987) divided the whole region of possible arrangements of two cylinders into four: (i) the proximity interference region, where the flow around one cylinder affects the other; (ii) the wake interference region, the near-wake flow of the upstream cylinder is unaffected by the downstream one; however, the downstream one is significantly affected by the upstream cylinder; (iii) the proximity and wake interference region, where both proximity and wake interference are significant; (iv) the no-interference region, where the wake of one cylinder does not affect the other. Sumner et al. (2000) conducted flow visualization and particle image velocimetry (PIV) measurements for T/D =
Flow states and transitions in flows past arrays of tandem cylinders
Journal of Fluid Mechanics, 2021
Direct numerical simulations at Re = 200 have been conducted of the flow past rows of tandem cylinders. Local stability analysis shows that the wakes of two tandem cylinders are characterised by the formation of a region where the mean flow is locally absolutely unstable immediately behind the second cylinder, followed by a long region that is convectively unstable. The location where the flow changes from absolutely to convectively unstable provides a prediction of where the placement of a third body could trigger a global change, which is confirmed with simulations of the flow past three cylinders, and the flow past two cylinders followed by a short flat plate. A third body placed downstream of the absolute/convective instability transition location is effectively cloaked, its presence having virtually no impact on the flow both upstream and downstream. However, when a body is placed upstream of this location it triggers a global change in the flow, its presence being broadcast throughout the flow domain. However, a third cylinder placed well upstream of this location does not trigger the global change. Sensitivity analysis of the mean flow is conducted, and it is shown that the third cylinder does not simply act as a small perturbation that can excite sensitive regions, but when it is very close to the second cylinder it induces a mean flow correction that eliminates the sensitive regions which may explain why it does not trigger the global change.
Fluid dynamics around twin cylinders and interactions.
Multiple cylindrical structures are widely seen in engineering. Flow interference between the structures leads to a very high fluctuating forces, structural vibrations, acoustic noise, or resonance, which in some cases can trigger failure. Recently circular pins in various arrays have been using as fins to enhance the cooling effect. While the enhancement is directly connected to nature of flow around the pins, no much is known of physics of flow around the pins. The knowledge of flow around two cylinders is insightful for understanding the flow around an array of cylinders/pins. This paper presents results of an experimental investigation into interactions between flowing fluid and a cylinder that is neighbored by another cylinder of the same diameter. Strouhal number (St), time-mean and fluctuating forces on and flow structures around the cylinder are investigated while the gap-spacing ratio T/D is varied from 0.1 to 5 and the attack angle α from 0 to 180 where T is the gap width between the cylinders, and D is the diameter of a cylinder. A flow visualisation test was conducted to observe flow structures around the cylinders. Based on forces, St, flow structures and fluid-cylinder interaction mechanisms, 19 distinct flow categories in the ranges of α and T/D are observed, including one quadristable flow, three tristable flows and four bistable flows. The quadristable, tristable and bistable flows ensue from instabilities of the gap flow, shear layers, vortices, separation bubbles and wakes, engendering a strong jump/drop in forces and St of the cylinders. Six different interaction mechanisms are observed, namely interaction between boundary layer and cylinder, shear layer/wake and cylinder, shear layer and shear layer, vortex and cylinder, vortex and shear layer, and vortex and vortex. While the interaction between vortex and cylinder results in a very high fluctuating drag, that between vortex and shear layer results in a high fluctuating lift. On the other hand, the interaction between shear layer/wake and cylinder suppresses mean and fluctuating forces as well as weakens flow unsteadiness for stationary cylinders but may cause violent galloping vibration when the cylinders are elastic. The interaction between boundary layer and cylinder also may generate galloping vibrations.
Two interacting cylinders in cross flow.
Cylindrical structures in a group are frequently seen on land and in the ocean. Mutual flow interaction between the structures makes the wake very excited or tranquil depending on the spacing between the structures. The excited wake-enhancing forces in some cases cause a catastrophic failure of the structures. This paper presents results of an experimental investigation of Strouhal number (St), time-mean, and fluctuating forces on, and flow structures around, two identical circular cylinders at stagger angle α = 0 • –180 • and gap-spacing ratio T /D = 0.1–5, where T is the gap width between the cylinders, and D is the diameter of a cylinder. While forces were measured using a load cell, St was from spectral analysis of fluctuating pressures measured on the side surfaces of the cylinders. A flow visualization test was conducted to observe flow structures around the cylinders. Based on forces, St, and flow structures, 19 distinct flow categories in the ranges of α and T /D investigated are observed, including one quadristable flow, three kinds of tristable flows, and four kinds of bistable flows. The quadristable, tristable, and bistable flows ensue from instabilities of the gap flow, shear layers, vortices, separation bubbles, and wakes, engendering a strong jump or drop in forces and St of the cylinders. The two cylinders interact with each other in six different mechanisms, namely interaction between boundary layer and cylinder, shear layer or wake and cylinder, shear layer and shear layer, vortex and cylinder, vortex and shear layer, and vortex and vortex. While the interaction between vortex and cylinder results in a very high fluctuating drag, that between vortex and shear layer results in a high fluctuating lift. On the other hand, the interaction between shear layer or wake and cylinder weakens mean and fluctuating forces and flow unsteadiness. A mutual discussion of forces, St, and flow structures is presented in this paper.
Three-dimensionality effects in flow around two tandem cylinders
2006
The flow around two stationary cylinders in tandem arrangement at the laminar and early turbulent regime, (Re = 10 2-10 3), is studied using two-and three-dimensional direct numerical simulations. A range of spacings between the cylinders from 1.1 to 5.0 diameters is considered with emphasis on identifying the effects of three-dimensionality and cylinder spacing as well as their coupling. To achieve this, we compare the twodimensional with corresponding three-dimensional results as well as the tandem cylinder system results with those of a single cylinder. The critical spacing for vortex formation and shedding in the gap region depends on the Reynolds number. This dependence is associated with the formation length and base pressure suction variations of a single cylinder with Reynolds number. This association is useful in explaining some of the discrepancies between the two-dimensional and three-dimensional results. A major effect of three-dimensionality is in the exact value of the critical spacing, resulting in deviations from the two-dimensional predictions for the vorticity fields, the forces on the downstream cylinder, and the shedding frequency of the tandem system. Two-dimensional simulations under-predict the critical spacing, leading to erroneous results for the forces and shedding frequencies over a range of spacings where the flow is qualitatively different. To quantify the three-dimensional effects we first employ enstrophy, decomposed into a primary and a secondary component. The primary component involves the vorticity parallel to the cylinder axis, while the secondary component incorporates the streamwise and transverse components of the vorticity vector. Comparison with the single cylinder case reveals that the presence of the downstream cylinder at spacings lower than the critical value has a stabilizing effect on both the primary and secondary enstrophy. Systematic quantification of three-dimensionalities involves finding measures for the intensity of the spanwise fluctuations of the forces. This also verifies the stabilization scenario, suggesting that when the second cylinder is placed at a distance smaller than the critical one, threedimensional effects are suppressed compared to the single-cylinder case. However, when the spacing exceeds the critical value, the upstream cylinder tends to behave like a single cylinder, but three-dimensionality in the flow generally increases.
A finite volume method based on a velocity-only formulation is used to solve the flow field around a confined circular cylinder in a channel in order to investigate lateral wall proximity effects on stability, Strouhal number, hydrodynamic forces and wake structure behind the cylinder for a wide range of blockage ratios (0.1Ͻр0.9) and Reynolds numbers (0ϽReр280). For blockage ratios less than approximately 0.85 a first critical Reynolds number is identified at which a supercritical Hopf bifurcation of the symmetric solution occurs. For blockage ratios greater than about 0.687 and at Reynolds numbers exceeding the first critical Reynolds number a second curve of neutral stability is seen, representing a pitchfork bifurcation of the steady symmetric solution to one of two possible steady asymmetric solutions. Either side of the neutral stability curve for the pitchfork bifurcation our linear stability analysis and direct numerical simulations demonstrate that although the flow is linearly stable it is unstable to finite two-dimensional perturbations. At blockage ratios larger than about 0.82 the steady asymmetric solutions also become unstable through a Hopf bifurcation. In contrast with the first Hopf bifurcation of the symmetric solution at lower Reynolds numbers numerical calculations of the lift coefficient reveal that the oscillations are no longer symmetric in the rising and falling parts of each cycle. Very strong vortices shed from the cylinder and the wall cause drastic increases in the amplitudes of the lift and drag coefficients. A co-dimension 2 point where pitchfork and Hopf bifurcations occur simultaneously has been located in parameter space. Altogether, four distinct regions in the parameter space (,Re)(0,0.9͔ϫ(0,280͔ have been identified, each corresponding to a different class of flow: ͑i͒ Steady symmetric flow, ͑ii͒ symmetric vortex shedding, ͑iii͒ steady asymmetric flow, and ͑iv͒ asymmetric vortex shedding, where a periodic-in-time flow is classed as symmetric or asymmetric depending on whether the time-average over one cycle of the lift coefficient is zero or not. Numerical solutions are computed on meshes having up to 1.8 million degrees of freedom. Extensive comparisons are made with the results available in the literature.
Physics of Fluids
Flow past two wall-mounted square cylinders in a tandem arrangement are simulated through direct numerical simulation to investigate the effects of the gap between the two cylinders on the wake. Numerical simulations are conducted for a constant Reynolds number of 500, height to width length ratio H ¼ 4, and gap to width ratios of G ¼ 1 to 8 with an interval of 1. The flow in the wake of the downstream cylinder is found to be significantly affected by the free shear layers from the top and sides of the upstream cylinder. At G ¼ 1 and 2, the freeshear layer generated from the upstream cylinder reattaches the top surface of the downstream cylinder and further develops into a downwash behind the downstream cylinder. At G ¼ 3 to 8, the downwash behind the downstream cylinder disappears because flow separation from the top upstream edge of the downstream cylinder does not occur for G ¼ 3 to 6 and is very weak for G ¼ 7 and 8. The disappearance of downwash in the wake of the downstream cylinder further results in very weak variation of flow along the span of the downstream cylinder. The single, reattachment, and binary wake modes at the mid-span of the cylinder occur at G ¼ (1 and 2), (3 and 4), and (5 and above), respectively.