“…This phenomenon has been documented in a number of bluff-body wakes that form this two-row vortex structure, e.g. see Hosseini, Griffith & Leontini (2021), and is associated with an absolute instability of the mean wake profile. Figure 13.Deflected state : ( a ) time history of the angular displacement; ( b ) frequency spectrum of the angular displacement; and ( c ) an image of the vorticity field at , for the case and .…”
Section: Description Of Dynamic States and Their Interactionmentioning
An ‘inverted flag’ – a flexible plate clamped at its trailing edge – undergoes large-amplitude flow-induced flapping when immersed in a uniform and steady flow. Here, we report direct numerical simulations of a related single degree-of-freedom mechanical system: a rigid plate attached at its trailing edge to a torsional spring. This system is termed a ‘rigid inverted flag’ and exhibits the dynamical states reported for the (flexible) inverted flag, with additional behaviour. This finding shows that the flapping dynamics of inverted flags is not reliant on their continuous flexibility, i.e. many degrees of freedom. The rigid inverted flag exhibits additional, novel states including a heteroclinic-type orbit that results in small-amplitude flapping, and a number of chaotic large-amplitude flapping regimes. We show that the various routes to chaos are driven by a series of periodic states, including at least two which are subharmonic. The instability and competition between these periodic states lead to chaos via type-I intermittency, mode competition and mode locking. The rigid inverted flag allows these periodic states and their subsequent interaction to be explained simply: they arise from an interaction between a preferred vortex shedding frequency and a single natural frequency of the structure. The dynamics of rigid inverted flags is yet to be studied experimentally, and this numerical study provides impetus for such future work.
“…This phenomenon has been documented in a number of bluff-body wakes that form this two-row vortex structure, e.g. see Hosseini, Griffith & Leontini (2021), and is associated with an absolute instability of the mean wake profile. Figure 13.Deflected state : ( a ) time history of the angular displacement; ( b ) frequency spectrum of the angular displacement; and ( c ) an image of the vorticity field at , for the case and .…”
Section: Description Of Dynamic States and Their Interactionmentioning
An ‘inverted flag’ – a flexible plate clamped at its trailing edge – undergoes large-amplitude flow-induced flapping when immersed in a uniform and steady flow. Here, we report direct numerical simulations of a related single degree-of-freedom mechanical system: a rigid plate attached at its trailing edge to a torsional spring. This system is termed a ‘rigid inverted flag’ and exhibits the dynamical states reported for the (flexible) inverted flag, with additional behaviour. This finding shows that the flapping dynamics of inverted flags is not reliant on their continuous flexibility, i.e. many degrees of freedom. The rigid inverted flag exhibits additional, novel states including a heteroclinic-type orbit that results in small-amplitude flapping, and a number of chaotic large-amplitude flapping regimes. We show that the various routes to chaos are driven by a series of periodic states, including at least two which are subharmonic. The instability and competition between these periodic states lead to chaos via type-I intermittency, mode competition and mode locking. The rigid inverted flag allows these periodic states and their subsequent interaction to be explained simply: they arise from an interaction between a preferred vortex shedding frequency and a single natural frequency of the structure. The dynamics of rigid inverted flags is yet to be studied experimentally, and this numerical study provides impetus for such future work.
“…Provided that this third body was placed within the region of absolute instability of the downstream cylinder, vortex shedding could be entirely suppressed in the gap, resulting in a reattachment regime instead. We are inclined to believe in the view put forward by Hosseini et al (2021). This suggests that the gap vortices for relatively small gap ratios are driven by the vortex shedding from the downstream cylinder, and not the other way around.…”
Section: Discussionmentioning
confidence: 89%
“…2022 b ), similar to the ‘lock-in’ effect found for straight tandem cylinders within the co-shedding regime. This hypothesis was implicitly challenged by Hosseini, Griffith & Leontini (2021). In their investigation, a third body was placed in the near wake of tandem cylinders at and , a combination of gap ratio and which normally results in co-shedding.…”
Section: Discussionmentioning
confidence: 99%
“…Some previous studies of straight tandem cylinders have suggested that, within the reattachment regime, it is the frequency of the gap vortices that drives the frequency of the wake vortices (Xu & Zhou 2004;Aasland et al 2022b), similar to the 'lock-in' effect found for straight tandem cylinders within the co-shedding regime. This hypothesis was implicitly challenged by Hosseini, Griffith & Leontini (2021). In their investigation, a third body was placed in the near wake of tandem cylinders at L/D = 5 and Re = 200, a combination of gap ratio and Re which normally results in co-shedding.…”
Flow around curved tandem cylinders in the convex configuration has been studied by means of direct numerical simulations, for a Reynolds number of 500 and a nominal gap ratio of 3.0. Spanwise variation of flow regimes, as well as curvature-induced axial velocity, leads to an exceedingly complex vortex dynamics in the wake. Both parallel and oblique vortex shedding are observed. Oblique shedding is connected to repeated occurrences of dislocations. The dislocations are caused by two main mechanisms: frequency differences in the upper part of the curved geometry and shedding of gap vortices into the lower near wake. Both types of dislocations are closely associated with a mode switch in the gap. In parts of the gap, there is low-frequency quasi-periodic asymmetry of the gap vortices, where the flow is biased to one side of the gap for intervals of several wake vortex shedding periods. The switch from side to side is associated with a surge of the vertical velocity, and the frequency of the switch is similar to that of long-term variation of the recirculation length in the lower gap.
“…Whereas a large amount of studies have been performed in the past regarding the fluid-dynamic behaviour of two 2D (i.e. "infinite") smooth circular cylinders (Zdravkovich, 1977;Okajima, 1979;Igarashi, 1981;Pearcy et al, 1982;Arie et al, 1983;Igarashi, 1984;Zdravkovich, 1985Zdravkovich, , 1987Meneghini et al, 2001;Alam et al, 2003;Zhou and Yiu, 2006;Alam and Zhou, 2007;Carmo et al, 2010;Sumner, 2010;Alam and Meyer, 2011;Alam, 2014;Griffith et al, 2017;Alam et al, 2018;Schewe and Jacobs, 2019;Hosseini et al, 2021;Schewe et al, 2021) or sharp-edged prisms with square cross sections (Reinholds et al, 1977;Shiraishi et al, 1986;Kareem, 1987;Sakamoto et al, 1987;Sakamoto and Haniu, 1988;Luo, 1989;Luo and Teng, 1990;Ohya et al, 1989;Luo and Teng, 1990;Takeuchi and Matsumoto, 1992;Tatsutani et al, 1993;Hangan and Vickery, 1999;Luo et al, 1999;Alam et al, 2002;Liu and Chen, 2002;Kim et al, 2008;Yen et al, 2008;Sohankar and Etminan, 2009;Etminan et al, 2011;…”
Section: Two Square-section Prisms With Finite Edge Roundness Arrange...mentioning
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