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Position diagrams of flap arrays for (top to btm:) \(\mu ^*=\{0.66,0.8,1.0,1.6,4.0\}\), where flaps \(1-5\) are coloured corresponding to legend on . (Color figure online)
Source publication
The fluid-structure interaction mechanisms of a coating composed of flexible flaps immersed in a periodically oscillating channel flow is here studied by means of numerical simulation, employing the Euler-Bernoulli equations to account for the flexibility of the structures. A set of passively actuated flaps have previously been demonstrated to deli...
Citations
... O'Connor and Revell (2019) implemented a lattice Boltzmann immersed boundary finite element model to study steady flow over an array of wall-mounted flexible flaps at low Reynolds number. The model was first validated with laboratory experiment data of oscillatory flow over a row of flexible flaps Revell et al., 2017) and then applied to investigate the coherent waving interactions between the vegetation and flow. Tschisgale and Fröhlich (2020) developed an efficient and robust semi-implicit immersed boundary method for the fluid-structure interaction of slender flexible structures in viscous fluid. ...
An immersed boundary‐finite element with soft‐body dynamics has been implemented to study steady flow over a finite patch of submerged flexible aquatic vegetation. The flow structure interaction model can resolve the flow interactions with flexible vegetation, and hence the reconfiguration of vegetation blades to ambient flow. Flow dynamics strongly depend on two dimensionless parameters, namely vegetation density and Cauchy number (defined as the ratio of the fluid drag force to the elastic force). Five different flow patterns have been identified based on vegetation density and Cauchy number, including the limited reach, swaying, “monami” A, “monami” B with slow moving interfacial wave, and prone. The “monami” B pattern occurred at high vegetation density and is different from “monami” A, in which the passage of Kelvin‐Helmholtz billows strongly affects the vegetation interface. With soft‐body dynamics, blade‐to‐blade interactions can also be resolved. At high vegetation density, the hydrodynamic interactions play an important role in blade‐to‐blade interactions, where adjacent vegetation blades interact via the interstitial fluid pressure. At low vegetation density, direct contacts among vegetation blades play important roles in preventing unphysical penetration of vegetation blades.
... Such continuously oscillating motions are commonly observed in vegetation and are known as "homami" or "monami" (O'Connor and Revell, 2019); they can be modeled in terms of the interactions between the oncoming flow and multiple wall-clamped flexible flags. Revell et al. (2017) numerically explored the physical coupling between multiple flexible flags in an oscillation flow. The oscillations of flags are closely linked to the mixing layer instability (Ghisalberti and Nepf, 2002) and to the drag fluctuations in intermittent flows (Nepf, 2012) that arise due to the presence of the wall-clamped flexible flags. ...
The flapping dynamics of vertically clamped three-dimensional flags in a Poiseuille flow was studied numerically by using the immersed boundary method. First, the flapping dynamics of a single flag was explored for comparison. Two distinct flow modes were observed: a flapping mode and a deflected mode. In the flapping mode, periodic vortices shed from the flag are formed, leading to alternating upstroke and downstroke flapping motions induced by the hydrodynamic and restoring forces. In the deflected mode, the flag is initially deflected by the hydrodynamic force and reaches a stationary state; the hydrodynamic force is balanced by the restoring force. For tandem flags, when the gap distance is small, the flags behave as one single flag with a higher bending rigidity. When the gap distance is intermediate, the front flag deflects the oncoming flow away from the rear flag. The flapping motion of the front flag is significantly confined by the presence of the rear flag, which results in an attenuation of more than 50% in its flapping amplitude. When the distance is large, the impact of the rear flag on the upstream flow field is negligible, so the front flag exhibits a flapping amplitude and frequency that are similar to those of a single flag. The vortices shed from the front flag induce the formation downstream of a low pressure region, which results in active flapping in the rear flag with a strong amplitude. There are two vortices shed from the tandem flags in each flapping period. When they are far apart, the phase difference is linearly dependent on the gap distance.
The coupled interactions between fluidsO’Connor, Joseph and slender structures play a number of critical roles in a broad range of physical processes. In flow control applications, poroelastic coatings consisting of arrays of passive slender structures have been shown to provide beneficial aerodynamic characteristics when applied to bluff bodies.Revell, Alistair This effect has been linked to the appearance of a travelling wave through the array which locks in to the shedding frequency of the wake. Through a simplified test case, which reduces the problem complexity while retaining the essential physics of the behaviour, the present work aims to further elucidate this phenomenon via numerical simulations. A range of array lengths are tested and the appearance and propagation of the travelling waves are monitored. The results show that for small arrays there exists one clearly defined wave, which is attributed to the advection of the primary bulk vortex over the array. However, for larger arrays, secondary vortices are generated at the tips which also induce a wave-like behaviour. These secondary vortices are smaller in size and intensity than the primary vortex and induce a smaller deflection in the flaps which dissipates quicker as it propagates though the array.
Coherent waving interactions between vegetation and fluid flows are known to emerge under conditions associated with the mixing layer instability. A similar waving motion has also been observed in flow control applications, where passive slender structures are used to augment bluff body wakes. While their existence is well reported, the mechanisms which govern this behaviour, and their dependence on structural properties, are not yet fully understood. This work investigates the coupled interactions of a large array of slender structures in an open-channel flow, via numerical simulation. A direct modelling approach, whereby the individual structures are fully resolved, is realised via a lattice Boltzmann-immersed boundary-finite element model. For steady flow conditions at low–moderate Reynolds number, the response of the array is measured over a range of mass ratio and bending rigidity, spanning two orders of magnitude, and the ensuing response is characterised. The results show a range of behaviours which are classified into distinct states: static, regular waving, irregular waving and flapping. The regular waving regime is found to occur when the natural frequency of the array approaches the estimated frequency of the mixing layer instability. Furthermore, after normalising with respect to the natural frequency of the array, the frequency response across the examined parameter space collapses onto a single curve. These findings indicate that the coherent waving mode is in fact a coupled instability, as opposed to a purely fluid-driven response, and that this specific regime is triggered by a lock-in between the fluid and structural natural frequencies.
