Dynamic interactions of multiple wall-mounted flexible flaps

Abstract

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.

On the turbulence dynamics induced by a surrogate seagrass canopy

“…Another characteristic frequency of interest is the undampened natural blade frequency, estimated by with and (Luhar & Nepf 2016), which results in Hz. Recent numerical simulations by O'Connor & Revell (2019) indicate that canopy motion is a coupled response between the natural structure (vegetation blade) and coherent flow motions. Both and may coexist if they differ sufficiently; however, a lock-in phenomenon and canopy waving may occur when and are similar.…”

“…Various studies have quantified coherent vortices and isolated blade motions, and distinct differences in motion and turbulence dynamics have been linked to canopy morphology. Singular flexible vegetation elements deflect to a greater extent than when located within a canopy (O'Connor & Revell 2019), thus altering the vertical distribution of stresses and canopy motion. Wilson et al.…”

“…2019). O'Connor & Revell (2019) showed that monami behaviour is a function of the natural blade frequency and the mixing layer instability frequency, resulting in the spatial and temporal canopy dynamics being associated with combined fluid–structure interaction. The spatial configuration of the vegetation element within a canopy is also an important factor; Liu et al.…”

Understanding the Role of Endothelial Glycocalyx in Mechanotransduction via Computational Simulation: A Mini Review

“…Macroscopic simulations deal with problems at cell-or tissue-scales, like endothelial cell deformation, force distribution on glycocalyx ultrastructures and flow shear stress over the EG layer (Tarbell and Shi, 2013;Dabagh et al, 2014). Mesoscopic methods can further explore detailed fluid-structure interactions (O'connor and Revell, 2019) between blood flow and endothelium surface structures but at slightly higher computational cost. Microscopic in silico experiments have been successfully applied to capture the atomic/molecular interactions in a single proteoglycan unit (Jiang et al, 2017).…”

“…At the mesoscales, more features of the glycocalyx can be included, which enables detailed comparison with experiments. By coupling the LBM with immersed boundary methods, alongside structural solvers, researchers revealed a broad range of behaviours for slender structures, from a single flap in a periodic array, to a small finite array of flaps, and finally to a large finite array (Yin and Zhang, 2013;O'connor et al, 2016;O'connor and Revell, 2019). The findings suggest that the flow instability over the slender array depends on the natural frequencies of the flow and the surface structures (O'connor and Revell, 2019).…”