Flow past a rotationally oscillating cylinder with an attached flexible filament

Abstract

Experimental studies are conducted on a rotationally oscillating cylinder with an attached flexible filament at a Reynolds number of 150. Parametric studies are carried out to investigate the effect of cylinder forcing parameters and filament stiffness on the resultant wake structure. The diagnostics are flow visualization using the laser-induced fluorescence technique, frequency measurement using a hot film, and characterization of the velocity and vorticity field using planar particle image velocimetry. The streamwise force and power are estimated through control volume analysis, using a modified formulation, which considers the streamwise and transverse velocity fluctuations in the wake. These terms become important in a flow field where asymmetric wakes are observed. An attached filament significantly modifies the flow past a rotationally oscillating cylinder from a Bénard–Kármán vortex street to a reverse Bénard–Kármán vortex street, albeit over a certain range of Strouhal number, $St_{A} \sim 0.25\text {--}0.5$ , encountered in nature in flapping flight/fish locomotion and in the flow past pitching airfoils. The transition from a Kármán vortex street to a reverse Kármán vortex street precedes the drag-to-thrust transition. The mechanism of unsteady thrust generation is discussed. Maximum thrust is generated at the instants when vortices are shed in the wake from the filament tip. At $St_{A} > 0.4$ , a deflected wake associated with the shedding of an asymmetric vortex street is observed. Filament flexibility delays the formation of an asymmetric wake. Wake symmetry is governed by the time instant at which a vortex pair is shed in the wake from the filament tip.

Dynamics of a rigid-flexible coupling system in a uniform flow

“…A splitter plate, attached downstream, inhibits the interaction between the separating shear layers, thereby delaying vortex shedding and resulting in a weaker Kármán vortex street in the wake (Sunil et al. 2022). This reduces the fluctuating lift and drag acting on the bluff body.…”

“…2014; Sunil et al. 2022). It is found that a filament, with proper flexibility and length, may bring about more performance benefits in suppressing/weakening the vortex shedding as compared with a rigid splitter plate with the same length (Wu et al.…”

“…2014; Sunil et al. 2022). In addition, some studies have found that installing two or multiple flexible filaments behind a bluff body, referred to as ‘hairy coating’, can offer more drag reduction under specific conditions than a single filament (Favier et al.…”

“…As a passive flow control technique, installing splitter plates behind a bluff body has been investigated by numerous experiments and simulations in the past (Anderson & Szewczyk 1997;Gilliéron & Kourta 2010;Wu et al 2014;Sunil, Kumar & Poddar 2022). A splitter plate, attached downstream, inhibits the interaction between the separating shear layers, thereby delaying vortex shedding and resulting in a weaker Kármán vortex street in the wake (Sunil et al 2022). This reduces the fluctuating lift and drag acting on the bluff body.…”

“…Earlier studies were mainly limited to the rigid plates. Recently, inspired by the drag-reducing mechanism of passive flexibility in natural systems, particular attention has been devoted to the flexible splitter plate (Lācis et al 2014;Wu et al 2014;Sunil et al 2022). It is found that a filament, with proper flexibility and length, may bring about more performance benefits in suppressing/weakening the vortex shedding as compared with a rigid splitter plate with the same length (Wu et al 2014;Sunil et al 2022).…”