Flow behind a cylinder forced by a combination of oscillatory translational and rotational motions

Abstract

The flow behind a cylinder undergoing forced combined oscillatory motion has been studied. The motion consists of two independent oscillations: cross-stream translation and rotation. Previous studies have extensively investigated the effect of these motions individually on cylinder wakes; however, the investigation of their combined effect is new. The motivation lies in its application to vortex-induced vibration and its suppression and to biomimetic motion. The focus is on the effect of the phase difference between the two motions. The results show that there is an unexpected loss of synchronization of the wake for a finite range of phase differences.The primary goal of this research is to understand the physical mechanisms behind the response of a cylinder wake to the combined forcing mechanisms of cross-stream translation and rotational oscillations. With an in-depth understanding of the flow physics it may be possible to propose a novel means of actively or passively suppressing the lock-on between vortex shedding and transverse oscillation. Also, we are interested in the application to biomimetic motions and, in particular, to carangiform motion. 1 There has been considerable research on the effect of either transverse or rotational oscillations on cylinder wakes, as discussed in the extensive reviews. 2,3 Primarily, these have focused on the translational oscillation due to their focus on vortex-induced vibration. There have also been studies of the effect of rotational oscillation on wakes. 4,5 Previous numerical work has also been performed on the effect of the combined motions in quiescent fluids 6 and when there is a flow past the cylinder; 7 however, the influence of an important parameter was not considered. Indeed, previous interesting results 6,8 indicate that the phase difference between the two motions is of considerable importance and this is the focus of the research discussed here. This work is part of a more extensive set of experiments that considers the full range of independent variables.The experiments were conducted in the FLAIR freesurface closed-loop water channel at Monash University. A schematic of the problem is given in Fig. 1. The cylinder used was 800 mm in length and with an outer diameter of D = 20 mm, giving an aspect ratio of 40. The experiments were performed for a fixed upstream velocity U ϱ = 0.0606 m / s giving Re= U ϱ D / = 1322. Two sinusoidal motions were imposed, namely, translational ͑cross stream͒, given by y͑t͒ = A t sin͑2f t t͒ / D, and rotational, given by ͑t͒ = A sin͑2f t + ⌽͒. The frequencies are fixed close to that of the natural frequency ͑T −1 = f t = f = 0.6 s −1 Ϸ f N ͒. The natural frequency was found to be equal to f N Ϸ 0.6154 s −1 .The Strouhal number based on this frequency is about St Ϸ f N D / U ϱ = 0.203 and the Strouhal number of the forcing is St t Ϸ f t D / U ϱ = 0.198. The experiments presented are for fixed amplitudes of oscillation, A t = D / 2 and A = 1. These amplitudes combined with the equal frequencies provide equal maximum velocities fro...

A device to achieve low Reynolds numbers in an open surface water channel

“…Its velocity range is 0.045-0.5 m/s, with a flow quality comparable to other world class facilities, as has been shown in a variety of experiments (Carberry et al 2005;Nazarinia et al 2009;Nemes et al 2012).…”

Flow over a cylinder subjected to combined translational and rotational oscillations

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a b s t r a c tThe experimental research reported here employs particle image velocimetry to extend the study of Nazarinia et al (2009a), recording detailed vorticity fields in the near-wake of a circular cylinder undergoing combined translational and rotational oscillatory motions. The focus of the present study is to examine the effect of the ratio between the cross-stream translational and rotational velocities and frequencies on the synchronization of the nearwake structures for multiple phase differences between the two motions.

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Streamwise forced oscillations of circular and square cylinders

“…This facility has been previously used for similar bluff-body wake studies. 23,24 Square/circular cylinders of 25 mm width/diameter were oscillated using a high resolution stepper motor connected to a traverse, which allowed for linear translational motion. 25 Position was confirmed using a hollow bore optical encoder attached to the shaft of the stepper.…”