Flow past a rotationally oscillating cylinder

Abstract

Flow past a circular cylinder executing sinusoidal rotary oscillations about its own axis is studied experimentally. The experiments are carried out at a Reynolds number of 185, oscillation amplitudes varying from π/8 to π, and at non-dimensional forcing frequencies (ratio of the cylinder oscillation frequency to the vortex-shedding frequency from a stationary cylinder) varying from 0 to 5. The diagnostic is performed by extensive flow visualization using the hydrogen bubble technique, hotwire anemometry and particle-image velocimetry. The wake structures are related to the velocity spectra at various forcing parameters and downstream distances. It is found that the phenomenon of lock-on occurs in a forcing frequency range which depends not only on the amplitude of oscillation but also the downstream location from the cylinder. The experimentally measured lock-on diagram in the forcing amplitude and frequency plane at various downstream locations ranging from 2 to 23 diameters is presented. The far-field wake decouples, after the lock-on at higher forcing frequencies and behaves more like a regular Bénard-von Kármán vortex street from a stationary cylinder with vortex-shedding frequency mostly lower than that from a stationary cylinder. The dependence of circulation values of the shed vortices on the forcing frequency reveals a decay character independent of forcing amplitude beyond forcing frequency of ∼1.0 and a scaling behaviour with forcing amplitude at forcing frequencies 1.0. The flow visualizations reveal that the far-field wake becomes two-dimensional (planar) near the forcing frequencies where the circulation of the shed vortices becomes maximum and strong three-dimensional flow is generated as mode shape changes in certain forcing parameter conditions. It is also found from flow visualizations that even at higher Reynolds number of 400, forcing the cylinder at forcing amplitudes of π/4 and π/2 can make the flow field two-dimensional at forcing frequencies greater than ∼2.5.

Vortex shedding and heat transfer in rotationally oscillating cylinders

“…The mean wake profile and wake modes were found to be similar at spanwise locations located at mid-span and ∼0.75D above and below mid-span, as would be expected if the rotary forcing aligned the shedding with the cylinder. Based on these results, and the results of Poncet (2002), Jian et al (2007) and Kumar et al (2013), it is reasonable to speculate that the shedding is parallel and nominally two-dimensional everywhere along the span of the cylinder, except very close to the boundaries due to end effects. This does not exclude the possible existence of small-scale three-dimensional structures that have not been captured due to experimental limitations.…”

“…Experimental studies have been performed by Tokumaru & Dimotakis (1991), Fujisawa, Kawaji & Ikemoto (2001), Fujisawa, Tanahashi & Srinivas (2005), Lee & Lee (2006), Saad, Lee & Lee (2007), Lee & Lee (2008), Nazarinia et al (2012) and, most recently, Kumar et al (2013) 552 P. Sellappan and T. Pottebaum and Sellappan & Pottebaum (2014). Tokumaru & Dimotakis (1991) conducted an experimental study at Re = 15 000, performing flow visualization and wake profile measurements on circular cylinders undergoing forced rotary oscillations in a steady uniform flow and found that the drag forces on the cylinder were reduced at high oscillation frequencies while undergoing rotary forcing.…”

“…One study that looked at the behaviour of both stationary and rotationally oscillating cylinders was by Saad et al (2007), with the oscillating cylinder experiments performed at Re = 3700, and comparisons were made between the wake of an unforced and forced cylinder and it was also shown that the formation length decreased with increasing frequency of rotary oscillations. Recently, Kumar et al (2013) experimentally studied vortex shedding from rotationally oscillating cylinders at Re = 185 using the hydrogen bubble technique, hot-wire anemometry and PIV and identified the oscillation conditions under which the shedding is synchronized to the forcing. They also studied the possibility of forcing the wake to become two-dimensional through rotary forcing at Re = 185 and Re = 400 and identified forcing conditions under which it is possible to do so.…”

“…However, it has been shown that rotationally oscillating the cylinder suppresses instabilities, thereby bringing the flow back to a nominally two-dimensional state (Poncet 2002;Jian, Xue-ming & An-lu 2007;Kumar et al 2013). In particular, Kumar et al (2013), through an extremely well-designed set of experiments using the hydrogen bubble technique and utilizing a unique end view perspective of the wake structure, have shown that rotary oscillations are capable of bringing the shedding back to a nominally two-dimensional state under certain forcing conditions even for relatively small-aspect-ratio cylinders at moderately high Re. Evidence in favour of two-dimensionality was obtained during the set up and testing phase of the present study as the wake was measured at different spanwise locations.…”

Flows produced by the combined oscillatory rotation and translation of a circular cylinder in a quiescent fluid

“…Research has also been done with cylinders oscillating rotationally (Tokumaru & Dimotakis 1991;Thiria, Goujon-Durand & Wesfreid 2006;Du & Dalton 2013;Kumar et al 2013) as well as transversely (Ongoren & Rockwell 1988;Williamson & Roshko 1988;Gu, Chyu & Rockwell 1994;Blackburn & Henderson 1999;Guilmineau & Queutey 2002;Lam, Liu & Hu 2010b;Leontini, Jacono & Thompson 2011) in a cross-flow at varying amplitudes and frequencies below, equal to and above the natural shedding frequency of a stationary cylinder. Others have investigated the effects of prescribed sinusoidal translation in quiescent fluids (Williamson 1985;Justesen 1991;Dütsch et al 1998;Iliadis & Anagnostopoulos 1998;Lam, Hu & Liu 2010a).…”