Near-wake structure of an oscillating cylinder: effect of controlled shear-layer vortices

Abstract

The instantaneous structure of the near wake of a cylinder subjected to small-amplitude perturbations is characterized using high-image-density particle image velocimetry. Emphasis is on control of the small-scale shear-layer vortices, which feed into the Kármán vortices. Modifications of the Kármán vortex formation are classified according to patterns of modulated and locked-on shear-layer vortices. The formation length of the Kármán vortices can be dramatically shortened and, in the limiting case, occur adjacent to the base of the cylinder when it is perturbed at the inherent instability frequency of the shear layer and its subharmonics. Moreover, the induced shear-layer vortices can lead to large-amplitude transverse undulations of the entire near-wake region during formation of the Kármán vortices.These variations of the near-wake structure are further elucidated by considering the transient response of the wake, induced by abrupt cessation and onset of periodic motion of the cylinder. Distinctive intermediate states of the wake arise during relaxation to its asymptotic state; such relaxation requires a very large number of periods of the inherent instability of the shear layer.

Origin and Control of Unsteady Loading of Aerodynamic Surfaces Due to Vortex Breakdown and Stall

“…Such features are evident for the case of a single cylinder, as shown by Chyu & Rockwell (1996). Moreover, within the gap region between the two cylinders, the possibility of instantaneous and averaged asymmetries of the flow patterns, and their consequence on the near-wake of the downstream cylinder, is of interest.…”

“…The history of unstable free shear flows is wide and diverse, but recent theoretical developments provide a basis for unifying our perspective. Huerre and Monkewitz (1990) Then, this link to the vorticity-based model of Stuart (1967), can be employed to physically interpret the magnitude of the Reynolds stress due to the small-scale vortices, as shown by Chyu and Rockwell (1996). According to the model of Stuart (1967), a single row of vortical structures exists in a mixing layer of velocity ratio R = (Ui -U2)/ (Ui + U2), in which Ui and U 2 are the velocities on the high and low speeds of the shear layer.…”

“…Further aspects of the relationship between the development of the small-scale shear layer vortices and the Karman vortices, depicted in the schematic of Figure 4, have been pursued by Chyu and Rockwell (1996). Since the small-scale, shear-layer vortices are highly sensitive to small amplitude disturbances, it is possible to indirectly manipulate the formation length of the large-scale vortices by application of suitable perturbations that alter the small-scale vortices.…”

Reynolds number effects on flow over twisted offshore structure with drag reduction and vortex suppression

“…증되었다 (Ahmed and Bays-muchmore, 1992;Nebres et al, 1993;Chyu and Rockwell, 1996;Lam et al, 2004a;Lam et al, 2004b;Lee and Nguyen, 2007;Lam and Lin, 2008 (Unal and Rockwell, 1988;Prasad and Williamson, 1997;Norberg, 1998, Dong et al, 2006 (Breuer, 1998;Breuer, 2000;Jordan and Ragab, 1998;Jordan, 2002). (Keefe, 1962;Bishop and Hassan, 1964;White, 1974;Moeller, 1982;Moeller and Leehey, 1984;Norberg, 1987;Tadrist et al, 1990;Gopalkrishnan 1993 있다 (Unal and Rockwell, 1988;Prasad and Williamson, 1997;Norberg, 1998, Dong et al, 2006.…”

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ABSTRACT: We investigated the Reynolds number effects on the flow over a twisted offshore structure in the range of -muchmore, 1992;Nebres et al, 1993;Chyu and Rockwell, 1996;Lam et al, 2004a;Lam et al, 2004b;Lee and Nguyen, 2007;Lam and Lin, 2008

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