Orthogonal Plane Space-Time Imaging of Patterns of Vorticity in the Near-Wake of an Oscillating Cylinder
Submitted by ldb217 on Thu, 2013-10-10 12:57
Orthogonal plane space-time imaging of patterns of vorticity in the near-wake of an oscillating cylinder. Transverse oscillations of a cylinder are assumed to promote two-dimensionality of the near-wake; the role of small-scale three-dimensionality, and its influence on the cylinder forces, has remained unclear. Time sequences of instantaneous images are acquired in orthogonal planes to yield spanwise ωz and streamwise ωx patterns of vorticity. Patterns of ωz (side views) exhibit Kármán-like vortex formation; patterns of ωx (end views) take the form of pronounced concentrations along the span of the near-wake. Maximum instantaneous values of ωx can be as high as one-third those of instantaneous ωz. (ωz)min = 3 sec-1; ) ωz = 3 sec-1. (ωx)min = 1 sec-1; ) ωx = 2 sec-1.
Abstract:
Streamwise oscillations of a circular cylinder in a steady uniform flow are investigated experimentally using a technique of high-image-density particle image velocimetry, in conjunction with instantaneous force measurements. This approach allows insight into the relationship between the loading and the patterns of vorticity and streamline topology in the near-wake.
In analogy with the classical locked-on state arising from transverse oscillations of a cylinder in uniform flow, it is possible to attain either locked-on or quasi-locked-on states due to streamwise oscillations. In these cases, however, the repetitive signature of the transverse force is not sinusoidal; rather, it is strongly modulated and the corresponding spectra can exhibit several sharply defined peaks. The predominant peak can vary over a remarkably wide range, extending from the subharmonic to the third harmonic of the cylinder oscillation frequency; for certain locked-on states of the transverse force signature, the spectral peak at the cylinder oscillation frequency is actually suppressed. Corresponding instantaneous traces and spectra of the in-line force simply show dominance of the spectral peak at the cylinder oscillation frequency. Further interpretation of the loading is provided in terms of Lissajous patterns of the transverse and in-line force coefficients.
All of these features are related to the instantaneous patterns of vortex formation in the near-wake. During a typical cycle of the cylinder oscillation, these patterns can be synthesized into two broad categories: Kármán-like shedding; and a nearly "frozen" array of shed vortices. The order of occurrence of these basic patterns during an oscillation cycle dictates the instantaneous signatures and time-averaged spectra of the transverse force.
Album:
Vortex Formation from an Oscillating Cylinder
