Streamwise oscillations of a cylinder in a steady current. Part 1. Locked-on states of vortex formation and loading

Streamwise oscillations of a circular cylinder in a steady uniform flow are investigated experimentally using a technique of high-image-density partic le 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 oscil lations of a cylinder in uniform flow, it is possible to attain either lock ed-on or quasi-locked-on states due to streamwise oscillations. In these ca ses, however, the repetitive signature of the transverse force is not sinus oidal; rather, it is strongly modulated and the corresponding spectra can e xhibit 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 sp ectra of the in-line force simply show dominance of the spectral peak at th e 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 f ormation in the near wake. During a typical cycle of the cylinder oscillati on, these patterns can be divided into two broad categories: Karman-like sh edding; and a nearly 'frozen' array of shed vortices. The order of occurren ce of these basic patterns during an oscillation cycle dictates the instant aneous signatures and time-averaged spectra of the transverse force.