Vortex formation from a forced system of two cylinders .2. Side-by-side arrangement

The wake from a system of side-by-side cylinders is subjected to forced excitation over a range of dimensionless frequency and phase angle between the oscillating cylinders. The extent of the frequency range of locked-on response of the vortex patterns is approximately the same as for a single cylinder, but displaced to higher frequencies and reaching to lower amplitudes. For in-phase motion of the two cylinders, the upper extent of the lock-on region is amplitude limited. Within each of the regions of lock-on, different phase angles lead to a variety of patterns of interacting vortices; nevertheless, the general form of the near-wake velocity spectrum is the same for all patterns. Outside the region of locked-on response, the pattern of vortex formation is repetitive at a multiple of the period of the cylinder oscillation. At a given frequency of excitation, several classes of modulated vortex patterns are attainable by altering the phase angle; these patterns have, however, the same modulation period. Moreover, these vortex patterns are all associated with similar velocity spectra of the near-wake. (C) 1996 Academic Press Limited.