Vortex formation from a forced system of two cylinders .1. Tandem arrangement

The wakes between and downstream of a tandem arrangement of two cylinders are investigated at a Reynolds number of 160. The cylinder system is subjected to controlled excitation over a range of excitation frequency and phase angle between the oscillating cylinders. Use of the hydrogen bubble technique allows visualization of the wake regions and laser-Doppler anemometry leads to wake spectra. It is possible to attain locked-on response, such that the near-wake structure is phase-locked to the motion of the cylinder system, over a much wider range of excitation frequency than for the corresponding single cylinder. Outside the region of locked-on response, modulated, highly repetitive wake patterns occur; these patterns repeat at a multiple of the period of the cylinder oscillation. For closely spaced cylinders, it is possible to attain a regime sensitive to the phase angle between the cylinders; transformation between locked-on and modulated response occurs when the phase angle is varied. The foregoing types of modulated and locked-on response can be classified according to the type of spectrum of the velocity fluctuation in the near-wake. These spectra can exhibit a large number of peaks, each of which can be traced to one or more of the following origins: the frequency of the cylinder oscillation and its harmonics; the inherent vortex formation frequency from the two-cylinder system; a modulation frequency; and sum and difference components of the foregoing. (C) 1996 Academic Press Limited.