Galloping response of a cylinder with upstream wake interference

A compliantly-mounted rigid cylinder was towed at Re = 3 x 10(4), 4.75 diameters behind a stationary leading cylinder of the same size. An in-line configuration and a 12-degree staggered arrangement each produced large-amplitude galloping responses, and an upward extension of the frequency lock-in range to a reduced velocity of at least 17. The frequency lock-in begins at nearly the same free-stream reduced velocity as a single cylinder, while a large phase change in the lift force occurs at higher reduced velocities, which can be extrapolated from the single-cylinder lock-in point. Force spectra indicate that shedding from the upstream cylinder is completely unaffected by motions of the trailing cylinder. Furthermore, the motion-coupled peaks suggest that only one lift force cycle and one or two drag force cycles occur per oscillation, the latter depending on the offset. (C) 2001 Academic Press.