Excitation by Vortices Shedding Periodically from Two Cylinders in Cruciform Arrangement.

Abstract

The behavior of the excitation caused by longitudinal vortices shedding from two cruciform circular cylinders with an equal diameter d is investigated using a wind tunnel. When the gap s between two cylinders is less than d/2, longitudinal vortices shed periodically and a fluctuating lift force is exerted on the upstream cylinder. The nondimensional frequency, StM, of the longitudinal vortices is independent of the Reynolds number. The value of StM is 0.06<StM<0.09 when s/d<0.25 (region A) and StM≒0.04 when 0.25<s/d<0.5 (region B). The fluctuating lift force on the upstream cylinder is estimated from the pressure on the cylinder surface using a phase-averaging technique. The result shows that the flow near the crossing of the cylinder is dominated by four trailing vortices in the region A and by two necklace vortices in the region B. These vortices shed an anti-phase on the opposite side of the upstream cylinder, causing an alternative lift force to be exerted on it. The abrupt change of StM at s/d=0.25 is caused by the fact that these two types of vortices interchange their role to excite the upstream cylinder.