Three-dimensional vortical structures behind a normal or inclined cylinder with or without running water rivulets

Abstract

As a step to identify the excitation mechanism of rain-wind induced cable vibration, three-dimensional vortical structures have been measured in a circular cylinder wake using Particle Imaging Velocimetry (PIV) for the Reynolds number range of 2 x 103 to 1 x 104. The PIV was modified, compared with the conventional one, in terms of its light sheet arrangement to capture reliably streamwise vortices. While in agreement with previous reports, the presently measured spanwise structures complement the data in the literature in the streamwise evolution of the near-wake spanwise vortex in size, strength, streamwise and lateral convection velocities, shedding new light upon vigorous interactions between oppositely signed spanwise structures. The longitudinal vortices display mushroom patterns in (x, z)-plane, where x represents free flow direction and z is in line with cylinder axis, in the immediate proximity to the cylinder. Their most likely inclination in the (x, y)-plane, where y is perpendicular to both x and z directions, is inferred from the measurements in different (x, z)-planes. The longitudinal vortices in the (y, z)-plane show alternate change in sign, though not discernible at x/d > 15, where d is the diameter of cylinder. They decay in the maximum vorticity and circulation rapidly from x/d = 5 to 10 and slowly for x/d > 10, and are further compared with the spanwise vortices in size, strength and rate of decay. Then the effects of water rivulets running along an inclined circular cylinder on the near-wake were experimentally investigated. Water was released from the upper end of the cylinder at a volume flow rate Q. At an incoming wind speed U = 8 - 15 m/s, two water rivulets were observed near the flow separation points, running along the cylinder inclined at a = 45ofor 0o< b < 90o where b is the cylinder yaw angle, and both oscillating circumferentially. The quasi-periodical vortex street is observed intermittently with and without the presence of the rivulets. The rivulets lead to a significant increase in the drag coefficient, which is consistent with the violent vibration associated with the rain-wind-structure interactions. It is found that the vortex strength grows by up to 60% as b increases from 0o to 30o. A mechanism for the rain-wind-induced cable vibration is proposed.