Unstable aerodynamic phenomena around the resonant velocity of a rectangular cylinder with small side ratio

This paper investigated the unstable aerodynamic phenomena of a rectangular cylinder, on the basis of the results obtained by the experiment of flow visualization. Especially we focused on low-speed galloping whose possibility in occurrence is recognized for a rectangular cylinder less than the critical depth section (D/B<0.6; D, depth of a rectangular cylinder in the wind direction; B, breadth of a rectangular cylinder) [Y. Nakamura, Y. Ohya, The effects of turbulence on the mean flow past two-dimensional rectangular cylinders, J. Fluid Mech. 149 (1984) 255–273; T. Mizota, Doctoral Thesis, Faculty of Engineering Kyushu University, Japan, 1984 (in Japanese)] at the wind velocity lower than the resonant velocity. When a plate with very small side ratio can be elastically mounted without large damping in a uniform flow, it is well known that low-speed galloping appears. In order to understand the physical mechanism for unstable aerodynamic phenomena of a rectangular cylinder, we examine the differences of two wake patterns, which are observed around the rectangular cylinder in low-speed galloping and vortex-induced oscillation. Also, we carry out experiment of measurement of wind pressure on the side surface of a freely oscillating rectangular cylinder. Taking into consideration the phase lag of pressure to the oscillation, we clarify the negative damping unsteady force acting on the rectangular cylinder in low-speed galloping.