Interference Effect of Wake Body on the Cross Flow Oscillation of a Square Cylinder in Uniform Flow

Wind tunnel experiments are carried out to investigate interference effect of wake body on cross flow vibration of a square cylinder in uniform flow. The side length d of the square cylinder is 26∼40 mm and the length l_e = 315 mm. As the wake body, a strip-plate of width w = d is set downstream the square cylinder with a gap s in cruciform arrangement. The length of the plate l_d is varied from infinity, i.e. full measuring section height, to l_d/d = 1. Both the Karman vortex excitation (K-VIV) and the galloping are suppressed by the l_d/d = ∞ plate in the non-dimensional gap range of 1.6<s/d ≦ 4, although the mechanisms are completely different between the two oscillations. The longitudinal vortex excitation (L-VIV) found in the previous work is confirmed to be induced by the plate at around s/d = 1.4 for the systems with various dimensions and structure parameters. The K-VIV suppression effect is virtually the same for the wake plates with l_d/d≧10, and becomes less definite for shorter plates when l_d/d ≦ 6. The galloping suppression effect persists up to the shortest wake plate of l_d/d = 1 at s/d<2. The L-VIV is observed for plates of l_d/d≧6, with weaker degree for shorter plates. The K-VIV seems to be enhanced by l_d/d = 2, 4, 6 plates at 1≦s/d≦2 (EK-VIV). By setting the wake plate with l_d/d = 1 or 2 at s/d around 0.3, a new type of fluid-elastic vibration is induced as an interference effect of wake body (WBI-FEV). A method is presented to predict fluid-elastic vibrations to which the Van der Pol equation does not apply. The prediction by this method agrees well with measured WBI-FEV for the l_d/d = 2 plate.