Effects of Oscillation Amplitude on Nonlinear Motion-Induced Force for 5 ∶ 1 Rectangular Cylinder

Investigations on the nonlinear motion-induced force acting on a rectangular cylinder with aspect ratio 5∶1, which is significant in the analysis of nonlinear aeroelastic behaviour for bluff sections, are part of fundamental and cutting-edge research on bluff body aerodynamics. Combined with surface synchronous pressure measurement, forced motion wind tunnel tests were carried out to investigate the influence of oscillation amplitude on the spectral characteristics of motion-induced force and pressure distribution for a 5∶1 rectangular cylinder. After analyzing the pressure distribution mode based on the proper orthogonal decomposition (POD) method, the mechanism for nonlinear motion-induced force was discussed. Experiments and analysis indicate that the higher harmonics in the motion-induced force acting on a 5∶1 rectangular cylinder only shows significant proportion when the section is under pitching motion and the amplitude exceeds 8°, while the linear components grow nonlinearly with the increase of oscillation amplitude. When the rectangular cylinder rotates with amplitude less than 8° or oscillates vertically, the reattaching point is near the trailing edge and remains stable during the period of motion. Vortices on the top surface shed directly into the wake flow and the corresponding pressure mode is symmetric, all of which suggest that the motion-induced force is determined by a main vortex with single frequency. When the rectangular cylinder rotates with amplitude greater than or equal to 8°, the first symmetric pressure mode along with the second and third anti-symmetrical pressure mode appear simultaneously. The corresponding reattaching point is concentrated at the leading edge, so that vortices continue to develop along the model surface. These suggest the existence of several vortices with different frequencies. The secondary vortex with a frequency larger than the motion frequency leads to the occurrence of higher pressure mode, and results in higher harmonics in motion-induced force.