Unsteady Lifts and Wakes of Oscillating Rectangular Prisms

Measurements are made of the unsteady lifts and wakes of rectangular prisms oscillating transversely in a uniform flow with the short sides normal to the flow direction. Rectangular prisms tested have three different side ratios of l, 2, and 4, respectively, and the tests are carried out in a wind tunnel. The phase angles of the frequency response components of both the unsteady lift and the near-wake velocity show abrupt change when the critical reduced wind velocity for vortex shedding is approached. This is suggested to be one of the key phenomena to solving the problem of vortex excitation of bluff structures. For all of the three rectangular prisms tested, the linear quasi-steady aerodynamic theory of galloping is found to be applicable until reduced wind velocity is lowered fairly closely to the aforementioned critical reduced wind velocity. However, the reason why the applicability of the theory to such low reduced wind velocities may be fortuitous is also mentioned. Further analyses are made of the instability occurring in a restricted range of reduced wind velocity well below the critical.