Measurement and verification of unsteady galloping force on a rectangular 2:1 cylinder

To investigate the unsteady galloping instability of a rectangular cylinder with an aspect ratio of 2:1 or that under interaction with vortex-induced vibration (VIV), the nonlinear unsteady galloping force (UGF) was measured in this study through free-vibration wind tunnel tests of a spring-suspended sectional model (SSSM). The measurement precision of UGF was remarkably enhanced by adopting an improved measurement technique of aerodynamic force, which includes an interior installation of force balances and an optimization of sectional model arrangement. The nonlinear parameters of non-wind-induced aeroelastic force were identified by an energy equivalent principle (EEP) from the free-decay response in still air. The observed galloping instabilities were found to have strong aerodynamic nonlinearity and unsteady effect from the measured UGF, and comparison with classical quasi-steady prediction. The mechanical nonlinearity in the vibration system of SSSM was identified by the equivalent linearization approximation (ELA) to further verify the measured UGF. The accuracy and reliability of measured UGF was verified by comparing the calculated galloping response with experimental one. Finally, it was found by comparing the measured UGF results by the forced and free vibration tests that the forced vibration is inapplicable to investigate the nonlinear behavior of the UGF of bluff bodies.