UNSTEADY PRESSURES DURING FREQUENCY LOCK-IN OF A CYLINDER EXPERIENCING NON-SYNCHRONOUS VIBRATIONS
When an unsteady aerodynamic instability coalesces with a natural mode of vibration, Non-Synchronous Vibrations (NSV) can occur. Often in turbomachinery, vortex shedding, or tip gap flow causes frequency lock-in to the compressor or turbine blade bending or torsional natural mode of vibration. This can cause limit cycle oscillation (LCO) and ultimately blade failure. There is no current design tool for NSV that engine manufacturers can use; rather, expensive wind tunnel testing and time-consuming 3D computational fluid dynamics (CFD) are conducted to search for it. While studies have been done on turbomachinery geometry to study NSV, simpler configurations need to be studied first to understand the underlying flow physics. In this work, a cylinder undergoes enforced oscillations transverse to oncoming flow in the von Karman Vortex Street flow regime. The unsteady pressures are studied in both unlocked and lock-in conditions. In this context, this paper attempts to model the unsteady pressures of the lock-in region as a linear combination of unlocked amplitudes. Once the frequency content is collected for multiple cases, the unlocked data is fed into a Multiple Linear Regression to predict the lock-in data, with very good agreement. Previously shown was the coefficient of lift linearly increases with increasing amplitude of oscillation while locked in. The first study validates this finding. However, when unlocked first, the pressure contributions from various frequency ratios linearly combine to recreate the pressure loading for the lock-in cases. Finally, pressure amplitudes for various unlocked amplitude ratios linearly combine to form the same lock-in pressure loading. Unsteady pressures are not measured and do not often appear in literature. In addition, the unsteady pressure contributions have not previously been split up between the enforced motion and the vortex shedding, making this work novel.