Unsteady Pressure Analysis of a Three Stage Turbine - Searching for Nonsynchronous Vibrations and Lock-In

When an unsteady aerodynamic instability nears a natural mode of a rigid body, the phenomenon known as Nonsynchronous Vibrations (NSV) can occur. These vibrations cause large amplitude oscillations and ultimately blade failure in jet engines and steam turbines; however, the underlying flow physics are much less understood compared to other aeroelastic phenomenon such as flutter or forced response. When the buffeting frequency of the flow around a body such as a blade nears its structural natural frequency, the flow frequency merges to the natural frequency in a phenomenon known as “lock-in”. Within this “lock-in” region, there is only one frequency, while outside of it there are two. Although this region of “lock-in” is well documented both experimentally and computationally, the pressure content associated with this phenomenon have not been fully understood. Using the geometry for a 3-stage turbine rig, computational simulations are performed to study the unsteady pressure frequency content on the rotors and stators. A Fast-Fourier Transform is performed on the time-series pressure measurements to analyze frequency domain pressure content. These frequencies are determined to be different than the blade passing frequency, while they are also located far away from the traditional flutter and forced response frequencies, so they must be related to Nonsynchronous Vibrations. At these NSV frequencies, the amplitude and behavior of the steady pressures provides insight into the flow physics previously not understood.