Non-Synchronous Vibration: Lock-in Region and Unsteady Pressure Analysis on NACA0012 Airfoil
Non-synchronous vibration (NSV) in turbomachinery is a complex phenomenon of interest that has been studied but not yet fully understood. The interaction between fluid dynamic instabilities and natural vibration of the blades are the main reason NSV occurs. When the natural instability frequency/shedding frequency is close to the natural frequency of the body, the system is locked in, namely the shedding frequency “locks in” to the natural frequency of the body, catastrophic turbine or wing failure can potentially occur. The research done in this thesis report consists of both experimental studies performed on a symmetric NACA0012 airfoil in Duke University Subsonic Wind Tunnel and computational studies using Computational Fluid Dynamics software ANSYS Fluent, under various flow and airfoil motion conditions. Utilizing data acquisition system and LabVIEW control software, pressure data along the upper and lower surfaces of the airfoil were collected in time domain and transformed into frequency domain data with Fast Fourier Transform and analyzed in MATLAB. To understand the underlying flow physics and relationships between unsteady pressure contributed by shedding and natural vibration, the region where the two frequencies are locked in is more accurately identified. A preliminary model to predict the unsteady pressure distribution under lock-in condition from unlocked pressure data is defined. The solitary experiments and computational simulations done on NACA0012 airfoil, and the results found in this thesis provide a better understanding of lock-in condition and its relationship to flow conditions and serve as footstone for future studies on other geometries of interest under various flow conditions. The goal of steady/unsteady pressure analysis as part of the research is to visualize the pressure distribution on the surface of the airfoil in both locked-in and unlocked conditions. From the pressure distribution, the lock-in phenomenon can be better understood, as of when and why it occurs, and ultimately, how to avoid it in real-world operations.
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