Fiber Optic Sensing of Strain and Temperature for Structures in a Hypersonic Flow

Abstract

Research into multi-sensor fiber Bragg grating (FBG) networks has the potential to benefit aircrafthealth monitoring systems. The overall goal of this thesis research is to demonstrate that FBG sensors can be used to measure temperature and strain data of a structure in a hypersonic flow. To that end, the design and testing of a five-fiber Bragg grating sensor array mounted on a thin rectangular compliant panel in a Mach 6 hypersonic flow is pursued. Each FBG sensor can capture both panel vibration and temperature data. The hypersonic wind tunnel facilities at the NASA Langley Research Center (LaRC) were used to test the sensor array mounted on a thin panel surface, which formed the boundary of a plenum cavity. This thin panel is subjected to a shock wave generated by a finned flat plate structure at certain flow speeds and specific orientations of this finned structure. FBG sensors were used to collect the resulting strain and temperature response data on the panel surface due to the associated fluid-structure interactions. The panel formed one boundary of the plenum box, and the generated backpressure influenced the (acoustic) stiffness of the system. A variety of testing conditions were studied, and 19 experiments were conducted at Mach 6 flow speed. The following parameters were varied in the experiments: fin angle, plenum backpressure, and Reynolds number. The FBG results were validated by using measurements from pressure transducers mounted in the plenum and an infrared (IR) camera imaging of the surface. Based on the results obtained in the high-speed wind tunnel experiments, it is shown that multiplexed fiber Bragg grating sensors can be successfully used to obtain strain and temperature data on a structure in a dynamic, high-temperature environment experienced in a hypersonic flow. The pressure fluctuation frequencies obtained from the pressure transducers were found to fall within the expected range of the structural panel vibrations. By using the IR camera imaging, temperature distribution data over time were captured, and this information was iii compared to the FBG data obtained from five distinct sensor locations. Collectively, the information obtained from the pressure sensors, IR camera imaging, and multiplexed FBG sensors is used to form a cohesive picture of the response exhibited by a structure in hypersonic flow. The results provide a basis for using FBG sensors on aircraft structures for monitoring structural responses in hypersonic flows.