Scaling and aspect ratio effects on the frequency-modulated ultrasonic signal generated via fluidic oscillator

Traditional ultrasonic non-destructive testing methods in civil engineering require the use of coupling agents, leading to prolonged and labor-intensivemeasurement procedures along with the risk of surface damage. To alleviate these concerns, air-coupled ultrasonic actuation has been introduced as an effective alternative. However, the power loss due to impedance mismatchs at the interfaces remains an important limitation. To mitigate the power losses at the interfaces while characterizing the specimen thickness, we employ the fluidic oscillator as an ultrasonic source, wherein air acts as both the operating and coupling medium. The fluidic oscillator generates signals through self-sustained oscillations of the exiting air jet under continuous pressurized air supply, and therefore, eliminates the need for intricate design and manufacturing processes. The frequency spectrum characteristics and time-averaged acoustic fields of the fluidic oscillators are shown for the several configurations that were designed by varying the aspect ratio and scaling. Our preliminary investigations highlighted the dependence of the dominant spectral characteristics of a given fluidic oscillator on the mass flow rate of input air. Leveraging this observation, frequency-modulated chirp signals are produced by rapidly varying the flow rate, enhancing the signal-to-noise ratio for a reliable assessment of material characteristics.