Fundamental research for actuation of low-frequency ultrasound by means of fluidic components
Structural Engineering, Building Informatics and Construction Operation
Final Report Abstract
Ultrasonic Non-Destructive Testing (NDT) is widely used for detecting internal defects of concrete-based structures in civil engineering. It relies on low-frequency ultrasonic waves, ranging from 50 to 200 kHz, to detect internal defects without damaging the material. A majority of the currently employed devices for testing require direct contact with the surface, which is time-consuming and cumbersome for large-scale structures. These challenges have led to the exploration of alternative actuation mechanisms, such as air-coupled ultrasound generation, where air serves as the propagating medium. These devices can reduce the required measurement time by a factor of 50 or up to 100. This would enable the inspection of entire structures like tunnels or bridges. These measurements would be a valuable data base of a detailed and reliable condition assessment of the transport infrastructure. However, the substantial impedance mismatch between the transducer and the air coupling medium results in significant power loss, thereby reducing signal strength. Other devices based on thin membranes or laser induced signal generate lack of robustness to be used in the harsh environment on the construction site. Feedback-type sweeping jet fluidic oscillators are widely used in flow control, cooling, and mixing operations due to their ability to generate oscillating flows without moving parts. These oscillators operate via inherent instabilities in the flow, where the jet alternates between chamber walls through interactions with feedback channels, producing a temporally and spatially oscillating flow at the outlet. The small the oscillator geometry, the higher the generated frequencies. Thus, sweeping frequencies up to 50 kHz are possible. If the geometry is fixed, the sweep frequency can be controlled by the supply pressure. If the supply is varied by means of a valve, the sweeping frequency varies as well. This enables the generation of frequencymodulated signal. This system is applied to concrete specimens to determine structure thickness and internal delaminations. Moreover, these devices are small and can be made of ceramics or stell. Therefore, they are very robust and ideally suited for the construction site. As part of this project, fluidic oscillators were developed, manufactured and tested at BAM. The frequency content, frequency bandwidth and sound pressure amplitude were analysed experimentally. Acoustic measurements with microphones and initial tests on concrete samples were carried out. The nozzles were geometrically scaled in order to optimise the centre frequency and the sound pressure amplitude. Frequency-modulated excitation signals (chirps) were generated with the aid of high-frequency pressure rules. The currently achieved frequency bandwidth of approx. 10 % turned out to be too low for reproducible and statistically verified ultrasonic measurements through concrete.
