Passive Multi-Degree-of-Freedom Piezoelectric Rods for Continuous Monitoring of Scour Holes

Local scour is the leading cause of bridge collapse in the United States and, therefore, poses a great threat to public safety, disrupts local commerce, and costs millions of dollars in repairs. Flow-driven scour monitoring technologies have emerged, where they detect frequency shifts in buried rods placed nearby the structure of interest. As scour erodes sediment in the location of the sensor, the exposed length of the sensor will increase, and the subsequent shift in frequency is detected. Ideally, the shift in resonant frequency is related only to the change in length. However, it was found that the velocity of the excitation fluid can significantly affect the fundamental frequency output. Higher mass ratio rods are known to be less susceptible to the influences of flowing-water excitation. Therefore, this letter aims to increase the mass ratio of the scour sensor while also introducing more frequency features into the signal signature that can be used to determine length. Two variations of passive piezoelectric sensors are developed and then optimized to have greater participation of high-order frequencies under free vibration conditions using weights as localized masses. The results show that a vibrational sensor could be made to output up to five modal frequencies, related to the number of additional participating weights.