A method and apparatus for measuring at least one parameter of a fluid flowing through an internal passage of an elongated body is provided. The internal passage is disposed between a first wall and a second wall, and the first wall and the second wall each include an interior surface and an exterior surface. The method includes the steps of providing an array of at least two ultrasonic sensor units, operating the sensor units to transmit ultrasonic signals at one or more frequencies substantially coincident with at least one frequency at which the transmitted ultrasonic signals resonate within the first wall, receiving the ultrasonic signals with the sensor units, and processing the received ultrasonic signals to measure the at least one parameter of fluid flow within the internal passage.