Predicting Pipe Wall Vibration in Frequency Bands With Low Acoustic and Vibration Modal Coincidence

Abstract

Abstract Accurate predictions of pipe wall vibration from internal dynamic fluid excitations are critical for assessment of damage mechanisms such as acoustic induced vibration. A common method to relate the excitation of the pipe wall vibration to the internal acoustic excitation is through relationships between the modes in the fluid and modes in the pipe wall. At coincidence, the transfer of acoustic energy to the pipe structural vibration is most efficient. Because of variations in the pipe dimensions and the internal acoustic field, the methods to predict the transfer of energy from the internal acoustic field to the pipe wall vibration often uses a frequency band averaged relationship. These methods rely on having a sufficient number of modes and a known coupling behavior in the frequency band over which the average is calculated. However, at low frequencies, the modal density can be such that there are few internal acoustic or structural modes which causes fewer modal coincidences between them to define the coupling behavior. Further, with finite length pipe segments, the coincidence may not occur as with an infinite length duct. In such cases, the radiation efficiency in the mode-based approach does not include coincidence and is augmented with a mass law approach. This methodology was implemented in statistical energy analysis methods. The basic formulation along with an example will be given.