Dynamic Forcing Function for Flow-Acoustic-Induced Vibration

Abstract

Vibration problems in piping systems can manifest themselves in two forms: excessive noise generated from wall flexural vibrations, that in combination with jet noise itself may exceed OSHA limits, and piping system vibration, which could lead to fatigue failure at stress risers. Thus, a comprehensive dynamic forcing function for flow-acoustic-induced vibration on piping system needs to consider both the flexural and system excitation sources. Jet column instabilities and acoustic standing waves are utilized to identify the vibration sources in a large-capacity steam piping system. Initial noise generation originates from approximately six jet diameters downstream of a control valve. It consists of two sharply defined high-frequency sources, the shock cells and the large-scale axisymmetric coherent turbulent structure generated from the nonlinear shear layer instability. These sources effectively excite wall flexural vibrations because of their nonzero net dynamic forcing on pipe “shells.” The compact shock and instability wave noise sources further excite the low-frequency acoustic standing wave in the acoustic duct formed by the discharge piping. The low-frequency acoustic standing wave excites the piping system vibration axially. The piping system was also excited transversely by a more potent vibration source incurred by the spiral mode provoked by a piping elbow. Field observation and measurement of the vibration problem of a large-diameter piping system confirm the prediction.