Measurement of Sound Power Due to Flow Noise at the Outlet of a Straight Pipe

Intake, exhaust, and heating / air conditioning systems in automobiles consist of various common duct elements. Noise arises primarily due to the source and is attenuated using common elements like expansion chambers and resonators. This attenuation is straightforward to predict using plane wave simulation and more advanced numerical methods. However, flow noise is often an unexpected important noise source. Predictions require computer intensive analyses. To better understand the aeroacoustic sources in duct systems, a flow rig has been developed at the University of Kentucky. The flow rig consists of a blower, a silencer to attenuate blower noise, external noise sources, and then the test duct. The flow rig can be equipped with an anechoic termination to measure transmission loss or may be used to measure insertion loss directly. In the latter case, the sound power is measured from the pipe outlet inside of a hemi-anechoic chamber. In this research, the external noise sources are excluded, and the flow noise is measured directly. An experimental investigation is performed to determine the sound power for a range of different duct lengths, diameters, and flow rates. Sound pressure level results are reported versus frequency and versus the Strouhal number. The objective of this work is to provide data for validating future computational fluid dynamics predictions and to provide engineers with some expectation of the anticipated flow noise for common situation.