Measurement and analysis of the noise radiated by low Mach number centrifugal blowers

Abstract

The wave equation was integrated numerically by the finite-element method, using a number of different combinations of the acoustic impedance and the scaling parame. ters. For comparison, the acoustic field of the same propeller in free space was also computed.The calculations show that at flow speeds much less than that of sound, the wall must be lined with acoustical absorbers to make the free-field and ducted-propeller models agree in acoustic-pressure levels and directionality. The best agreement Would occur where the ratio of wall radius to propeller radius is large and the measurements are made away from the walls.At a flow speed half that of sound, the acoustic-pressure magnitude and directivity along the duct wall near the propeller are similar for a hard wall and for a variety of soft walls. However, the hard-wall results may be fortuitous, and additional studies are needed. Upstream or downstream from the propdie, r, soft wall liners having suitable admittances can suppress the usual duct acoustic m•c]es. In this case, the duct and free-field acoustic characteristics can agree over greater sideline distances. At high flow speeds, different upstream and downstream wall admittances are necessary for the equal suppression of duct modes on both sides of the propeller. Further information may be found in NASA TM-87333 [N86-29630/ NSP ], "Modeling the Effects of Wind Tunnel Absorption on th• Acoustic Radiation Chfiracteristics of Propellers." Copies may be purrchased (prepayment required) from the National Technica.[. Tel.: (703) 487-4650. Rush Orders may be placed for an extra fee by cal!ing (800) 336-4700. Advanced-degree dissertations in acousticsEdito?s note: Abstracts of Doctoral and Master's degree theses in acoustics will be welcomed at all times. Please note, however, that they must be double-spaced, formatted as shown below (don't make us retype them, please!) with PACS numbers, and not over 200 words in length.

Historical developments in the control of noise generated by small air-moving devices

“…108 A fundamental study of sound sources in low-Machnumber centrifugal blowers was done by Yeager. 109 He extended the correlation theory of Siddon to include sound intensity, thereby allowing investigations of the local radiation field from turbulent interactions with rigid surfaces. Correlation studies were performed on stationary blade sections immersed in turbulent flow using probe microphones and miniature pressure sensors.…”