A scale model study of an engine test facility for alleviating low-frequency howl.

This article focuses on an experimental study investigating low-frequency howling noise generated during engine tests in the Propulsion Systems Laboratory (PSL) at NASA Glenn Research Center. Using a 1/25th scale model of the PSL duct system, the study demonstrates that jet exhaust interacting with the ducts excites acoustic modes, causing significant low-frequency noise, which increases with the addition of a secondary duct. Various acoustic treatments were tested, revealing that wire-mesh screens placed at the downstream end of the primary duct effectively reduce the noise, with finer mesh sizes providing greater attenuation but potentially introducing high-frequency noise and flow blockage. Based on these findings, the study recommends implementing an equivalent of a 4-mesh screen—corresponding to water-cooled tubes spaced about 6 inches apart—as a practical solution to mitigate howling noise in the PSL facility.