Study on the Flow and Acoustic Characteristics of Submerged Exhaust Through a Lobed Nozzle

Abstract

Submerged exhaust noise is a significant contributor to the acoustic signature of submerged equipment using thermal power engines. Recently, it has been shown that the structure of a submerged exhaust nozzle has a great impact on the radiated noise levels. However, what kind of structure can effectively reduce the noise and how it works are still being studied. This paper investigates the influence of the lobed structure on the submerged gas jets. The submerged exhaust gas signatures are examined using a round nozzle and a lobed nozzle at gas flow rates from low to high $$(30-210\,\hbox {m}^{3}/\hbox {h})$$ . Hydrophones are used to measure the sound pressure emitted from the gas jet. A high frequency pressure transmitter is applied to reveal the effect of the lobed structure on the pressure fluctuations in the upstream pipeline. A high-speed digital video camera serves to examine the gas behavior near the nozzle exit. The sound pressure, pressure and image signals are synchronized using an originally designed synchronous system. The experiment results demonstrate that low frequency (less than 1000 Hz) sounds and pressure fluctuations always dominant the spectrum in a submerged exhaust process. These strong sounds and pressure fluctuations are closely associated with the jet contractions. Gas exhausted from the lobed nozzle would smoothly flow along the lobed edge, and have a smaller jet width than that from the round nozzle in the downstream area. The Strouhal number would also be reduced by the lobed nozzle, which means that gas exhausted from the lobed nozzle is less obstructed. Therefore, the lobed nozzle would not only reduce the low frequency sounds associated with the jet contraction (reduce 3.3–8.6 dB), but also significantly reduce the pressure fluctuations in the upstream pipeline.