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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. [ABSTRACT FROM AUTHOR]

This paper studies the dynamics of intra-acceptor hole relaxation in Be δ-doped GaAs/AlAs multiple quantum wells (MQW) with doping at the centre by time-resolved pump-probe spectroscopy using a picosecond free electron laser for infrared experiments. Low temperature far-infrared absorption measurements clearly show three principal absorption lines due to transitions of the Be acceptor from the ground state to the first three odd-parity excited states respectively. The pump-probe experiments are performed at different temperatures and different pump pulse wavelengths. The hole relaxation time from 2p excited state to 1s ground state in MQW is found to be much shorter than that in bulk GaAs, and shown to be independent of temperature but strongly dependent on wavelength. The zone-folded acoustic phonon emission and slower decay of the wavefunctions of impurity states are suggested to account for the reduction of the 2p excited state lifetime in MQW. The wavelength dependence of the 2p lifetime is attributed to the diffusion of the Be atom δ-layer in quantum wells. [ABSTRACT FROM AUTHOR]