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Being able to predict ship and marine propulsion noise is an important issue for naval architectures and the international maritime community. The main objective of this paper is the numerical investigation on the noise propagation by the high skew marine propeller working in a non-uniform inflow via RANS solver in the broadband frequency range. The pressure fluctuations were monitored at three points on the propeller blade, then by using the FFT operator we computed the blade passing frequency (BPF) for different propeller loading conditions. Based on these pressure pulses and adopting the Fowcs Williams-Hawking model we calculated noise radiated at the monitoring points. The results showed the BPF and noise level increased by increasing the load on the blades and we also observed that the noise generated at the leading edge was greater than at other points. Furthermore, the study of pressure fluctuations showed the propeller tip has more pressure variations in one revolution than other regions of the propeller surface.

Investigations and observations on fluid flow and performance characteristics (numerically and experimentally) and sound generation (experimentally) of single and double outlet squirrel cage fans are performed in this study. The main objective is to survey the performance of double outlet fans and the effect of two volute tongues as main sound sources. Fan performance and sound experiments are conducted using an in-duct experimental setup. The efficiency and pressure curves show that each outlet channels of the double outlet fan operates similar to a single outlet one. As a criterion for evaluating the sound generation, total sound pressure level (SPL) and the noise component at blade passing frequency (BPF) in the power spectrum are considered. A comparison between the total sound pressure levels of the fans shows that in both of them the BPF noise increases with flow rate, while higher SPL is found for the double outlet one. An exactlyhigher velocity jet/wake flow from the rotor in double outlet fan is responsible for the higher BPF noise. [ABSTRACT FROM AUTHOR]

A variety of centrifugal compressors are used in various fields of industry such as aircraft, home appliances, and vehicles. Comfort and quietness are important in these uses. As a result, noise has become an important consideration in compressor design besides the conventional performance parameters such as efficiency and pressure ratio. However, compressor noise has been difficult to understand because of the lack of information. The aim of this paper is to investigate the aero-acoustic characteristics of a centrifugal compressor for the fuelcell vehicle by experiments. The existing compressor system is modified to measure the internal pressure fluctuation at the impeller inlet, the impeller outlet and the diffuser outlet. Four microphone probes are also installed to determine the external noise levels and spectra of the compressor in an airtight room according to the RPM and mass flow rate. The test results show the possibility to tell the relative noise level of a centrifugal compressor with the internal pressure data. The external microphone signals have relation to the internal pressure signals. They have similar patterns and spectra. It is a noteworthy phenomenon because it is easier and inexpensive to predict pressure behaviors than noise characteristics of centrifugal compressors. The dominant noise source is the tonal noise during normal operation. But the broadband noise component due to the turbulent flow in the compressor increases during low flow rate operation. Computational simulations are carried out to describe these phenomena and to identify noise indicators. The turbulence kinetic energy and the pressure distribution obtained from CFD results may be indicative of the relative noise intensity of the compressor. The experimental facility, instrumentation and simulation conditions are described, and the results are presented in this paper. [ABSTRACT FROM AUTHOR]

Multirotor drone generally controls its attitude and position through the variation of the angular speed of each rotor, which makes the sound signature from it considerably different from the one of single-rotor flight vehicle. Specifically, the sound spectrum of single-rotor vehicles is dominated by the blade passing frequency (BPF) components, determined by the angular speed of the rotor. In contrast, the variation in the angular speed in the multirotor makes it challenging to predict the effects of the multirotor noise. In this study, the effects of the variation in the angular speed of each rotor on the noise of a hovering multirotor were quantified and predicted through uncertainty quantification and single-rotor stochastic analysis. Multirotor experimental results indicated that the Revolution per second (RPS) uncertainty of the multirotor is sensitive to maximum tilt angle and payload. The experimentally quantified angular speed variation is used as input to single-rotor stochastic analysis, and the measured aerodynamic and acoustics results show good agreement with the analytically/numerically predicted results. On comparing multirotor noise, the single-rotor noise with RPS uncertainty results exhibit similar spectral characteristics, especially near the BPF harmonics. Furthermore, the results reveal that the single-rotor stochastic simulation with angular speed uncertainty can predict angular speed variation effects on the multirotor noise in the hovering condition. This study concludes that the sound characteristics of a hovering multirotor can be predicted by considering a single-rotor case with RPS uncertainty. [ABSTRACT FROM AUTHOR]

This paper develops and proposes an Inverse Tracing Method (ITM) in the time domain to tackle the problem of the radiating noise caused by unsteady propeller sheet cavitation, in which the relationship between the time of the noise source and the time of the propagated sound can be inversely traced by the piecewise cubic-spline functions. By applying the inversely traced relationship into the solution of the wave equation, the Dirac delta function is assumed to simplify to δ (0), so the time integral is assumed to vanish. In addition, the unlimited differential terms also can be replaced by differentiating the piecewise cubic-spline functions. In order to validate the corrections of the ITM, the far-field noise calculated by the ITM is compared with that predicted by the far-field method proposed by Nobel et al. (1986), and good agreement is shown. The near-field noise obtained by the ITM is checked by the experimental results of a container ship, and the ITM overestimates the experimental data by about 2 dB. A workboat is also taken as a computing sample to predict the radiating noise caused by unsteady propeller sheet cavitation. Because the cavity volumes are over-predicted, the predicted noises by the ITM are obviously stronger than the measured noises. As the numerical cavity volumes are modified to meet the measured volumes, by adjusting the cavitation number, the predicted Sound Pressure Level (SPL) at the first blade passing frequency (i.e. the maximum SPL) by the ITM is smaller than the measured value, by 4 dB. At higher blade passing frequencies, the measured SPLs are remarkably stronger than the predicted levels, due to the appearance of tip vortex cavitation in the model test. • Highlight for Reviews. • The proposed scheme (ITM) is a new method, and can predict noises in both near and far fields. • The unlimited differential term can be expressed by the coefficients of the piecewise third-order polynomial. • The validation tests are carried out in both near and far fields for this method (ITM), and good agreements are obtained. • The predicted maximum sound pressure level (SPL) is smaller than the experimental data by 4 dB for the sampled workboat. [ABSTRACT FROM AUTHOR]