Blade Passing Frequency Component Research Articles

Influence of skew angle on the cavitation dynamics and induced low-frequency pressure fluctuations around a marine propeller

This study numerically investigated the effects of the skew angle on cavitation and the induced low-frequency pressure fluctuations. The numerical results compare well with available experimental data. The skew angle significantly impacts the cavitation dynamics as well as the pressure fluctuations. As the skew angle increases, the dominant cavity type gradually changes from sheet cavitation to tip vortex cavitation and the cavitation on the blade passing through the wake flow is delayed. Fast Fourier Transforms combined with Wavelet Transforms were applied to explore the relationship between the cavity volume acceleration and pressure fluctuations for various skew angles. The low-frequency pressure fluctuations around the propeller are mainly related to the first blade passing frequency (BPF) and its harmonics. The sheet cavity collapse significantly enhances the interaction of the low BPF components, while the tip vortex cavity collapse has the opposite effect. In addition, large skew angles weaken the interaction of the low BPF components which reduces the low-frequency pressure fluctuations.

Vibro-acoustics of a pipeline centrifugal compressor: Part II. Control with the micro-perforated panel

Structural vibration and noise radiation of a single-stage centrifugal compressor excited by the internal unsteady flow is investigated experimentally in the first part of our work [Zhou et al. Appl Acoust 2018;131:112–28]. Experimental results for acceleration of vibration and sound pressure under different rotational speeds indicate that the straight discharge pipe connecting the volute is forced to vibrate the most intensively at the blade passing frequency (BPF) and acts as the major contributor to the compressor noise emission. In this paper, a pipe micro-perforated panel (PMPP) is optimally designed with the genetic algorithm to suppress vibration and noise radiation of the straight discharge pipe section. A maximum sound pressure level (SPL) reduction of 20 dB for the BPF component and 8 dB for the overall noise are observed under the tested working conditions, while the influence of the PMPP on the aerodynamic performance of the compressor is negligible. The PMPP may be a promising method for controlling the tonal component of vibro-noises radiated from the centrifugal compressor.

Examination of the predictions versus measurements of an acoustic interaction model for spinning modes, and concurrent low frequency amplitude modulation acoustic signatures from a 3MW wind turbine

A recently presented hypothesis and model relating to the generation of spinning modes from wind turbines, as a direct result of acoustic interaction involving the tower, results in a far field infrasound sound pressure level prediction, which is higher than that predicted by point source method. The model also predicts a significant attenuation of the fundamental blade passing frequency component relative to the second and higher harmonics. The model concurrently predicts a low frequency (~20 Hz), amplitude modulated harmonic series as a side effect of the acoustic interaction on a 1.6 MW 80 m diameter wind turbine. This study examines the model predictions of a 3.0 MW 90 m diameter wind turbine, and compares the predictions to measurements of the low frequency harmonic series and blade passing frequency harmonics at several different distances from the wind turbine.

Computational Estimation of Fan Casing Noise at Blade Passing Frequency Component Noise

This paper studies both vibroacoustics and aeroacoustics of a centrifugal fan casing; the aim of this study is to explore a methodology to make quantitative predictions of fan casing noise. The spectra of the fan noise and casing vibration were firstly presented; discrete components related to the rotational frequency protrude in the spectra, especially the blade passing frequency (BPF). Computational fluid dynamics (CFD) technique was used to obtain the three-dimensional unsteady turbulent internal flow. Attention was paid to the pressure fluctuations on the volute wall; the shapes of pressure fluctuation were nearly sinusoidal in nature, with the BPF as the primary frequency. On the vibroacoustic side, Fast Fourier Transform (FFT) was applied to the time series of pressure fluctuations to extract the BPF component. A finite element analysis (FEA) model of the casing structure was constructed, and was validated by experimental modal analysis. The harmonic dynamic response of the casing structure was calculated with the BPF pressure fluctuation component as the excitation. The vibration results were then taken as the velocity (Neumann) boundary condition for the noise radiation model which was built in boundary element method (BEM), and the sound radiation was calculated. On the aeroacoustic side, the BPF component of pressure fluctuations was modeled as acoustic dipole source, and sound radiation was also solved by BEM. Results show that the sound pressure level (SPL) of vibroacoustics is fairly small compared to the aeroacoustic counterpart. This study shows that CFD, FEA together with BEM can be used to numerically predict BPF casing noise of turbomachinery successfully.

Study of the Tonal Casing Noise of a Centrifugal Fan at the Blade Passing Frequency

This paper studied the casing vibration and noise radiation of a centrifugal fan at the blade passing frequency (BPF) induced by the internal unsteady flow. Three-dimensional numerical simulation of the complete unsteady flow in the whole impeller-volute configuration was carried out using computational fluid dynamics (CFD) techniques. Pressure fluctuations on the casing were obtained, and the BPF component was extracted after Fast Fourier Transform (FFT). In Part I of this two-part study, the BPF pressure fluctuation was modeled as aeroacoustic dipole source according to Lighthill's acoustic analogy theory, and sound propagation was simulated regarding the casing as a rigid body. In this part, the casing was modeled as an elastic structure in order to study the tonal vibroacoustics of the casing structure with the BPF pressure fluctuations as excitation. Time harmonic response of the casing was calculated using the finite element method (FEM) and vibroacoustic noise radiation was analyzed by the boundary element method (BEM). The primary objective is to make an attempt to quantitatively evaluate the flow-induced casing vibration and noise radiation in fluid machinery. Some results were obtained casting light on the characteristics of casing vibration and noise radiation.

원심압축기의 공력소음 저감에 관한 설계연구 Part I : 성능해석 및 소음예측

The objective of this research is to suggest anoise prediction method for a centrifugal compressor. It is focused on the Blade Passing Frequency component which is regarded as the main part of the rotating impeller noise. Navier-Stokes solver is used to simulate the flow-field of the centrifugal compressor, and the time-dependent pressure data are calculated to perform the near-field noise prediction by using Ffowcs Williams - Hawkings formulation. Indirect Boundary Element Method is applied to consider the noise propagation effect. Pressure fluctuations of the inlet and the outlet in the centrifugal compressor impeller are presented and the sound pressure level prediction results are compared with the experimental data.

NOISE SOURCE IDENTIFICATION IN A PROPFAN MODEL BY MEANS OF ACOUSTICAL NEAR FIELD MEASUREMENTS

For the exploration of the dominant aerodynamic noise sources, the pressure fluctuations in the exit plane (near field) of the propfan model CRISP (Counter Rotating Integrated Shrouded Propfan) were measured with conventional 1/4 inch microphones. The pressure field of the tone components was resolved into a distribution of duct modes. Knowledge of the dominant modes allows conclusions about the dominant noise generation mechanisms, because the different noise sources inside the propfan create different sets of modes. The CRISP concept is developed by Motoren- und Turbinen-Union München (MTU). The experimental propfan has two counter-rotating rotors of 0·4 m diameter and equal speed. The shroud is supported by seven struts located downstream of the second rotor. Measurements were made with equal (B1=B2=10) as well as unequal (B1/B2=10/12) blade numbers and under different operational conditions. The highest overall harmonic levels were found for the configuration with equal blade numbers. In this case, the blade passing frequency component is generated mainly by the rotor 2/struts interaction, and the higher blade tone harmonics, which dominate the overall tone noise level, are produced by the interaction of the two rotors. In the case of unequal blade numbers, all even harmonics of the shaft frequency (H=2, 4, 6 ...) can be generated by the rotor 1/rotor 2 interaction. The harmonics belowH=22, however, are excited as non-propagational modes only and were found to have small amplitudes in the exit plane. The rotor 1/rotor 2 interaction is the main noise generation mechanism for the configuration with a short axial distance between the rotors. When the rotor distance is enlarged, the rotor/rotor interaction noise is reduced and, as a consequence, the contributions from the rotor/struts interactions become important. In addition to the experiments, a theoretical method is described for the prediction of the frequencies and azimuthal modes generated by two rotors with arbitrary speeds, directions of rotation and blade numbers. This method is helpful for the design of low noise rotor systems.

Noise in Centrifugal Compressor. 1st Report. Influence of the Diffuser Vane on Noise.

Noise in compressor stages with two vaned diffusers and a stage with one vaneless diffuser are measured. The overall noise level and the blade-passing frequency (BPF) component level in the vaned diffuser stages are higher than those of the vaneless diffuser stage. The difference in the BPF components of the two types of diffuser varies with the rotatinal speed of the impeller. Under certain conditions, the large noise which the BPF component is extremely high and sounds like a whistle is observed. Measurements of pressure fluctuation in the vaned diffuser passage show a standing wave in the flow passage, and so the loud noise is considered to be caused by resonance in the vaned diffuser. The resonance frequency is affected by the diffuser vane shape.

Active Noise Control to Reduce the Blade Tone Noise of Centrifugal Fans

This paper describes an active noise control method to suppress the blade tones of centrifugal fans. Two secondary sound sources are mounted into the cutoff region of the fan casing. These sources are driven with electrical signals that are synchronized with the rotation of the impeller, and their amplitudes and phase are adjusted to give maximum reduction for the blade tone levels in the inlet and outlet duct of the fan. With this design, the sound emitted by the secondary sources is introduced into the interior of the casing near the source region where the blade tone is generated, i.e., the cutoff. The present experiments were concentrated on the reduction of the fundamental of the blade tone for centrifugal fan with impeller diameters between 280 mm (11 in.) and 710 mm (28 in.). Two different designs of secondary sources were investigated. In the first, two loudspeakers are contained within an enclosure which has an open end made of a curved perforated plate which replaces part of the original cutoff. The second design incorporates two vibrating plates which replace portions of the outlet duct side and the volute side of the cutoff. Reductions in tone sound pressure level of up to 23 dB have been observed for a variety of aerodynamic loading conditions and fan inlet geometries. To obtain a better understanding of the physical mechanism of this active noise control method, sound pressure measurements were also made on the inner surface of the fan casing along the volute. Both amplitude and phase of the blade passing frequency component were measured relative to a reference signal derived from the impeller rotation. The result of this experiment is that the sound field inside the casing is dominated by the pressure pattern rotating together with the impeller. Since the impeller tip Mach number is well below sonic speed, however, the radiation efficiency of the rotating pressures is very low. The blade tone noise measured in the far-field is generated by the unsteady pressures at the cutoff which in turn are produced by the flow leaving the impeller. This aerodynamic noise generating mechanism is modified by the active sources located in the cutoff.