Fan Noise Research Articles

Blade retrofit design to reduce the noise of an axial flow fan by mitigating the tip leakage vortex

In the context of controlling axial fan aerodynamic noise, one effective approach involves the redesign of the blade. In the present study, a novel blade design was formulated for a specific model of axial flow fan, targeting the mitigation of the tip leakage vortex (TLV) and subsequent reduction of noise levels. The assessed noise-reduction blades have perforations drilled from the leading edge (LE) to the pressure surface (PS) of the blades. By impeding the tip leakage flow (TLF), these perforations effectively weaken the TLV. The three-dimensional non-constant flow field within the channel is computed employing the shear stress transport (SST) turbulence model. The aerodynamic noise prediction, which is based on the Lighthill’s acoustic analogy theory, shows that the designed blades can effectively reduce the aerodynamic noise of the axial fan. Specifically, enhancing the ratio of inlet area to outlet area of the ventilation holes leads to an improved noise reduction effect. Notably, for ratios of 0.44 and 1.06, the designed blades exhibit noise reductions of up to 3.3 dB(A) and 3.9 dB(A), respectively. Meanwhile, the static efficiency of these two fans is reduced by 0.50% and 0.26%, respectively. The study thus provides good theoretical support for the design of axial flow fans in the context of noise reduction.

Direct-Noise of an Ultrahigh-Bypass-Ratio Turbofan: Periodic-Sector vs Full-Annulus Large-Eddy Simulations

The tonal and broadband noise of an ultrahigh-bypass-ratio fan stage, which has been developed at École Centrale de Lyon, is studied using large-eddy simulations (LES). Wall-modeled LES of a periodic sector and a 360-deg full fan stage have been performed at approach conditions, which is a relevant operating point for aircraft noise certifications. The fan noise is directly obtained from the fully compressible LES, using a well-refined unstructured mesh, and compared with state-of-the-art analytical models. The impact of the periodic boundary conditions, which are often used for high-fidelity simulations of turbofan engines, is assessed. The results from the 360-deg and periodic-sector LES are compared from aerodynamic and acoustic perspectives, including an analysis of the mean and turbulent flow quantities and sound-pressure spectra. Aerodynamic parameters show similar results for both configurations. However, the fan blade loading is slightly reduced in the 360-deg- LES near the blade tip. Acoustically, lower sound power levels at the intake and exhaust sections of the fan stage are obtained in the 360-deg LES, when compared to the periodic sector LES, particularly at low and middle frequencies. This can be associated with lower coherence levels in the fan wakes and smaller spanwise correlation lengths at the trailing edge of the blades. The modal content of the acoustic field has also been analyzed in detail and shows that the periodic sector LES cannot correctly simulate the modal content of the fan noise.

Effect of impeller structure on aerodynamic performance and noise reduction of a small multi-blade centrifugal fan

In this study, the numerical simulation and orthogonal test are combined to study the effects of blade outlet angle, blade inlet angle, and blade number on the centrifugal fan flow characteristics, stress distribution, and noise control. The experimental tests were carried out to confirm the accuracy of the computational model. The computational fluid dynamics (CFD) technology was used to establish 16 groups of impeller models and derive the optimal parameter combination form. The results show that the blade inlet angle and outlet angle have more significant effects on efficiency and flow rate of fans. Among different optimize structures, the A8 fan with an inlet angle of 60°, an outlet angle of 180°, and a blade number of 45, has the best aerodynamic performance, with a 17.4% increase in flow rate, a 3.3% increase in efficiency, a 7.6% reduction in noise and a 23.0% reduction in impeller stress, compared to the original fan. Reasonable adjustment of the inlet and outlet angles greatly improves fluid distribution and suppresses flow separation. Furthermore, the noise can be reduced by lessening the disturbance between the volute tongue and wake flow, so the flow state gets greatly improved at the volute tongue region.

Evaluating an Additive Manufactured Acoustic Metamaterial Using the Advanced Noise Control Fan

This paper examines the performance of a 3D printed acoustic metamaterial as an acoustic treatment for aircraft engine nacelles in the Advanced Noise Control Fan. As the level of air travel continues to increase, so too does the demand for better noise-reduction technologies for aircraft. Engines are one of the two main sources of noise generated by aircraft, with fan noise, in particular, being of concern due to its broadband and tonal contributions. Small and lightweight methods of addressing both broadband and tonal noise are necessary due to the limitations presented by the current engine design. Presented in this paper is a novel acoustic metamaterial that has undergone design optimization for broadband noise reduction. The final design was produced using 3D printing and tested using the Advanced Noise Control Fan at the University of Notre Dame. It was found that the material is capable of reducing the first harmonic of the blade passing frequency by up to 18.5 dB, with an overall noise reduction of 3.7 dB.

Smart Blade Count Selection to Align Modal Propagation Angle with Stator Stagger Angle for Low-Noise Ducted Fan Designs

The rotor–stator interaction noise is a major source of fan noise. Especially for low-speed fan stages, the tonal component is typically a dominant noise source. A challenge is to reduce this tonal noise, as it is typically perceived as unpleasant. Therefore, in this paper, we analytically, numerically and experimentally investigate an acoustic effect to lower the tonal noise excitation. Our study on an existing low-speed fan indicates a reduction in tonal interaction noise of more than 9 dB at the source if the excited acoustic modes propagate parallel to the stator leading edge angle. Moreover, a design-to-low-noise approach is demonstrated in order to apply this effect to two new fan stages with fewer stator than rotor blades. The acoustic design of both fans is determined by an appropriate choice of the rotor and stator blade numbers in order to align the modal propagation angle with the stator stagger angle. The blade geometries are obtained from aerodynamic optimization. Both fans provide similar aerodynamic but opposing acoustic radiation characteristics compared to the baseline fan and a significant tonal noise reduction resulting from the impact of the modal propagation angle on noise excitation. To ensure that this effect can also be applied to other low-speed fans, a design rule is derived and validated.

Aerodynamic noise characteristics of a centrifugal fan in high-altitude environments.

In high-altitude areas, the air is thin and the atmospheric pressure is low, which can affect the performance of centrifugal fans and aerodynamic noise. In this paper, steady and unsteady simulations of a centrifugal fan flow field are performed at altitudes of 0, 1000, 2000, 3000, 4000, and 5000 m, and the Ffowcs Williams-Hawkings equation is used to predict the aerodynamic noise of the fan. The results indicate that the tonal and broadband noise generated by the fan decrease with increasing altitude, and the A-weighted sound pressure level of each frequency band of the fan decreases when the air volume is held fixed. The maximum sound power level Lwmax, sound pressure pulsation interval, and total noise sound pressure level Lp decrease linearly with increasing altitude. For every 1000 m increase in altitude, Lwmax and Lp decrease by 0.45 dB and 1.05 dB respectively. The fan noise characteristics, performance parameters, and human auditory perception are the main factors that affect the establishment of fan noise standards in high-altitude areas.

Mirror-based multimodal straightforward method for impedance eduction using a zigzag array

The Mirror-based Multimodal StraightForward Method (MM-SFM) is proposed herein to realize accurate and reliable liner impedance eduction covering the wide frequency range of aeroengine fan noises. It adopts a novel zigzag array in the flow duct, and unfolds them by a coordinate transformation into a mapping diagonal array in an extended duct, which can increase the array transverse spacing Δz considerably. Then, it invokes the M-SFM incorporating the trust-region and extended Prony's algorithms, to decompose the three-dimensional multimodal aeroacoustic field and educe the impedance. A parametric study based on the Monte-Carlo uncertainty analysis is conducted through finite-element numerical experiments on a 51 × 51 mm2 flow duct, to survey the method performance under random perturbations. It indicates that, the educing accuracy and reliability are promoted by increasing Δz or the axial spacing Δx properly, and the decomposed-mode number M×N=10 is appropriate herein. In our studied cases, using the recommended array of Δz=25.5mm and Δx=22mm locating at the nodes of transverse mode m = 2, the MM-SFM generally produces accurate averaged educed impedances in a considerably extended frequency range from 100 Hz up to nearly 10 kHz, and has small educing uncertainties at most absorptive frequencies and thus a good reliability.

Investigation of acoustic comfort, productivity, and engagement in naturally ventilated university classrooms: Role of background noise and students’ noise sensitivity

Optimal classroom acoustics enhance students' engagement and learning efficiency. In warm-humid climate, ceiling fans are used at different speeds to enhance thermal comfort in naturally ventilated classrooms. Although ceiling fans enhance thermal comfort, using fans at higher speeds significantly increases Background Noise (BGN), interfering with acoustic comfort. To achieve an optimal acoustic environment, it is important to understand the effect of BGN on acoustic comfort. Hence, focusing on ceiling fan noise in classrooms, this study investigates the effect of acoustic environment and students' noise sensitivity on acoustic comfort, productivity, and engagement. A four-month-long field study was conducted in 11 naturally ventilated classrooms, obtaining 828 responses. Sound pressure levels in active classrooms were measured to calculate BGN and Signal-to-noise ratio (SNR). Reverberation time (RT) was measured both in students' presence and absence. Acoustic comfort conditions, productivity, and engagement of students at every seat were obtained through questionnaire survey. BGN levels and RT ranged from 58.2 to 65.3 dBA and 0.7–2.1 s, respectively. A positive correlation was found between acoustic comfort and engagement level and productivity. Regression analysis showed that students with high noise sensitivity reported acoustic discomfort when the BGN exceeded 62.4 dBA, RT was above 1.7 s and SNR was below 3.7 dBA. Chi-square test involving the acceptability vote indicated that students were more likely to accept the BGN ≤61 dBA, RT ≤ 0.7 s, and SNR ≥6.5 dBA. This study establishes the relation of BGN, RT, and SNR with acoustic comfort in naturally ventilated classrooms, offering insights for classroom acoustic environment enhancement.

The aerodynamic and noise characteristics of centrifugal fan with partially tilted tongue

Tilted tongue is one of the very effective solutions to reduce the noise of centrifugal fans. In this study, the accuracy of the numerical simulations is verified by experimentally measured aerodynamic performance and acoustic spectra. Two localized tilting designs of the tongue are proposed with the aim of reducing the aerodynamic noise of the centrifugal fan. Numerical results show that the different approaches significantly affect the noise reduction of the fan, with the best design exhibiting up to 2.33 dB of noise reduction. The internal flow and acoustic fields are analyzed in conjunction with the dynamic modal decomposition method to reveal the effect of the local tilt of the tongue on the flow and acoustic characteristics of the fan. The structural change induces the separation vortices at the trailing edge of the blades and the generation of secondary flow at the outlet wall of the volute shell, which is the main reason for the slight degradation of the aerodynamic characteristics of the fan. The causes of the blade passage frequency (BPF) noise reduction are not identical for different orders. BPF noise reduction stems from diminishing source intensity and dispersing source phase. Noise reduction in 2BPF is mostly due to weakening sound source intensity, with less conspicuous phase dispersion. This study provides good theoretical support for the design of backward curved centrifugal fans in the context of noise reduction.

Performance improvement and noise reduction analysis of multi-blade centrifugal fan imitating long-eared owl wing surface

This research was inspired by the long-eared owl's ability to fly silently. For the first time in this study, a wind turbine blade is designed to mimic the wing surface and the leading edge of the long-eared owl. The commonly used two-dimensional blade profile in previous studies is replaced by a more effective three-dimensional profile. This change leads to improved aerodynamic performance of the multi-blade centrifugal fan and reduced noise levels. The airfoil and leading edge profile parameters of the long-eared owl were extracted and utilized. These parameters were used to develop a fitting formula based on their correlation. This formula facilitated the design and optimization of a bionic blade (B-Blade). The results indicate a 4.1% enhancement in the maximum flow rate compared to the original blade fan, alongside a noise reduction of 1.3 dB(A) under identical static pressure conditions. An examination of the internal flow, noise, and sound source characteristics of both fan types was conducted, elucidating the aerodynamic noise mechanism. Fan noise propagation showed pronounced dipole sound source traits. The sound source area at the B-Blade fan's inlet and the volute tongue was more compact, leading to a decrease in mid-low frequency discrete noise. The sound source intensity was also diminished. The B-Blade fan also ameliorated the flow distribution at the impeller outlet, reducing the unstable interaction between the impeller and volute tongue, thereby effectively diminishing noise.

Acoustic measurement for resin-based acoustic liner on turbofan engine testbed

Acoustic liner is one of the most effective devices for suppressing fan noise emitted by aircraft engines. IHI and Japan Aerospace Exploration Agency (JAXA) have developed a resin-based acoustic liner for fan noise attenuation that is lighter than conventional acoustic liners in production. In this study, an acoustic measurement of the full-scale resin-based acoustic liner was conducted on the turbofan engine testbed installed at the JAXA test facility to evaluate its noise reduction capability in actual operating environment. The in-duct and in-cell acoustic measurements were performed during the test campaign. A baseline hardwall liner and the production acoustic liner were also tested for comparison. The tonal and broadband fan noise reduction of the resin-based acoustic liner relative to the hardwall liner was evaluated as well as that of the production acoustic liner. According to the test results obtained in the enclosed testbed, it was found that the resin-based acoustic liner has good noise reduction performance as expected for typical acoustic liners. The maximum noise reduction is equivalent to the production acoustic liner for both tonal and broadband components over the wide range of engine power settings.

Perceptual dimensions of fan sounds with different tonal characteristics

Fan noise is a common tonal noise source in people's everyday life. While tonal sounds at high frequencies often contribute to annoyance, the perception and evaluation of low-frequency tones is not fully understood. In the present study, the perceptual dimensions of 24 tonal prototype fan sounds were explored using a semantic differential consisting of 21 adjective scales that were rated by 40 participants. The synthesized sounds had the same sound pressure level of 55 dB(A) but differed in fundamental frequency and spectral envelope of the embedded tonal signatures Using a factor analysis, four perceptual dimensions were found that explain 59 % of the variance. Fan sounds with a fundamental frequency of 60 Hz were rated to be more "humming" (1st dimension) and more "pleasant" (2nd dimension) but less "buzzing" and less "beeping" (3rd dimension) in comparison to sounds with higher fundamental frequencies of 180 Hz and 300 Hz. An analysis of the relationships between the adjective scales and psychoacoustic metrics show that the "humming" ratings were highly correlated to a metric describing the relative contribution of low frequencies to the overall loudness. In contrast, psychoacoustic sharpness and tonality had smaller correlation coefficients with the "humming" ratings.

Development of higher back-pressure application of cooling fan test plenum per ISO 10302-1

International standard, ISO 10302-1 for measuring the airborne noise of individual fans (air-moving devices) for cooling information technology and telecommunications equipment, is being revised. While the existing test specification limits the maximum back pressure applied to the fan under noise measurement to 750 Pa, this revision attempts to cover up to 3000 Pa depending on the size of the test plenum. This paper describes the background to the specification changes and various technical studies.

A framework to simulate and to auralize the sound emitted by aircraft engines

In civil aviation, the well-established Effective Perceived Noise Level (EPNL) has remained, so far, a simple but reliable psycho-acoustic metric, representing the backbone of noise regulation politics and leading to quieter aircraft turbofan engines. However, the recent changes induced by further acoustic optimization of engines, or the introduction of innovative designs, suggest that a more refined assessment than the sole EPNL metric is needed, which is based on sound auralization. The paper presents a framework of simulation tools that are currently being developed at the DLR Engine Acoustics Department with the goal of assessing novel designs at a very early development stage. This framework allows to simulate acoustically an engine flyover and to produce audible signals as perceived by a virtual observer standing on the ground. The source calculation is performed by the tool PropNoise and relies on analytical physics-based models for the fan noise sources, and Stone's semi-empirical jet noise model. The subsequent tool VIOLIN simulates the propagation of sound through the atmosphere and the ground reflection effects. Finally, the spectrograms of the broadband and tonal noise components are converted to audible sound by the auralization tool CORAL. The procedure is applied to a realistic engine test case and compared with experimentally recorded sound.

Multi-channel active noise reduction system realizes the local area sound field control of the subway driver's cabin

Aerodynamic noise, air conditioning fan noise, and wheel-rail noise during subway operation have an adverse effect on drivers' physical and emotional well-being and can jeopardize the safe operation of vehicles. On the basis of the original passive acoustic treatment, multi-channel active control technology is used to manage the local sound field in order to further enhance the acoustic comfort of subway drivers' working environment. The multi-channel active noise reduction system installed on the driver's seat is described in this paper. In order to properly prevent the issues of ear pain and obstruction brought on by prolonged use of noise-canceling headphones, the system integrates reference sensors, error sensors, speakers, and a power amplifier within the headrest. The noise in the carriage is picked up using a multi-channel reference signal, and a low delay and anti-interference algorithm is investigated in accordance with the wide frequency band's characteristics and the abrupt change in sound pressure level when the vehicle is running. The test results demonstrate the system's ability to stabilize under situations of abrupt changes in driving noise and in-car broadcast, as well as to successfully minimize noise in the head area of subway drivers.

Outdoor engine test using acoustic liners combined with Fine-Perforated-Film

Resonant-type liner panels are one the primary countermeasures for the fan noise of aircraft engines, though the noise reduction performances of the liners are known to be tolerated by the flow streaming on the liner surface. Therefore, suppressing the effect of the flow on the liners is greatly important for larger noise reduction. In this study, a liner panel with a special surface structure was manufactured and applied to an outdoor test using a small turbofan engine, DGEN 380. The surface structure consisted of a special thin film, Fine-Perforated-Film (FPF), and a gap. The gap was fixed between the FPF and liner surface. In addition, a typical resonant-type liner and a hard wall were used for comparison, to investigate the effect of the structure. During the test, the samples were installed to the exhaust bypass duct of the DGEN 380 engine. The acoustic pressure was measured with a far field microphone array. Analysis and comparison of the results showed that the new structure suppressed the effect of the grazing flow and caused a larger amount of noise reduction, compared to the typical liner sample.

Reducing the aerodynamic noise of the axial flow fan with perforated surface

In this study, the effect of utilizing a perforated surface as an innovative flow control method to reduce fan noise is numerically investigated. The case study consists of a shrouded axial flow fan with a perforated surface placed on its shroud behind the trailing edge. Using the LES model and FW-H analogy, the fan noise is determined in the frequency domain at a nominated location by performing computational aero-acoustic (CAA) simulations. In order to evaluate the penalty imposed on the aerodynamic performance by this control method, an aerodynamic simulation is performed using RANS equations and the k-ω SST turbulence model. The mentioned numerical models are experimentally validated. The results show that the coherent structures of the flow are disrupted, and the fan noise is reduced in the trailing edge area - an extremely noise-prone zone - thanks to the passive control method. The noise reduction values could potentially reach 6 dB specifically at the critical blade pass frequency, which significantly contributes to the overall noise produced by the fan. Moreover, it is shown that there are strong relationships between the Proudman, turbulence kinetic energy, and entropy indexes with the total aeroacoustic performance of the fan. In contradistinction to other applied methodologies, this control method does not impose any aerodynamic performance penalty, though it could be improved by around 1.5 % at the operating point.

Investigation on accuracy of numerical simulation of aerodynamic noise of single-stage axial fan

The prediction accuracy of turbomachinery aerodynamic noise, particularly in relation to broadband noise with uncertain factors, has long been a challenging issue. Previous studies have not fully comprehended the factors influencing its prediction accuracy, lacking an objective and comprehensive evaluation method. An improved approach combining orthogonal experiment design and principal component analysis is employed to address these limitations. The evaluation method expands the noise metrics and provides a comprehensive assessment of the accuracy of numerical simulation for aerodynamic noise. The evaluation method is utilized to optimize and quantitatively analyze the impact of the refinement size of the core area on noise prediction for single-stage axial fans. Subsequently, the three metrics, namely, Z1, Z2, and broadband noise Z3, are integrated using PCA to form a new integrated optimal metric Ztotal. The influence of different refinement sizes, particularly on Ztotal, is quantitatively examined. The findings reveal that the mesh size of the stator wake (D area) exhibits the most significant influence on noise prediction accuracy, with a calculated weight of 81.3% on noise accuracy. Furthermore, a comprehensive investigation is conducted on the influence of turbulence models and the wall Y+ value on aerodynamic noise. Detached-eddy simulation and large eddy simulation demonstrate effective capabilities in simulating both upstream and downstream turbulent flow characteristics of the stator, enabling accurate prediction of broadband noise. This study presents a set of numerical simulation schemes that achieve precise prediction of turbomachinery aerodynamic noise.

An Empirical noise model of centrifugal fans with different volute tongues based on Langevin regression

In the context of the rapidly evolving field of building ventilation, fan noise control stands out as a pivotal technology to enhance the quality of life. This study aimed to elucidate the inherent randomness, distinct characteristics, and potential low-dimensional noise structure essential for developing effective noise control methods. In this study, a stochastic dynamics model for the noise of centrifugal fans with various volute tongue structures is constructed using the Langevin regression method and experimental measurements. An empirical noise model capturing the peak characteristics of noise discrete tonal is developed through parametric mode expansion and an empirical mean-field model of turbulent wakes, facilitating accurate noise predictions. The research findings demonstrate the effectiveness of the Langevin equation employing the cubic Stuart-Landau equation and multiplicative noise to predict fan noise across different volute tongue structures. While stochastic dynamics models unavoidably lose the characteristic discrete noise peaks, predictions of the total A-weighted sound pressure level remain within a 2.2% error margin. The stochastic dynamics models applied to centrifugal fan aerodynamic noise and turbulent wakes exhibit striking resemblances, suggesting a fundamental connection between turbulence and noise generation. The extended empirical noise model maintains discrete pitch noise peaks by introducing shift modes, indicating that stronger Langevin deformations correspond to larger translational amplitudes of the shift modes. The advancement of stochastic dynamics and empirical noise models lays a foundational framework for the formulation of effective noise control strategies.

Duct modal identification considering statistical dependency via the Boltzmann machine

The fan noise is the primary component of aero-engine noise. Duct acoustic modal identification is essential for analyzing the mechanism of fan noise generation and propagation. The acoustic modal in the duct can be identified by the measurement of the microphone array mounted on the duct wall. It is generally assumed in the conventional modal identification methods that the modal coefficients are independent of each other. However, the realistic duct acoustic modal coefficients exhibit significant statistical dependence. A Boltzmann machine(BM) model considering the statistical dependence between modal coefficients is proposed for recovering the modal coefficients in this paper. The modal coefficients are estimated using a block coordinate optimization method in a Bayesian framework. Moreover, the modal Boltzmann parameters can be learned iteratively by the BM. The effectiveness of the proposed method is verified by numerical simulations and experimental tests. It turns out that the modal coefficients can be accurately identified by the proposed method, and a more significant sparsity is shown by the results. In addition, the statistical dependence between the modal coefficients can be captured by the BM.