Noise Control Fan Research Articles

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.

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.

Ducted fan noise control by turbulence grids for drones

A small ducted fan with a rotor-stator assembly is being tested in an anechoic chamber at Peking University. To attenuate fan noise, turbulence grids have been inserted before the rotor to eliminate large-scale ingested flow structures and, therefore, to minimize the flow interactions between the rotors and stators. Several different grids with various geometry and solidity configurations have been considered in the tests. To demonstrate the proposed noise control method, both near-field in-duct on-surface microphones and far-field microphones are used to measure acoustic performance. In addition, a microphone array is deployed to acoustic images at several working conditions. The corresponding thrust performance, with and without the grids in between the rotors and stators, is being measured simultaneously. The possible fluid mechanics is presumably caused by the reduced strength of ingested turbulence structures. More results help to reveal this physical understanding will be given in the final manuscript. Through this work, we hope to investigate a new strategy that could possibly reduce UAV fan noise at several representative working conditions.

Evaluation of metal foam liners for broadband noise reduction of turbofan engines

Previous research shows that open-celled metal foam liners have the potential to provide substantial aircraft noise reduction benefits. Our recent results show that their normal incidence sound absorption performance can be further improved by compressing them uniformly; this compression reduces their effective pore size and increases the thermoviscous losses. Further, their performance may be optimized by stacking foams with varying degrees of compression and creating a step-wise effective relative density gradient. Here, we present the results from our recent experiments, evaluating the attenuation performance of such a step-wise gradient metal foam liner using the Advanced Noise Control Fan (ANCF). Liner spools with two different step-wise configurations were fabricated and tested using the ANCF test rig at the Notre Dame Turbomachinery Laboratory. Farfield data was gathered with the liner spool mounted at the inlet and aft locations. In this paper, we present our results comparing the attenuation performance under both test conditions as compared to measurements obtained using a hard wall baseline configuration. Our results show that step-wise metal foam configurations can provide robust acoustical performance for aircraft noise reduction applications.

Application of immersed boundary method in turbomachines

Simulating unsteady turbulent flow in turbomachines is still challenging due to the complexity of blade geometry and relative motion between rotor and stator. This study presents an Immersed Boundary Method (IBM) for high-Reynolds turbomachinery internal flows, and shows the advantage of the automatic grid generation techniques and flexible moving boundary treatments. The wall functions are used in the present method to alleviate the wall resolution restriction of turbulence simulation. The Two-Dimensional (2-D) IBM solver, which was previously developed and tested for a low-speed compressor, is further validated for a well-documented Low-Pressure Turbine (LPT) cascade. Both the blade loading and the total pressure losses in the wake are well captured by the present 2-D solver. The complex Three-Dimensional (3-D) effects in turbomachines motivate the further development of an extended 3-D IBM solver by using a curvilinear-coordinate system that facilitates the hub and casing boundary treatment. The good performance of the 3-D solver is demonstrated through comparison with CFX solver solutions for the rotor configuration of Advanced Noise Control Fan (ANCF). Further effects of the grid resolution on capturing the blade wake are discussed. The results indicate that the present 3-D solver is capable of reproducing the evolution of the blade wake with suitable computational grid.

Enhanced micro-perforated wall-treatments for reducing the boundary layer noise in a low-speed wind-tunnel: modelling, characterization and optimization studies.

Micro-perforated aero-acoustic liners are robust non-fibrous wall-treatments able to achieve significant axial attenuation in low-speed ducted flows, if their input impedance is suitably optimized. They are also potential candidates for broadband reduction of the flow-induced noise, useful to increase the signal-to-noise ratio in wind-tunnels test section or for fan noise control when inserted in casing treatments. The current theoretical and experimental study determines under which range of frequencies, holes diameter and free stream flow velocity, micro-perforated partitions are able to efficiently reduce the boundary layer noise, either by absorption or transmission mechanisms, depending on the spectral content of the aero-acoustic excitation. The formulated analytical models, either modal- or wavenumber-based, are validated against wind-tunnel measurements of the overall power injected by a low-speed boundary layer into a micro-perforated partition and its aerodynamic transmission loss. A hole-based Strouhal number is found below which little back-scattering of the wall-pressure components occurs. Optimization studies are performed to determine the range of parameters that enhance the total absorption or transmission of boundary layer noise, with special emphasis on the low frequency content of the wall-pressures.

Numerical Study on the Duct Noise Control of Single-Stage Axial Flow Fan with Wave Leading Edge Stator Blade Configurations

Numerical Study on the Duct Noise Control of Single-Stage Axial Flow Fan with Wave Leading Edge Stator Blade Configurations

Low-Speed Turbofan Aerodynamic and Acoustic Prediction with an Isothermal Lattice Boltzmann Method

The objective of this paper is to assess the capability of the isothermal lattice Boltzmann method (LBM) to correctly capture aerodynamic and acoustic features from a low-speed turbofan. The evaluation is done on the Advanced Noise Control Fan model developed by NASA Glenn Research Center. An extensive comparison of the measured and computed aerodynamic results is presented for the first time on this configuration. The trends are well captured, but quantitative discrepancies are observed for some variables, leading to lower fan performance values in the simulations. This last point was also observed in the previous related studies, but remained unexplained. By validating in parallel the LBM aerodynamic results with a Reynolds-averaged Navier–Stokes simulation provided by the database, a likely uncertainty of the absolute values of some variables in the experimental mean flow data is shown (a result of the intent that experimental data were not acquired with absolute levels as an objective). Direct in-duct and far-field acoustic results are also investigated, taking advantage of the low-dissipative characteristics of the LBM. A good agreement with the measurements in terms of broadband noise is obtained, but the low frequencies tend to be overestimated. For the tonal noise, if the expected acoustic modes are recovered, quantitative differences are observed. Predictions based on a hybrid method, where the acoustic sources computed by the LBM are propagated analytically using Goldstein’s analogy (Aeroacoustics, McGraw–Hill, New York, 1976), are also made and compare well with the direct predictions, thus proving the correct propagation of acoustic waves by the solver.

Liner development at low technology readiness level utilizing the advanced noise control fan

The Advanced Noise Control Fan (ANCF) was utilized in the design, test, and evaluation for technical risk mitigation for many of the innovative ducted acoustic liner technologies developed by NASA over the past 20 years. The ANCF is a low-speed ducted fan test bed for measuring and understanding fan-generated aeroacoustics, duct propagation, and radiation to the farfield. It is considered a low Technology Readiness Level testbed. The international aeroacoustics research community employed the ANCF to facilitate advancement of liner concepts, liner measurement technologies, and for liner code validation. From 1994 to 2016, it was located in the NASA Glenn Research Center’s Applied Aero Propulsion Laboratory. In 2016 the ANCF was transferred to the University of Notre Dame where it is expected to continue to positively impact research in the liner area. This paper summarizes the capabilities and contributions of the ANCF to the field of liner development by documenting its history.

Numerical simulation on near-field aerodynamic noise of an adjustable-blade axial-flow fan

A numerical simulation method based on the Ffowcs-Williams and Hawkings model is employed to predict the mechanisms of the near-field aerodynamic noise distribution characteristics of an adjustable-blade axial-flow fan with different installation angles of moving blades (Δβ). The simulated results reveal that with Δβ changing from −12° to 12°, the changing curves of the maximum total sound pressure level (MTSPL) at the tip clearance region (A region), the leading edge region (B region), and the trailing edge region (C region) exhibit an apparently rising trend, which increase by 4.0 dB, 5.7 dB, and 4.3 dB, respectively. Besides, the MTSPL at the C region is always smaller than that at A and B regions within the studied installation angles. Additionally, the acoustic energy ratio (Cpi) is the ratio of the sound energy density of a certain frequency to the total sound energy density, which shows the various frequency distribution characteristics under studied angles. It is found that when Δβ deflects from −12° to 0°, Cp1 (the acoustic energy ratio at the low-frequency in the range of 20–500) decreases from 0.71 to 0.59, Cp2 (the acoustic energy ratio at the intermediate-frequency in the range of 500–2000) increases from 0.18 to 0.25, and Cp3 (the acoustic energy ratio at the high-frequency in the range of 2000–3000) rises from 0.1 to 0.16. This study derives the aerodynamic distribution characteristics of the TSPL and acoustic energy in the near field of moving blades, which reveals its changing rules and frequency distribution under various installation angles. The conclusions may provide guidance for the research regarding the technology of the noise control of the adjustable-blade axial-flow fan.

Design Study of Fan Gas-Dynamic and Aeroacoustic Characteristics

The NUMECA FINE/Turbo package was used to simulate gas-dynamic flow in a model Advanced Noise Control Fan; the calculation results were compared with experimental data. Fan tonal noise was simulated for two geometry options (14 and 13 outlet guide vanes) in FINE/Acoustics, and the calculated and experimental parameters were compared.

Fan Noise Control and Heat Dissipation Optimization of Commercial Vehicle Cooling System

Aiming at the problem of matching fan noise and heat dissipation in a commercial vehicle cooling system, it is proposed to reduce noise by changing the fan structure and to increase the windshield to block the hot air reflux. By changing the number of blades, wheel hub ratio, blade bending angle fan performance parameters, and the cooling fan mass flow rate and aerodynamic noise were simulated by CFD, the fan performance parameters were optimized. Then, the flow field and temperature field of engine cabin are analyzed. According to the analysis, the hot air reflux exists in the radiator, and the baffle is added to the upper part of the radiator and to the left and right sides of the radiator to block the reflux. The results show that the fan noise is reduced by 2.7 dB and the maximum temperature in the engine cabin is reduced from 417k to 392k.

Noise predictions of the advanced noise control fan model using lattice Boltzmann method and Ffowcs Williams–Hawkings analogy

The lattice Boltzmann method (LBM) is applied to simulate the complete three-dimensional transient flow and noise generated on the Advanced Noise Control Fan (ANCF) geometry. The ANCF is a model developed by NASA Glenn composed by a rotor–stator system enclosed by a duct. The experimental noise spectra provided by NASA at 30 microphones around the fan are used to validate the results obtained with the LBM. Overall, the predicted sound spectra pattern and the main tonal frequencies agree well with the noise measurements; in terms of tonal noise levels, the agreement varies depending on the frequency and microphone position. The noise at the 30 microphone positions is also computed by employing the porous Ffowcs Williams and Hawkings (FW–H) formulation with noise sources located on two integral surfaces placed at the ANCF openings. The good agreement obtained between the sound levels calculated with the FW–H formulation and those computed by the LBM show that the FW–H analogy may be applied in place of the LBM numerical simulation. In addition, the FW–H formulation is applied to compute the sound generation with sources located on different physical surfaces of the ANCF model such as the rotor, stator, hub, and nacelle, procedure known as noise source breakdown. A comparison of the FW–H results applied on different surfaces of the ANCF model with the noise computed by the LBM shows that most of the tonal noise levels observed at the far-field microphones (predicted by the LBM simulation) correspond to the noise sources generated on the stator vane surfaces. Such a study of the noise sources breakdown also evidences the interaction effect between the rotor and the stator, whose mechanism is the main contributor to the tonal noise generation content.

Acoustic analysis of two small axial-flow fans in series

The noise signatures of two identical small axial-flow cooling fans installed in series with a flow straightener between them are experimentally measured and analyzed. A technique of noise source component decomposition is used to decompose the raw sound signals into tonal and broadband components, both further attributed to either the upstream or the downstream fan. Two major inlet flow conditions (unobstructed inlet and a distorted inlet flow case by the simplified circuit board) and a range of operating conditions under which the tested fan works are investigated. The tonal and broadband noises radiated by the upstream fan are greatly increased by inlet flow distortion, especially in the low workload region, while the two noise components radiated by downstream fan are hardly affected. The upstream fan and the flow straightener flush mounted between the two fans both play a significant role in keeping the downstream fan free from the influence of inlet flow distortion. Themagnitude relations among all the noise source components under different inlet flow and operating conditions are compared, which would definitely offermuch help in designing a quiet two-stage or even multiple-stage fan and adopting an appropriate method to efficient fan noise control and reduction.

Effect of Weak Outlet-Guide-Vane Heterogeneity on Rotor–Stator Tonal Noise

Four numerical simulations of the NASA Active Noise Control Fan rig with some modifications have been performed to investigate the influence of the heterogeneity of the stator row on the tonal noise radiation for a realistic turbofan with a high hub-to-tip ratio. These simulations achieved with the Powerflow solver based on the lattice Boltzmann method provide a direct acoustic prediction for both tonal and broadband noise. The numerical simulations are used to evaluate the noise contributions of the rotor-wake interaction with the stator vanes, and the rotor interaction with both the inlet distortion and the potential field generated by the stator row. Analytical models are evaluated on this configuration using the flow description provided by the simulations. Rotor-wake and inlet-distortion interaction noise generation with in-duct propagation uses the classical Amiet’s response, whereas Parry’s model is used for the rotor response to the potential field. The simulation is used to evaluate simple excitation models for the potential field and the rotor-wake evolutions. The analytical results for homogeneous and heterogeneous configurations compare well with the detailed acoustic modal powers extracted from the direct acoustic field simulated with Powerflow. The wake interaction remains the dominant source in the present heterogeneous configurations.

Sound quality control of axial fan noise by using microperforated panel housing with a hollow tube

This study presents a novel passive noise control approach to directly suppress sound radiation from an axial-flow fan, which involves micro-perforated panels (MPP) backed by cavities and a hollow tube. Apart from the sound suppression performance in terms of insertion loss, sound quality of axial fan with a dipole nature is also investigated which serves as a significant supplementary index for assessing the noise control device. The noise suppression is achieved by the sound cancelation between sound fields from the fan of dipole nature and sound radiation from a vibrating panel via vibro-acoustic coupling and interference from the hollow tube boundaries, as well as by sound absorption in micro-perforations. A two-dimensional theoretical model, capable of dealing with strong coupling among the vibrating micro-perforated panel, sound radiation from the dipole source, and sound fields inside the cavity and the duct is developed. The theoretical results are validated by both finite element simulation and experiment. Results show that an addition of hollow tube enhances the sound suppression performance in the passband region of the MPP housing device. The findings of the current research have the potential to control ducted-fan noise effectively, to enhance the quality of products with a ducted-fan system.

Prediction of tonal ducted fan noise

This study introduces an analytical model aiming at predicting the tonal acoustic sources generated and radiated by the rotor–stator interaction in a fan stage. This model is able to cope with complex three-dimensional stator geometries and it fully accounts for cascade effects, characteristic of modern fan stages. It is based on a proper description of the rotor wake coupled with an analytic cascade response function and with an acoustic analogy. The proposed model is validated for the first time against acoustic sources and sound power measurements, on the Advanced Noise Control Fan from NASA. On this configuration representative of an actual fan stage, the model is shown to predict tonal sources and powers accurately, in function of the rotational velocity and of the stator-vane count. Another realistic configuration, namely the low-pressure CME2 research compressor, is then considered in order to demonstrate the suitability of the model to be used as a design tool in an industrial context. A parametric study concerning both the stator vane sweep and lean angles is performed on this rotor–stator stage. The model produces predictions consistent with studies from literature, quantifying the effectiveness of swept and leaned vanes as a tonal noise reduction mechanism. This parametric study allows defining an optimal stator design for minimal noise emission.

Passive control of centrifugal fan noise by employing open-cell metal foam

Experimental and numerical studies were conducted to investigate passive control of a forward-curved centrifugal fan noise by employing the open-cell metal foam (OCMF). The results indicated that a reduction of about 5dB in overall A-weighted sound pressure level (OASPL) was successfully achieved with a decline of about 200Pa in total pressure rise (TPR). For the OCMF investigated in the present paper with the pores per inch (PPI) of 20 and the porosity of 97%, the noise control mechanism was to suppress the source strength of tonal noise generated from the periodic impeller–tongue interaction rather than to absorb the broadband acoustic energy. The decline of the aerodynamic performance was mainly caused by reducing the work capacity of the impeller rather than caused by flow loss owing to the flow resistance of the OCMF. An improved design of the OCMF was suggested to achieve more reduction in noise level with less decline of aerodynamic performance.

Reactive control of subsonic axial fan noise in a duct.

Suppressing the ducted fan noise at low frequencies without varying the flow capacity is still a technical challenge. This study examines a conceived device consisting of two tensioned membranes backed with cavities housing the axial fan for suppression of the sound radiation from the axial fan directly. The noise suppression is achieved by destructive interference between the sound fields from the axial fan of a dipole nature and sound radiation from the membrane via vibroacoustics coupling. A two-dimensional model with the flow effect is presented which allows the performance of the device to be explored analytically. The air flow influences the symmetrical behavior and excites the odd in vacuo mode response of the membrane due to kinematic coupling. Such an asymmetrical effect can be compromised with off-center alignment of the axial fan. Tension plays an important role to sustain the performance to revoke the deformation of the membrane during the axial fan operation. With the design of four appropriately tensioned membranes covered by a cylindrical cavity, the first and second blade passage frequencies of the axial fan can be reduced by at least 20 dB. The satisfactory agreement between experiment and theory demonstrates that its feasibility is practical.