Fan Noise Generation Research Articles

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.

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.

Flow field analysis and noise characteristics of an automotive cooling fan at different speeds

As the core component of the automotive air conditioning cooling system, the automotive cooling fan directly affects the cooling performance of the entire system. Therefore, it is necessary to study the flow field and noise of the automotive cooling fan. This study focuses on the analysis off low field characteristics and noise generation in an automotive air conditioning systems, specifically related to an automobile cooling fan at different speeds. The flow field properties of the fan are examined through the distribution off low line trajectory, pressure distribution on the blade surface, and velocity distribution characteristics at various speeds. The analysis reveals that the maximum air velocity occurs at the point where the fan blade meets the wind guard, while the maximum pressure is observed at this point and at the edge where the fan blade cuts through the cavity. Furthermore, a monitoring point located 1 m in front of the fan is selected to study the average value of fan pneumatic noise pulsation and generate a noise cloud map, aiming to identify the main noise source of the fan. To better understand the noise distribution and variation at different speeds, various noise metrics are utilized, including noise Sound Pressure Level (SPL) spectrum, 1/3-octave SPL spectrum, and 1/3-octave A-weighted SPL spectrum. The noise characterization results indicate that the primary areas of fan noise generation are located at the tip of the leaf margin of the forward swept wing. This research provides valuable insights for the improvement and optimization of the cooling fan design, as well as subsequent noise reduction efforts.

Experimental investigations on the performance and noise characteristics of a forward-curved fan with the stepped tongue

This work presents an experimental study to investigate the influence of step volute tongue on aerodynamic performance and aeroacoustic behavior of a forward-curved fan. The noise characteristics are analyzed and controlled based on measured acoustic pressures for various stepped tongues; meanwhile, fan performance is detected and optimized. The design parameters of the stepped tongues are presented to provide significant physical insight into increasing the static pressure as well as the efficiency of static pressure and reducing the fan noise generation. The comparison of the test results indicates that the improved static pressure and its efficiency of the HLHL model have increased by 15.67 Pa and 3.57%, respectively, by comparing with those of the baseline model. At 740 m3/h, a great correlation between different arrays of stepped tongue and the noise generation was observed. The tonal noise level of the HLHL model is a better optimization scheme because it decreases as much as 1.2 dB for the noise generation of forward-curved fan. In particular, it is found that some stepped tongues of the volute tongue achieved the goal of reducing noise generation and improving the performance of fan by experimental measurement at the same time.

Hybrid numerical model for acoustic propagation through sheared flows

Simulating the propagation of sound in non-uniform flows remains challenging, especially for large, three-dimensional problems. To account for the sound refraction due to gradients of velocity and temperature, one has to solve the Linearised Euler Equations (LEE) which can be computationally expensive in three dimensions. Alternatively, one can use the Linearised Potential Equation (LPE) which is much cheaper but is limited to potential, isentropic flows. In this paper, a hybrid model combining the LEE and the LPE is proposed in order to simulate the sound propagation in sheared flows at a reasonable computational cost. The LEE are applied only in regions with strong sheared mean flows, the LPE is used everywhere else. The coupling between the LEE and the LPE consists in imposing relations for the characteristic waves propagating at the interface between the LEE and the LPE regions. In this study, the hybrid model is implemented in a high-order finite element solver in the frequency domain. Its performance is first assessed by simulating the propagation of planes waves in a uniform mean flow and the acoustic radiation from a semi-infinite duct in a strongly sheared non-isothermal jet flow. No significant spurious noise is produced at the LEE-LPE interfaces. The applicability of the hybrid model to industrial problems is then demonstrated by simulating the propagation of fan noise through the jet flow exiting from a turbofan exhaust. The use of the proposed hybrid model does not affect the propagation of the sound field while reducing the memory footprint by more than one order of magnitude compared to a full three-dimensional LEE simulation.

Large-scale unsteady flow structures in the leakage flow of a low-speed axial fan with rotating shroud

The leakage flow in low-speed fans provided with rotating shroud has been surveyed by means of two-dimensional LDV and PIV. Measurements have been taken upstream of the rotor and in the gap area at different operating conditions. The flow contains both periodic and non-periodic large-scale flow structures, which are responsible for the noise generation. The former structures have been extracted by means of the ensemble average technique, and, in order to study the latter ones, the non-periodic part of the instantaneous velocity field has been analysed by means of the proper orthogonal decomposition technique.In the present fan, the leakage flow consists in a jet which leaves the gap in the diagonal direction. Depending on rotational speed and blade loading, the jet may form a recirculation bubble anchored to the rotor ring or it may turn radially, forming a large recirculating flow zone.Large-scale, periodic vortical structures leaving the gap are convected by the leakage flow, either wrapping around the recirculation bubble or flowing radially outwards with a spiral pattern. In the former case, such structures, though weakened, likely reach the rotor, while, in the latter case, they have not been found at the rotor entrance. Hence, a different degree of mixing takes place which is consistent with the extension of the blade part where the leakage flow is reingested. This is consistent with the differences in acoustic measurements.The POD technique has shown that, at high loading, large-scale non-periodic vortical structures convect towards the rotor inlet. They are weaker though comparable to the periodic ones released from the gap; at low rotational speed they are stronger than at high rotational speed. Non-periodic flow structures may be present in the flow leaving the gap also, but it has not been possible to identify them clearly. Likely, non-periodic flow structures are originated by the ripping of the periodic ones.A possible explanation for the noise generation in fans with rotating shroud is that the periodic pressure field anchored to the rotor results in the release of periodic flow structures from the gap. Then, such structures may reach the rotor or rip during convection, likely resulting in non-periodic ones which are ingested by the rotor. In both cases, the interaction with the rotor blades likely results in noise generation. The degree of prerotation of the leakage flow is consistent with the frequency shift of the narrowband humps observed in the sound pressure spectra.

Расчет генерации шума вентилятора авиационного двигателя методами высокого порядка на графических процессорах

Расчет генерации шума вентилятора авиационного двигателя методами высокого порядка на графических процессорах

Analysis and Experiment of the Compressive Sensing Approach for Duct Mode Detection

This paper describes a new mode detection method based on the compressive sensing approach, which has been developed in information technology to reduce samples required by the classical Nyquist–Shannon sampling theory. A revised approach is proposed here to detect azimuthal spinning modes from aeroengine fan noise by using a microphone array outside the bypass duct. A number of numerical simulations is prepared to examine the associated test performance, such as the reconstruction accuracy, robustness, background noise interference, and coherence impact. The mode sparsity is usually much smaller than the highest order of the modes, so the azimuthal mode is sparse at the frequency of interest to ensure a satisfactory compressive sensing-based mode detection. The current simulations show log sensors shall be generally sufficient rather than sensors required by the Shannon–Nyquist sampling theory. An experimental system is designed and implemented to demonstrate the proposed method. The test system contains a straightened and rigid duct with an enclosed spinning mode synthesizer to emulate fan noise propagation inside and scattering from a bypass duct. An outer sensor array is employed to demonstrate that the compressive sensing-based mode detection method can significantly reduce the required number of sensors, especially for modes of high order, which results in a much simplified sensor array design for aerospace noise tests.

Noise prediction of axial fan duct using a lattice Boltzmann approach

Large axial fans in fire power plants are noise source of the plants, they are required to reduce the noise level. Fan noise propagates in a fan duct, and since it is emitted to the exterior, silencers are used as countermeasure in general. It is important for noise reduction to predict noise generation and noise propagation in fan ducts. The Lattice-Boltzmann Method (LBM) based approach was applied for the unsteady simulation of axial fan and its duct to predict fan noise generation and sound propagation simultaneously. The LBM has minute numerical viscosity, and is suitable for the analysis of the sound propagation and the flow field. In this research, aeroacoustic analysis by using applicable analysis model in the design phase has been conducted and validated compared to experimental results. The results shows good agreement with experiment within 3dB at overall value and predicted spectra are also compared with experimental results.

Towards the computation of aircraft engine fan noise generation with high order methods on GPUs

The present paper considers the computation of noise generation by aircraft engine fan with the in-house solver for graphic processing units called GHOST CFD. The solver is based on Dispersion Relation Preserving schemes which have high order of approximation and high resolution. An Optimized Low Dispersion and Dissipation Runge-Kutta scheme was utilized for the time integration. Large Eddy Simulation based on Relaxation Filtering was used for the turbulence modeling. The solver implements overset (“CHIMERA”) meshes which were used as rotor–stator interface treatment. The speedup gained from graphic processing units utilization was about 12–20 times compared to modern 8-core CPU, allowing to perform computations in a reasonable time period. The computations with GHOST CFD were performed in full annulus formulation with fan and outlet guide vane blades. The results were compared with the experimental data as well as the results of similar computations in the commercial ANSYS CFX solver some of which also included inlet guide vane blades.

An Image Based Mathematical Model for the Propagation of Fan Noise in a Plenum with Large Side Openings

This paper presents another application of an images group model for a special enclosure geometry and source orientation. A previous work outlined the concept via application to a special tight-fitting enclosure. Application of the concept to a fan plenum requires different mathematical descriptions for the image groups. This paper describes the sound reverberation inside a sound enclosure with mostly open sides where the primary noise sources are the air inlets and exhausts of axial type fans located at the top of the enclosure, the sound transmission through the air inlet openings, and the radiation to wayside positions. The main reverberation between the floor and ceiling is determined with an image based mathematical model. The model considers how the main reverberant part image group is amplified by its images from two parallel bulkheads and any side wall frame members. The method of images approach allows the hard surfaces of an untreated plenum to be represented by perfectly reflecting surfaces with zero sound absorption coefficients, thus not requiring any estimate or measurement for these surfaces. Numerical results show excellent comparison to experimental results for an actual plenum. The image model is also shown to be significantly more accurate than the standard large room diffuse field reverberant model.

Random particle methods applied to broadband fan interaction noise

Predicting broadband fan noise is key to reduce noise emissions from aircraft and wind turbines. Complete CFD simulations of broadband fan noise generation remain too expensive to be used routinely for engineering design. A more efficient approach consists in synthesizing a turbulent velocity field that captures the main features of the exact solution. This synthetic turbulence is then used in a noise source model. This paper concentrates on predicting broadband fan noise interaction (also called leading edge noise) and demonstrates that a random particle mesh method (RPM) is well suited for simulating this source mechanism. The linearized Euler equations are used to describe sound generation and propagation. In this work, the definition of the filter kernel is generalized to include non-Gaussian filters that can directly follow more realistic energy spectra such as the ones developed by Liepmann and von Kármán. The velocity correlation and energy spectrum of the turbulence are found to be well captured by the RPM. The acoustic predictions are successfully validated against Amiet’s analytical solution for a flat plate in a turbulent stream. A standard Langevin equation is used to model temporal decorrelation, but the presence of numerical issues leads to the introduction and validation of a second-order Langevin model.

Application of frequency-domain linearized Euler solutions to the prediction of aft fan tones and comparison with experimental measurements on model scale turbofan exhaust nozzles

Although it is widely accepted that aircraft noise needs to be further reduced, there is an equally important, on-going requirement to accurately predict the strengths of all the different aircraft noise sources, not only to ensure that a new aircraft is certifiable and can meet the ever more stringent local airport noise rules but also to prioritize and apply appropriate noise source reduction technologies at the design stage. As the bypass ratio of aircraft engines is increased – in order to reduce fuel consumption, emissions and jet mixing noise – the fan noise that radiates from the bypass exhaust nozzle is becoming one of the loudest engine sources, despite the large areas of acoustically absorptive treatment in the bypass duct. This paper addresses this ‘aft fan’ noise source, in particular the prediction of the propagation of fan noise through the bypass exhaust nozzle/jet exhaust flow and radiation out to the far-field observer. The proposed prediction method is equally applicable to fan tone and fan broadband noise (and also turbine and core noise) but here the method is validated with measured test data using simulated fan tones. The measured data had been previously acquired on two model scale turbofan engine exhausts with bypass and heated core flows typical of those found in a modern high bypass engine, but under static conditions (i.e. no flight simulation). The prediction method is based on frequency-domain solutions of the linearized Euler equations in conjunction with perfectly matched layer equations at the inlet and far-field boundaries using high-order finite differences. The discrete system of equations is inverted by the parallel sparse solver MUMPS. Far-field predictions are carried out by integrating Kirchhoff's formula in frequency domain. In addition to the acoustic modes excited and radiated, some non-acoustic waves within the cold stream-ambient shear layer are also captured by the computations at some flow and excitation frequencies. By extracting phase speed information from the near-field pressure solution, these non-acoustic waves are shown to be convective Kelvin–Helmholtz instability waves. Strouhal numbers computed along the shear layer, based on the local momentum thickness also confirm this in accordance with Michalke's instability criterion for incompressible round jets with a similar shear layer profile. Comparisons of the computed far-field results with the measured acoustic data reveal that, in general, the solver predicts the peak sound levels well when the farfield is dominated by the in-duct target mode (the target mode being the one specified to the in-duct mode generator). Calculations also show that the agreement can be considerably improved when the non-target modes are also included, despite their low in-duct levels. This is due to the fact that each duct mode has its own distinct directionality and a non-target low level mode may become dominant at angles where the higher-level target mode is directionally weak. The overall agreement between the computations and experiment strongly suggests that, at least for the range of mean flows and acoustic conditions considered, the physical aeroacoustic radiation processes are fully captured through the frequency-domain solutions to the linearized Euler equations and hence this could form the basis of a reliable aircraft noise prediction method.

Benchmark of fan noise propagation tools

Various propagating codes based on different formulations have been developed over the last years to simulate the fan noise propagation into the nacelle of a turbofan engine and its radiation in the Far field. The abilities of the ACTRAN softwares (Potential formulation / FEM), SPACE (Linearised EULER equations / DGM) and ICARE (Rays-tracing method) to simulate typical industrial configurations in the frequential domain have been evaluated based on measurements carried out at the RACE acoustic test facility on a fan model at 1/2 scale. The results of the computations carried out with each of these codes on intake and exhaust configurations (axisymmetric assumption) in hard wall and lined duct will be compared to the measured far field directivities. These comparisons will be discussed depending on the establishment of patterns (sources types, mean flows types, boundaries conditions) considered in the simulations of each of these codes. The impacts of the considered convected mean flow types (Uniform, Euler, NS,...) and the capacities of these codes to predict noise attenuation of the treatments will be investigated in particular.

Reduction of the aerodynamic tonal noise of a forward-curved centrifugal fan by modification of the volute tongue geometry

In this work, an experimental study about the influence of some geometric features on the aeroacoustic behaviour of a squirrel-cage fan, used in automotive air conditioning units, has been carried out. The study focused on the effect of both the shape and the position of the volute tongue on the noise generated by the fan. Different geometric configurations were tested in order to compare the results. First of all, the performance curves were measured in a standardized test facility. Then, the acoustic behaviour of the fan was characterized by means of acoustic pressure measurements near the fan inlet. The comparison of the test results indicated a great dependence of both the shape and the position of the volute tongue and the noise generation. In particular, some geometric configurations of the volute tongue were able to reduce the fan noise generation without reducing the fan operating range.

Experimental Study on the Aeroacoustic Behavior of a Forward-Curved Blades Centrifugal Fan

In this paper, an aeroacoustic study on a forward-curved blades centrifugal fan has been carried out. As a first step, the fan performance curves, i.e., total pressure, power, efficiency and sound power level versus flow rate were obtained, showing its unstable behavior over a wide operating range. Second, the fan sound power level spectra for several working conditions were determined. For this purpose a normalized installation for testing in laboratory was designed and constructed. Afterwards, the velocity and pressure fields, both at the inlet and outlet planes of the impeller were measured using hot wire probes and pressure transducers, for different operating conditions. Finally, the aeroacoustic behavior of the fan was determined measuring the vorticity field at the impeller outlet, which is known to be related to tonal noise generation. This relation is worked out using the theory of vortex sound, developed by several authors during the second half of this century. The paper shows that the generation of tonal noise is produced at the blade passing frequency and it increases with the flow rate. Although the main contribution to fan noise generation is due to mechanical sources, the bands in which aerodynamic noise is generated by these fans correspond to frequencies especially unpleasant to the human ear. Therefore, the research presented in this paper may be of considerable interest, establishing a starting point for the design of quieter and more efficient fans.

Basic fan noise generation mechanisms and innovative aerodynamic design techniques for their reduction

For low‐speed fans, noise radiation results from fluctuating aerodynamic forces on the blades. These may be characterized by their origin as due to vortex shedding or inflow turbulence ingestion. Both of these force and noise mechanisms are nominally broadband in frequency. However, a specialization of the latter considering a standing pattern of inflow velocity distribution yields the familiar discrete frequency spectrum of harmonics of the blade passage rate. Beneficial changes in noise levels and spectral distributions are described and demonstrated for changes in fan blade form such as thickness distribution and sweep. Blade edge sweep is shown to be particularly useful in reducing noise from both aerodynamic phenomena and, with careful design, effective in reducing discrete frequency noise as well. Examples are presented of automotive type fans designed for low noise using these techniques. Not only is noise reduced, but because of the sophisticated aerodynamic considerations required on loading and load distributions, the efficiency is markedly improved.

Propagation of Supersonic Fan Noise in a Nonuniform Medium

Propagation of Supersonic Fan Noise in a Nonuniform Medium

Demonstrating the fundamentals of fan noise

A compact, portable demonstration rig will be shown which lends itself well to demonstrating the dipole nature of fan noise generation. The demonstration apparatus provides a quantitative insight into the interaction of the dipole source distribution and the acoustic impedance presented by the fan duct. The demonstration also brings out some of the factors giving rise to fluctuating blade lift forces which constitute the dipoles responsible for fan noise generation. Implications of the fundamentals demonstrated for the standardization of fan sound measurements and for the design of ducted fan systems will be summarized with reference to a more complete description [P. K. Baade, “Effects of Acoustic Loading on Axial Flow Fan Noise Generation,” Noise Control Eng. (1976) (in press)].

Rotary power lawn mower noise

The purpose of this study was to examine the noise of a typical rotary power lawn mower. Hearing loss and annoyance criteria were examined to determine acceptable mower noise limits. It was found that if reduced from its present level of 91 dBA at the operator to 85 dBA, mower noise would pose almost no damage risk for a typical exposure. When the spectral content of the mower noise had been determined it was found that structural radiation was responsible for most of the noise in the range 500–10 000 Hz and that both the blade and exhaust were major sources below 500 Hz. The spectral content of the noise when mowing was found to be similar to that when not mowing. The three major noise sources—exhaust, blade, and structural radiation—were studied seperately. Sources of blade noise were identified by applying the theories of propellor and fan noise generation. Of the blade parameters studied, which included fan lift, sharp trailing edge, and multiple blades, none reduced the A-weighted level of the blade noise. A 1-dBA reduction was observed when the blade was statically balanced. A vibration isolator was designed and built to reduce the vibrations of the blade enclosure and resulted in an additional 112-dBA reduction. Existing theories of exhaust system design were researched and used to design resonator-type systems. Within the assumptions of classical acoustics, no acceptable solutions were found. Applying the theory of acoustic radiation from a hole, a system was designed, built, and tested which resulted in an excess attenuation over the stock muffler of 212 dBA. With the isolator, a blade with less lift, and the improved muffler, the final mower noise level was 86 dBA at the operator. Cost of the solutions is estimated at less than $ 10.00 and none of them is expected to impair the mower's operation.