Fan Noise Research Articles

Quantifying the Tip Leakage Vortex Wandering in a Low-Speed Axial Flow Fan via URANS Simulation

The tip leakage vortex is a dominant noise and aerodynamic loss source of low-speed axial flow fans. The tip leakage vortex often has an unsteady behavior, like the periodic motion of the vortex core: the so-called vortex wandering. This periodic vortex wandering results in additional noise, aerodynamic loss, and an oscillation in the moment acting on the blading, which causes an unfavorable vibration of the rotor. In the research campaigns focusing on vortex wandering, complicated measurement techniques (PIV) or high-fidelity but computationally costly simulations are commonly used. The present paper aims to introduce a relatively cost-effective unsteady Reynolds-averaged Navier-Stokes simulation-based investigation method of vortex wandering. The capabilities of the proposed technique are presented through a low-speed axial flow fan case study.

On the evolution of the shock waves systems created by the fan blades

Fan is one of the noise sources in a modern engine. The fan noise is especially noticeable during takeoff at high angular rotation speeds. In such operating modes, supersonic flow around the tips of the fan blades is realized, which leads to the formation of shock waves that propagate upstream until they exit of the engine channel. As a result, a specific noise is emitted into the front hemisphere, consisting of harmonics that are multiples of the fan rotation frequency. The paper analyzes the described effect based on a simple model of the propagation of a shock waves system. By using energy analysis, it is shown that the system of shock waves with equal amplitude decays faster than a system of shock waves with a spread in amplitude.

Fan noise radial mode detection based on [formula omitted] regularization method

Radial mode detection has been widely employed in fan noise research. Many radial mode detection algorithms by conducting the convex L1 norm optimization have been proposed by taking advantage of the fact that the fan noise sound field is usually dominated by a limited set of radial modes. Nevertheless, L1 norm optimization has been shown to be suboptimal and cannot enforce further sparsity. A fan noise radial mode detection method based on L1/2 regularization algorithm is proposed in this paper to improve its robustness and dynamic range. This method employs the L1/2 regularization to directly solve the in-duct sound propagation model. Synthesized simulations indicates that the L1/2 regularization method can detect the correct radial mode order and amplitude under low signal-to-noise ratio. Moreover, the new method is insensitive to regularization parameter. An experimental system of radial rake is implemented to validate the new method. It is shown that the L-curve method, which do not require any prior information of the acoustic mode distribution or contaminating measurement noise, is an appropriate choice of the regularization parameter determination method for the L1/2 regularizer. It is demonstrated that the new L1/2 regularization method can enhance the mode sparsity, especially under low signal-to-noise ratio. Moreover, the resolution and dynamic range can be improved by the L1/2 regularization method, which makes it an effective and robust fan noise radial mode detection technique in practical applications.

Sources for railway community noise modeling

For rail traffic, the wheel-rail interaction is one of the most important sources of noise, and is included in different ways in the noise calculation methods being used in Sweden today. However, if the rolling stock exhibit other noise sources it is not clear how they are handled by the different calculation methods. Recent measurements in Sweden of the newly refurbished 'X2' passenger trains indicate that cooling fan exhausts mounted high up on the driving unit radiate noise at levels corresponding to the wheel-rail noise at speeds of 200 km/h. The NMT96 calculation method currently used in Sweden represents the whole train as one line source per octave band, each placed at different heights. The newer Nord 2000 method, applies three sources at low heights for rolling noise and allows additional sources but without defining how to accommodate them. The Cnossos-EU method puts all potential exhaust and fan noise at the higher of two sources, at 4 m above the rail. None of the models define how to apply a source to just the locomotive as sound power is given per meter train or per rolling axis. The impact on noise exposure and mitigation is discussed in this paper.

Elevated noise sources in the European noise prediction model

The prediction model of the Environmental noise directive 2002/49/EC was transposed into Austrian national regulations in 2021 and 2022. In comparison to the Austrian prediction model defined in ONR 305011, which was last updated in 2009, it allows far more acoustic details to be taken into account enabling a more realistic description of noise emissions and propagation than before. For instance, an additional, elevated noise source position is used to model traction noise of aggregates or fans at the upper part of locomotives or motor coaches and aerodynamic noise caused by pantographs at high speeds. For free sound propagation the effect of split source positions is usually negligible, while in case of noise barriers the elevated source position lowers the insertion loss. This article discusses this effect using exemplary calculations with the European prediction model under various conditions (e.g. different barrier heights, and train speeds). The results emphasize the importance of accurate model input parameters describing traction and in case of high speeds aerodynamic noise for predictions, which need high accuracy.

Sound quality evaluation of small fan noise by tonal components

As Covod-19 has made online meetings from home offices the norm, the noise from small fans installed in information devices has increasingly become a factor disrupting a comfortable environment. In addition, as global warming accelerates, the use of new small fans called wearable fans has expanded, and the distance between small fans and humans has become even closer. In this study, we investigated whether T-TNR and T-PR, which are two methods for evaluating the tone characteristics of small fans, can be applied to the noise of projectors and wearable fans. We investigated a method for selecting dominant components and searched for a method for determining the harshness threshold using T-TNR and T-TPR.

Direct aeroacoutic simulation of a ducted axial fan with acoustic liners using a volume penalization lattice Boltzmann solver on heterogeneous massively parallel platforms

Aeroacoustic noise of an axial fan is directly solved to understand its mechanism, using our in-house Lattice Boltzmann Method (LBM) solver. To make the LBM solver industrially successful, we must reduce calculation time while maintaining its accuracy. We adopt Volume Penalization method for moving boundaries to achieve independent parallelism, and to utilize GPU (Graphical Processing Units) for parallel computation. In this study, we present a comparison between numerical and experimental results of an axial fan in a lined duct. The acoustic liner is designed to reduce 2nd BPF noise, and the noise reduction of 9.8dB is observed in the experiment. With our LBM code, the noise reduction was reproduced within an accuracy of 2.5dB, and profound insights are provided through the detailed investigation of the numerical results.

Design and analysis of Conceptual acoustic metamaterials applied to mitigating cooling fan noise for heavy-duty vehicles

The noise generated from cooling fans in heavy-duty vehicles like trucks poses challenges regarding environmental pollution and overall user comfort. To solve this problem, this study develops an acoustic metamaterial (AMM) that is constructed with small Helmholtz resonators in two layers. This concept can attenuate the noise in a broad frequency range. Furthermore, the particle swarm optimization (PSO) algorithm based on artificial intelligence (AI) is used to optimize the AMM structure. The key design parameters are discussed and analyzed. The installation strategies are put forth to examine the performance of the AMM. The proposed metamaterials, combined with the installation strategies, are numerically simulated using the finite element method. The results show that the current AMM is effective in reducing both tonal and broadband noise of interest. The findings shed light on the development of acoustic metamaterials for cooling-fan noise reduction in heavy-duty vehicles.

Passive Blowing Strategy to Enhance Aerodynamic Performance and Noise of a Hollow-bladed Axial Fan: An Experimental Study

Passive Blowing Strategy to Enhance Aerodynamic Performance and Noise of a Hollow-bladed Axial Fan: An Experimental Study

Optimization of Cooling Fan Based on Unequal Angle and Leading-Edge Triangle Blade Design

In order to reduce the noise of the axial fan, an uneven blade spacing was used to optimize tonal noise and blade leading edge triangles were added to optimize the broadband noise. First, different angular distribution schemes were designed on the basis of ensuring the dynamic balance of the fan, and the noise of the different schemes was simulated by using the computational aeroacoustics (CAA) numerical simulation method. Then, the best optimal angular distribution design was selected for experimental verification, which confirmed the reliability of the numerical calculation. The test results also show that the uneven blade spacing has little effect on the fan pressure and efficiency. Additionally, triangular leading edges were added to each blade based on the uneven blade spacing design. Combining these two optimization schemes proved to reduce the fan noise by 13.4 dBA.

Research and design of low-noise cooling fan for fuel cell vehicle

Abstract The main noise source of fuel cell vehicles comes from the cooling vehicle, which has the characteristics of high voltage and large air volume. However, the large air volume makes the fan noise level significantly increase, reaching about 70dB. Even if the car adopts a variety of physical noise reduction methods, the noise level inside the car is still high, seriously affecting the passenger ride experience. In recent years, related research combined with bionic technology to improve the fan blade by designing a new shape structure, so that it has similar characteristics to some organisms to achieve the purpose of noise reduction. In this paper, ANSYS-CFX software is used to simulate the fan efficiency and noise level under different blade numbers, predict its performance, and optimize its structure.

On the impact of operating condition and testing environment on the noise sources in an industrial engine cooling fan

Engine cooling fan noise is a relevant issue for manufacturers. It is well known that both the operating point and testing environment can affect the noise generation mechanisms and, consequently, the measured noise may change. Therefore, these aspects are investigated on a reference industrial fan, for which experimental data exists, using high-fidelity numerical simulations based on the lattice-Boltzmann method. Two operating conditions, namely the free blowing and the maximum efficiency ones, and three testing environments are analyzed: (i) a conventional semi-anechoic room, (ii) an ideal free field environment, and (iii) a testing environment resembling an anechoic aeraulic facility. For cases (i) and (ii) no pressure difference across the fan is imposed, while, for case (iii), a pressure difference across the fan can be imposed. For the latter, the impact of a fully reflective and fully absorbing wall separating the two regions upstream and downstream of the fan is analyzed. At free blowing conditions, the flow over the blades is largely separated. When the blade passes through a blockage region, because of the presence of a honeycomb-like structure needed for structural purposes, it experiences a prominent loading hump. The far-field noise, at a listener located along the axis of rotation, is therefore highly tonal, with a clear peak at the blade passing frequency tone. When the same fan is tested in a free field environment, it is found that there is a difference in the acoustic pressure at higher harmonics of the blade passing frequency due to the presence of flow recirculations in the anechoic room. Placing a thin wall across the fan increases the mass flow rate, for a given rotational speed, which results in a more severe flow separation over the blades and, therefore a higher tone prominence at the blade passing frequency. If the thin wall is modeled as a sound-absorbing wall, there is a drop of the overall sound pressure level of about 2 dBA. When the fan is tested at its maximum efficiency, i.e., nonzero pressure difference across the fan, it is found that the blockage effect is less relevant. The main noise generation mechanism is the back-flow vortex induced by the pressure difference across the fan interacting with the blade tip leading edge.

Localization of acoustical sources rotating in the cylindrical duct using a sparse nonuniform microphone array

The Doppler effect caused by the rotation of turbine fans brings significant challenges to the identification of fan noise sources inside the nacelle. Current methods of rotating source localization inside the cylindrical duct are executed with tough requirements for the microphone array mounted on the duct wall, which includes a fair enough number of microphones and equal-space distribution of each microphone in the azimuthal direction. A methodology based on nonuniform measurements and duct spin modes superposition (NMDMS) is proposed to identify rotating sources with high spatial resolution and few side lobes even near cut-on frequencies, which requires much fewer microphones and no uniform distribution of each microphone in the circumference. The sparsity in the azimuthal domain of the duct field generated by multiple rotating sources is verified theoretically, after which the azimuthal mode in each duct cross-section is reconstructed by the inhomogeneous measurements through the orthogonal matching pursuit (OMP) method. Followed by the identification of mode amplitude in the rotation frame, the sound pressure distribution as well as the axial acoustic velocity distribution on the source plane is reconstructed through duct modes summation. Numerical simulations and experiments are implemented to validate the method. Localization results indicate that rotating sources identification using the proposed method could not only save more than half the microphones without requirements of equal-space distribution but obtain good accuracy of localization and remarkably fewer side lobes.

Experimental investigation on aerodynamic noise of a small-scale multi-blade centrifugal fan

In this study, the aerodynamic noise of a small-scale centrifugal fan was experimentally investigated using acoustic testing techniques and time-resolved stereoscopic particle image velocimetry (SPIV). During the acoustic experiments, both far-field noise and near-field pressure fluctuations of the test fan were measured. The overall far-field noise towards the fan inlet side was found to be higher than that of the back side. The pressure fluctuations on the fan upper casing exceeded those on the side wall due to the uncontracted volute tongue, indicating pronounced flow-to-wall interactions. Moreover, based on a simultaneous measurement, the coherence between the near-field pressure fluctuations and far-field noise highlighted the significant contributions of impeller rotation to noise radiation. SPIV measurements uncovered the time-averaged and transient flow fields at the fan's inlet and outlet. The time-averaged results demonstrated the concentrated inlet flow and outlet flow separation, leading to high flow unsteadiness. Transient flow fields displayed an asymmetric jet-wake region characterized by both quasi-steady flow and rotational flow behaviours. The instantaneous flow results were analyzed using the dynamic mode decomposition (DMD) method, which clearly recognized the jet-wake patterns with frequencies corresponding to the rotational frequency. The observed consistency in frequency characteristics among noise, pressure fluctuations, and unsteady flow affirms that flow dynamics are crucial to the primary noise mechanisms.

Flow-induced noise and vibration in axial fan: a case study

AbstractReducing axial fan noise is crucial in residential, commercial, and industrial applications. This study explores how attaching different add-on designs to axial fans reduces aerodynamic noise. Eight design cases were examined, including extending the outlet and inlet ducts, using Chevron nozzles, placing internal spherical balls in staggered and straight patterns, applying a wavy inner wall treatment, and combining some of these nozzle designs. CFD and acoustic analyses were performed using Ansys Fluent 2022 R1. Noise reduction was measured at the nozzle outlet in each design case. All nozzle modifications reduced noise levels, with noise reductions ranging from 3 to 10% at the blade tip and 26–55% at the outlet. As an add-on to existing fan cases, the design cases investigated in this study can potentially improve the acoustic environment in various settings. The findings of this study can contribute to the development of quieter axial fans.

Numerical modeling of axial fan noise in fuel-cell electric truck: from component to full-vehicle analysis

This paper presents a comprehensive approach to predict the noise generated by an axial fan in electric vehicles, which is more noticeable due to the absence of combustion engines. Such cooling fans require stronger cooling performance due to increased demand for heat dissipation, resulting in even higher noise levels. A numerical model for fan noise in a fuel-cell electric vehicle (FCEV) is proposed with two steps: component-level and full-vehicle level models. At component level, Lattice Boltzmann Method (LBM) is employed for aeroacoustics simulation, determining the component sound power level (SWL). At full vehicle level, statistical energy analysis (SEA) is utilized to estimate the sound pressure level (SPL) inside the vehicle cabin, particularly at the driver's ear locations. The noise sources in the full vehicle model, were characterized from the component level SWL, as monopole sources. The model is validated by comparing the numerical predictions with the experimental measurements of the SWL in a hemi-anechoic chamber and the SPL in the cabin. Good agreements were obtained between numerical results and measurement results, demonstrating the effectiveness of the model. This allows for faster and more accurate virtual NVH design validations on large and complex systems where prototype measurements are difficult to conduct.

Influence of uncomfortable environmental conditions on odor detection thresholds for n-butanol determined with a dynamic dilution olfactometer and in room air

Odor guide values (OGV) are used when a person is exposed to an odor in a room and complains about odor annoyance. OGV are derived from odor detection thresholds (ODT) measured under standard environmental conditions. The study question was whether uncomfortable environmental conditions alter odor perception and should therefore be considered when applying OGV. ODT for n-butanol were determined with an olfactometer and in room air. Twenty healthy, non-smoking volunteers (10 women/10 men, 19–50 years) were selected and trained compliant with the European Standard for Olfactometry EN 13725:2022. Standard conditions were an average temperature between 22 °C and 23 °C, warm light (2800 Kelvin), quiet fan noise (45 dB), 415 ppm carbon dioxide (CO2), and relative humidity between 34 % and 42 %. On each test day, one of five conditions was examined: high temperature (26°C), cold light (6500 Kelvin), traffic noise (70 dB with peaks up to 85 dB), 1000 ppm CO2 and 4000 ppm CO2. Comparability of log-transformed ODT values was assessed by Bland-Altman plot analysis. None of the five conditions systematically affected ODT, either when measured with an olfactometer or in room air. Compared to standard conditions (Limits of Agreement (± LoA) 1.03), the variability of ODT was higher at high temperature (± LoA 1.70) and traffic noise (± LoA 1.45), but not at cold light (± LoA 1.02), 1000 ppm CO2 (± LoA 0.87) or 4000 ppm CO2 (± LoA 0.93). The results show that OGV can be used in uncomfortable environmental conditions. However, because ODT were lower or higher in a few subjects at high temperatures and traffic noise, individual occupant’s perception of temperature and noise should always be considered when applying the OGV concept. Furthermore, the results confirm that the experimental determination of ODT should be performed under controlled and standardized environmental conditions.

Enhancing Axial Fan Noise Reduction through Innovative Wavy Blade Configurations

Enhancing Axial Fan Noise Reduction through Innovative Wavy Blade Configurations

Study on noise characteristics of centrifugal fan based on blade loading distribution

Abstract In order to explore the influence of different blade loading distribution on the noise characteristics of centrifugal fan, this paper takes the blade loading distribution of the hub and shroud of the centrifugal fan as the design parameters, and establishes four centrifugal fan models with different blade loading distribution. The aerodynamic noise and radiation noise of the centrifugal fan are calculated by the transient results. The research shows that the noise characteristics of the fan change with the change of the blade loading distribution. The maximum aerodynamic noise is generated at the volute tongue, and the total sound pressure level of the gas along the flow direction shows a downward trend. The area with large vibration displacement on the surface of the volute occurs at the back cover plate of the volute, and the noise sound pressure level radiated outward is also higher. The purpose of noise reduction can be achieved by controlling the blade loading distribution.

Analysis of influence of blade number on aerodynamic performance and noise of forward centrifugal fan

Analysis of influence of blade number on aerodynamic performance and noise of forward centrifugal fan