Fan Noise

This paper provides an overview of the design and performance of the new aeroacoustic wind tunnel facility at the University of Bristol. The purpose of the facility is to enable near-and far-field acoustic and aerodynamic studies on a variety of different aerodynamic components and to examine diverse noise control techniques. The facility comprises a large acoustic chamber, anechoic down to 160 Hz, and a temperature controlled closed-circuit wind tunnel with an open test section. The wind tunnel features two interchangeable rectangular nozzles with a partially shared contraction. Both nozzles are shown to possess a high flow quality with high flow uniformity and low turbulence intensity of 0.09% and 0.12% for the smaller and larger nozzle, respectively. The maximum attainable flow speeds are 40 m/s for the larger nozzle and 120 m/s for the smaller nozzle corresponding to Reynolds numbers of 2.7 million and 8.1 million per meter, respectively. In this paper, we will present various aerodynamic and acoustic results to characterize the performance of the facility. The background noise levels are found to be sufficiently low and the far-field noise measurements from a flat plate, a round cylinder and a NACA 0012 airfoil compare favorably to existing experimental observations.

This paper describes the generation of blade-tower interaction (BTI) noise from upwind turbines and pylon-mounted fans using a combination of experimental and numerical means. An experimental rotor-rig was used in an anechoic chamber to obtain BTI acoustic data under controlled conditions. A computational model, based on the solution of the unsteady Reynolds Averaged Navier Stokes (URANS) equations and Curle's acoustic analogy, was used to describe the generation of fan and simplistic model of wind turbine BTI noise by the rotor-rig. For both the fan and model wind turbine case, the tower was found to be a more significant source of BTI noise than rotor blades. The acoustic waveforms for both turbine and fan are similar; however, in the case of the turbine, the blade contribution reinforces that from the tower, while in the case of a fan, there is some cancellation between the tower source and the blade source. This behavior can be explained by the unsteady aerodynamics occurring during BTI.

Counter-Rotating Open Rotors (CROR) are an attractive alternative to turbofans, due to their advantageous propulsive efficiencies. However, prior to their widespread application their noise levels need to be decreased in order to meet the requirements of strict laws and regulations. Mounting methods – a streamlined pylon, for instance – could have significant effects on the noise generation mechanisms. Earlier research has shown that a pylon can have a large effect on the noise of CROR at blade passing frequencies, as a result of the wake of the pylon interacting with the rotating blade sets. However, these studies were focused only on the noise generation mechanisms of the rotors in the presence of a pylon, and not the noise of the pylon itself. In the research presented herein a phased array microphone system combined with beamforming technology has been used to investigate the noise associated with the pylon itself. The blades of the front and the aft rotors were therefore remove...

e tonal noise control of an axial low-speed fan system with ow obstruction has been achieved both numerically and experimentally. Its primary noise source caused by rotor-wake interaction has been directly and accurately simulated with a La ice Boltzmann Method. e la er has then allowed deciphering the noise control mechanism of an upstream sinusoidal obstruction: the vortex rings shed by the obstruction yield a second noise source at the rotor-blade leading edge. e obstruction itself does not create any signi cant noise and is acoustically transparent. An industrially-applicable numerical methodology has then been proposed to obtain the optimal obstruction design for a given fan geometry and operating condition, with a maximum of six simulations of the fan system without and with the obstruction being static and slowly rotating. Simulations with rotating obstructions provide the optimal lobe amplitude and an optimal obstruction angular position, which are found to be mm and about • respectively for the present fan system both numerically and experimentally. e frequency selectivity of the obstruction and the linear variation of the secondary source level with the lobe amplitude have also been con rmed.

This manuscript contains the development stages of a multi-purpose wind tunnel built at the Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia. The fully automated wind tunnel is named Pangkor after an island in Perak, Malaysia. The development of the wind tunnel consists of three stages namely the design, fabrication and testing & commissioning. The computational fluids dynamic (CFD) approach was employed to ascertain the main geometries to optimize space utilization. Calculations are made based on typical wind tunnel design guidelines. Pitot tubes-pressure transducer, hotwire anemometry, temperature, room humidity and barometric sensors were used to verify actual flow of our construction. A traverse installed at the wind tunnel is capable of a two dimensional movements. The 15 kW axial fan used is especially selected because of space limitation. A variable frequency drive (VFD) connected to fan’s motor allows velocity control from a computer. All devices ...

The development of technologies to reduce turbofan engine noise reveals the fan noise, the first stage of an engine, as a great contributor for the total noise of an airplane. So a better understanding of the fan noise generation came up and motivated the construction of a fan rig test facility at the University of São Paulo in São Carlos by a partnership between the university and EMBRAER S.A.. The fan rig is composed of a long duct (12 m long) comprising a 16-bladed fan rotor and 14-vaned stator. The rotor is powered by an 100 hp electrical motor allowing speed up to 4250 RPM resulting in 0.1 Mach axial flow. A 77-microphone wall-mounted array was designed for fan noise analysis. A cooperation with NASA-Glenn allowed data and information exchanging from their similar fan rig setup, the ANCF, grating then the validation of the in-house developed software. A short guide for duct-array is proposed in this work. Complex software was developed to process the data from the microphones array. We performed 3 different types of analysis: power spectral density, noise imaging obtained by acoustic beamforming and modal analysis. We proposed a different technique for modal analysis based on beamforming images in this work. We did not find any similar technique in the references. The results obtained by this technique were validated with data from ANCF-NASA. Comparative results are presented for both fan rigs, such as: power spectral densities for different fan speeds, modal analysis at the blade passing frequency (strong tones generated by the fan), noise imaging obtained by beamforming for rotating and static noise sources. Finally, results achieved in this work are in agreement with those observed in the references consulted.

This manuscript contains the development stages of a multi-purpose wind tunnel built at the Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia. The fully automated wind tunnel is named Pangkor after an island in Perak, Malaysia. The development of the wind tunnel consists of three stages namely the design, fabrication and testing & commissioning. The computational fluids dynamic (CFD) approach was employed to ascertain the main geometries to optimize space utilization. Calculations are made based on typical wind tunnel design guidelines. Pitot tubes-pressure transducer, hotwire anemometry, temperature, room humidity and barometric sensors were used to verify actual flow of our construction. A traverse installed at the wind tunnel is capable of a two dimensional movements. The 15 kW axial fan used is especially selected because of space limitation. A variable frequency drive (VFD) connected to fan’s motor allows velocity control from a computer. All devices ...

Turbulent boundary layer trailing edge noise is the dominant noise in airfoil self-noise. This noise generated by the trailing edge can be minimised by different edge treatments. By varying the acoustic impedance of the trailing edge, acoustic radiation can be varied. A NACA0012 symmetric airfoil with 0.3 mm thick solid and a perforated plate fitted at the trailing edge is used for this study. Experiments were carried out in an anechoic facility at different velocity varying from 20 m/s to 45 m/s and geometric angles of attack of 0 o . Solid trailing edge extension and perforated trailing edge extensions enhance the noise reduction in the low frequency range. As compared to solid extensions perforated extensions provide greater noise reduction. Maximum noise reduction of approximate 6.5 dB in the low frequency range is obtained for the perforate plate attachment at zero angle of attack. At higher velocities an increase in the high frequency noise is observed due to the roughness off...

The well-known dominant sources of airframe noise are associated with unsteadiness of separated and/or vortical flow regions around the high-lift system (flaps, slats) and the aircraft undercarriage (landing gear). Current practical landing gear noise prediction models are individual component based, which means that the various components are divided into groups according to the frequency range, in which they predominantly radiate noise. Since the far-field noise spectra are approximately Strouhal based, the emitted frequency is assumed to be directly related to their size: the large elements are responsible for the low frequency region of the spectra, and the small components for the high frequency region. On the basis of such understanding of the noise generation mechanism, the special configurations that lead to considerable noise suppression were proposed. One element of these configurations are rods with different shape and cross section. In this work the situation when circul...

The present paper objective is to describee ITA's wind tunnel characteristics, giving dimensions and design details of each element This is an open circuit wind tunnel with a closed test section, with 1.0 m height by 1.28 m width. The test section maximum velocity is approximately 80 m/s and the turbulence level was designed to be lower than 0.05%, at the maximum velocity. An eight blades fan, connected to a 200hp electric motor, is used to generate the flow. The wind tunnel entrance nozzle is located inside the Prof. Feng Aeronautical Engineering Laboratory. This feature is interesting because atmospheric wind variations are damped before entering in the wind tunnel. In the settling chamber there are one honeycomb and three screens, used to reduce the turbulent intensity of the test section flow. The test section is designed to be very flexible, in order to reduce the time and cost for implementing new experimental apparatus. Such flexibility was obtained by using interchangeab...

To foresee and then minimize the noise due to a ventilation circuit, it is necessary to deeply analyse each of its elements. Fans are the main noise sources of these systems. The aim of the work was to find the fan acoustic fingerprint of a typical medium size centrifugal fan, for industrial and civil uses; though a fan could look like a rather simple and standardized machine, it is known that its acoustic performance is the result of various, different phenomena. Thanks to intensity measurements, according to ISO 9614-1, and intensity maps, the different sources of the fan and their relative importance were identified and, in particular, inlet, scroll and outlet. Pressure measurements were performed in a semi-anechoic chamber, according to ISO 3745, focusing on aerodynamic noise. Discrete tonal components in the overall power spectrum appeared indeed to be limited, with respect to the broadband noise, thanks to the scroll cut-off optimisation. The results also confirmed that aerody...

To foresee and then minimize the noise due to a ventilation circuit, it is necessary to deeply analyse each of its elements. Fans are the main noise sources of these systems. The aim of the work was to find the fan acoustic fingerprint of a typical medium size centrifugal fan, for industrial and civil uses; though a fan could look like a rather simple and standardized machine, it is known that its acoustic performance is the result of various, different phenomena. Thanks to intensity measurements, according to ISO 9614-1, and intensity maps, the different sources of the fan and their relative importance were identified and, in particular, inlet, scroll and outlet. Pressure measurements were performed in a semi-anechoic chamber, according to ISO 3745, focusing on aerodynamic noise. Discrete tonal components in the overall power spectrum appeared indeed to be limited, with respect to the broadband noise, thanks to the scroll cutoff optimisation. The results also confirmed that aerodynamic noise is essentially dipole noise, associated to turbulent boundary layer scattering at the blade trailing edge; evidence was found that overall power level depends on N 6. Several fan noise studies demonstrate that broadband noise attenuation needs a careful blade design and minimum turbulence at the inlet, but a deeper view inside the signal can reveal hidden characteristics. In this way, apart from frequency analysis, time domain analysis and Wavelet transform were used. The latter, in particular, stressed the self-similarities of the signal, consequence of its chaotic nature.

This manuscript contains the development stages of a multipurpose wind tunnel built at the Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia. The fully automated wind tunnel is named Pangkor after an island in Perak, Malaysia. The development of the wind tunnel consists of three stages namely the design, fabrication and testing & commissioning. The computational fluids dynamic (CFD) approach was employed to ascertain the main geometries to optimize space utilization. Calculations are made based on typical wind tunnel design guidelines. Pitot tubes-pressure transducer, hotwire anemometry, temperature, room humidity and barometric sensors were used to verify actual flow of our construction. A traverse installed at the wind tunnel is capable of a two dimensional movements. The 15 kW axial fan used is especially selected because of space limitation. A variable frequency drive (VFD) connected to fan's motor allows velocity control from a computer. All devices are connected a computer with one single controlling software; Scilab-ensuring ease of operation. The project shows that, with a limited budget, a wind tunnel with full functionalities could be constructed.

Effects of screens and perforated plates (grids) on free-stream turbulence are studied in several test flow conditions. The level, structure and decay of the turbulence generated by such 'manipulators ' depend in part on their shear-layer instabilities, and can therefore be Modified by inserting additional devices immediately downstream. The performance of screens and some perforated plates is found to depend on the characteristics of the incoming flow such as velocity, turbulence level and spectra. Combinations of perforated plates and screens are found to be very effective flow manipulators. By optimizing the intermanipulator separation and carefully matching the scales between the manipulator pair, the turbulence decay rate downstream of a grid can be quadrupled.

To foresee and then minimize the noise due to a ventilation circuit, it is necessary to deeply analyse each of its elements. Fans are the main noise sources of these systems. The aim of the work was to find the fan acoustic fingerprint of a typical medium size centrifugal fan, for industrial and civil uses; though a fan could look like a rather simple and standardized machine, it is known that its acoustic performance is the result of various, different phenomena. Thanks to intensity measurements, according to ISO 9614-1, and intensity maps, the different sources of the fan and their relative importance were identified and, in particular, inlet, scroll and outlet. Pressure measurements were performed in a semi-anechoic chamber, according to ISO 3745, focusing on aerodynamic noise. Discrete tonal components in the overall power spectrum appeared indeed to be limited, with respect to the broadband noise, thanks to the scroll cutoff optimisation. The results also confirmed that aerodynamic noise is essentially dipole noise, associated to turbulent boundary layer scattering at the blade trailing edge; evidence was found that overall power level depends on N 6. Several fan noise studies demonstrate that broadband noise attenuation needs a careful blade design and minimum turbulence at the inlet, but a deeper view inside the signal can reveal hidden characteristics. In this way, apart from frequency analysis, time domain analysis and Wavelet transform were used. The latter, in particular, stressed the self-similarities of the signal, consequence of its chaotic nature.

Experimental investigation of flow-induced sound and sources of sound necessitates an adequate environment of low interfering noise and suppression of unwanted sources of sound. In order to investigate the sound from fluid-structure interactions, a low-noise wind tunnel has been designed and implemented. This paper gives an overview of the design criteria and specifications, along with an aeroacoustic and aerodynamic characterization of the facility. The suitability to airfoil self-noise studies is explored and first measurements with a model airfoil are presented. The paper concludes with suggestions for improvements and provides an outlook on future applications.

Experimental investigation of flow-induced sound and sources of sound necessitates an adequate environment of low interfering noise and suppression of unwanted sources of sound. In order to investigate the sound from fluid-structure interactions, a low-noise wind tunnel has been designed and implemented. This paper gives an overview of the design criteria and specifications, along with an aeroacoustic and aerodynamic characterization of the facility. The suitability to airfoil self-noise studies is explored and first measurements with a model airfoil are presented. The paper concludes with suggestions for improvements and provides an outlook on future applications.

Experimental investigation of flow-induced sound and sources of sound necessitates an adequate environment of low interfering noise and suppression of unwanted sources of sound. In order to investigate the sound from fluid-structure interactions, a low-noise wind tunnel has been designed and implemented. This paper gives an overview of the design criteria and specifications, along with an aeroacoustic and aerodynamic characterization of the facility. The suitability to airfoil self-noise studies is explored and first measurements with a model airfoil are presented. The paper concludes with suggestions for improvements and provides an outlook on future applications.