Discrete Frequency Noise Research Articles

Measurement of aerodynamic noise and unsteady flow field around a symmetrical airfoil

The purpose of this paper is to study the physics of aerodynamic noise generation from the symmetrical airfoil of NACA 0018 in a uniform flow. The relationship between the noise spectrum and the unsteady flow field around the airfoil is studied in an acoustic wind tunnel using flow visualization and PIV analysis. The discrete frequency noise was generated from the airfoil inclined at small angle of attack to the free stream. The flow visualization result indicates the presence of attached boundary layer over the suction side and the separated shear layer over the rear pressure side of the airfoil, when the discrete frequency noise is observed. It is found from the PIV analysis that a large magnitude of vorticity is generated periodically from the pressure side of the trailing edge and it develops into an asymmetrical vortex street in the wake of the airfoil. The periodicity of the shedding vortices was found to agree with that of the frequency of the generated noise.

ACTIVE CONTROL OF ENVIRONMENTAL NOISE, VII: PERFORMANCE OF MULTI-FREQUENCY, MULTI-CHANNEL, FREEFIELD SOUND CANCELLING SYSTEMS

The paper considers the performance of multi-frequency, multi-channel, free field sound cancelling systems for the reduction of discrete frequency and periodic noise. The approach uses a method of directional active noise control. Large sound reductions from these systems have been made possible through: (a) synthetically generating the cancelling sound and synchronizing with the primary source; (b) automatic alignment of all stability regions of the control system and; (c) avoiding instability produced by these multi-channel systems.

2002 大型の風洞吸込口に設けたスリット状膨張空洞の消音効果

In many cases of fan noise, discrete-frequency noise (DFN), which is usually caused by rotor-stator interaction, is observed. Hence, DFN is realized as one of the main target of fan noise reduction. For such a discrete component of noise, a slit--an expansion chamber with very short axial length in a duct--may be effective as a resonator muffler. Since the slit is easy to be attached to actual machinery for its compactness, some results of its performance on an actual fan have already been obtained by the authors. As one of them, in the case of the wind tunnel in our laboratory, which has a considerably large open end, multiple components of DFN radiated from its inlet were observed. In this report, in order to reduce these components simultaneously, multiple slits--each of them is expected to have a resonant frequency coincides with each DFN--was used as located side by side on the inlet of the wind tunnel. As a result of the measurement of noise, the effectiveness of them was confirmed.

Active Control of Discrete-Frequency Noise Generated by Rotor-Stator Interactions

Discrete-frequency tones generated by unsteady blade row interactions are of particular concern in the design of advanced turbine engines. In the annular fan inlet and exit duct, the acoustic waves generated at the multiples of the rotor blade pass frequency must have circumferential periodicity. The rotor-stator generated discrete-frequency noise is characterized as a summation of these circumferentially periodic acoustic waves or spatial modes over the multiples of the rotor blade pass frequency. Only the spatial modes that propagate to the far field represent the discrete-frequency noise received by an observer. A series of fundamental discrete-frequency noise source control experiments is described. Active airfoil source control is utilized to generate propagating spatial modes to interact with, and simultaneously cancel, the upstream and downstream propagating spatial modes generated by the rotor-stator interaction. The active airfoil source control is optimized for the control of the propagating spatial modes. The active noise control system incorporates the active airfoil source control with in-duct spatial mode measurements. Data are acquired and analyzed that demonstrate the viability of this unique active noise control technique. Significant simultaneous noise reductions are achieved for the upstream and downstream propagating spatial modes. The control system is successfully demonstrated over a range of rotor blade pass frequencies.

114 クロスフローファンの回転音の音源探査 : 回転音の時刻歴伝播挙動の再現

Discrete frequency noise of a cross flow fan is generated by periodic flow fluctuation according to the rotation of an impeller. The interference of an impeller and a stabilizer is known as the main noise source. In this work, the discrete frequency noise source and the characteristics of the sound propagation of a cross flow fan were visualized according to operation points. The time sequential propagation of the sound wave was reconstructed by using an acoustic holography method. From the visualization of the discrete frequency noise, the following results were obtained. When the flow rate coefficient is large, the dominant discrete frequency noise source was observed at the contiguous area of an impeller and a stabilizer. And it shifts to the contiguous area of an impeller and a casing wall according to the decrease of the flow rate coefficient.

The Influence of Rotor Detuning on Rotor-Stator Interaction Generated Acoustic Response

In a turbomachine, the stator row is excited by the upstream rotor and responds at multiples of rotor blade pass frequency, resulting in a discrete blade pass frequency tone. Rotor detuning, i.e., unequal circumferential rotor blade spacing, decreases this fundamental tone by breaking the fundamental periodicity of blade passing. However, the stator row responds at additional multiples of rotor shaft pass frequency. This paper is directed at investigating the influence of rotor detuning on discrete frequency acoustic response. Complete data were acquired including acoustic mode and stator vane unsteady surface pressure data. Microphones in a circumferential array upstream of the rotor and stator are sampled simultaneously over one rotor revolution, and an ensemble averaged data set is acquired. A dual transform is then utilized to determine the acoustic pressure amplitude as a function of frequency and spatial mode. The stator vane acoustic response is also calculated from unsteady stator vane surface pressure measurements, and the spectrum of the acoustic response is compared with data from the circumferential microphone array. Rotor detuning is shown to have a significant effect on the discrete frequency noise. Although the stator when forced by the detuned rotor responded at more frequencies, there is an overall reduction in the total intensity level for a range of rotor speeds. Nomenclature Symbol Description ax freestream speed of sound c stator chord I acoustic intensity EL intensity level k reduced frequency k = coc/W k axial wave number kg tangential wave number Ic ... criticalNyquistmode 1 ^ an eigenvalue M axial mean flow Mach number n rotor harmonic N number of microphones number of rotor blades number of stator vanes p acoustic pressure P spatial transform of acoustic pressure s stator pitch spacing SPL sound pressure level r radial coordinate Uoo mean axial velocity Ujj axial acoustic velocity v stator index, v = 0,1,..., NVmes W Freestream velocity in two dimensions x axial direction z blade chord coordinate a mean stator angle-of-attack ctyp Two dimensional axial wave number P Two dimensional tangential wave number s level of detuning Tw unsteady bound vorticity complex magnitude 6 circumferential coordinate r^jj tuned rotor tangential spacing freestream density freestream density frequency co = nNBIadesQ rotor circular frequency pressure pattern phase speed axial coordinate A6,

1222 音響ホログラフィ法を用いた空力騒音計測に関する研究第 2 報 : 離散周波数騒音への応用

1222 音響ホログラフィ法を用いた空力騒音計測に関する研究第 2 報 : 離散周波数騒音への応用

Characteristics of Discrete Frequency Noise and Flow around a Flat Plate with Semi-circle Edges.

Experimental investigations on the discrete frequency noise and flow around a flat plate were carried out. Each cross section of the leading and tail edges of the flat plate was a semi-circle and tripping-wires (T. W.) were set on the both surfaces. The length and thickness of the flat plate was 80mm and 10mm. respectively. The diameter of the tripping wire was 2mm. The Reynolds number based on the thickness ranged from 1.2×(10)4 to 3.1×(10)4. The effect of the position of the tripping wires on the discrete frequency noise and the fluid flow around the flat plate was investigated. It was found that : the discrete frequency noise was reduced by T. W., and Γf (Γ : circulation, f : vortex shedding frequency) is one of the control factors of the discrete frequency noise.

非対称性超音速噴流の自励振動と擬似衝撃波構造の解明

The powerful jet noise containing discrete frequencies had been studied for the supersonic jet issuing from asymmetric nozzle with the oblique outlet. The discrete frequency noise was discussed to clarify the noise characteristics and generation mechanism. The behavior of supersonic jet was visualized by the instantaneous shadowgraph method. The vibration of total pressure in supersonic jet was measured using the probe having the built-in pressure transducer. The frequency spectrum of total pressure was analyzed using FFT. As the result, it was found that the peak frequency of total pressure vibration was mostly in agreement with the first peak of discrete frequency noise.

513 ジェットファンの離散周波数騒音に関する研究(流体工学 : 乱れ・はく離流れ・騒音)

513 ジェットファンの離散周波数騒音に関する研究(流体工学 : 乱れ・はく離流れ・騒音)

ACOUSTIC RADIATION FROM ROTATING BLADES—THE KIRCHHOFF METHOD IN AEROACOUSTICS

This paper reviews the current status of discrete frequency noise prediction for rotating blade machinery in the time domain. There are two major approaches both of which can be classified as the Kirchhoff method. These methods depend on the solution of two linear wave equations called the K and FW–H equations. The solutions of these equations for subsonic and supersonic surfaces are discussed and some important results of the research in the past years are presented. This paper is analytical in nature and emphasizes the work of the author and co-workers at NASA Langley Research Center.

OPTIMISATION OF BEARING DIAGNOSTIC TECHNIQUES USING SIMULATED AND ACTUAL BEARING FAULT SIGNALS

In this paper, bearing fault vibrations are modelled as a series of impulse responses of a single-degree-of-freedom system. The model incorporates slight random variations in the time between pulses so as to resemble actual vibration signals. Although the bearing fault harmonics in the raw spectrum are caused by the random fluctuations to smear over one another, they remain quite clear in the spectrum of the envelope. However, the envelope spectrum is still prone to masking by discrete and random noise. Therefore, the simulated bearing fault signals were used to investigate the efficient application of self-adaptive noise cancellation (SANC) in conjunction with envelope analysis in order to remove discrete frequency masking signals. Two ways of combining these techniques have been suggested, both of which require the original signal to be band-pass filtered and frequency-shifted in order to reduce the number of samples to be processed by SANC. The subsequent envelope analysis can then be performed by using the Hilbert transform technique or band-pass rectification. Band-pass rectification is simpler but requires extra zero padding above and below the demodulation band, making the length of the signal processed by SANC twice as long as with the former method, but still only a fraction of the length of the original signal. On the other hand, the Hilbert technique requires an extra forward and inverse discrete Fourier transform operation compared with band-pass rectification. These two methods reduce the masking effects in the envelope spectrum by removing pseudo-sum frequencies or placing them outside the frequency range of interest. This is illustrated with examples of simulated and actual vibration signals. The removal of discrete frequency noise using SANC is also demonstrated for actual vibration signals. The threshold for which analysing the squared envelope or its higher powers gives an improvement in the envelope spectrum has also been defined using simulated and actual vibration signals. The treatment in the paper is qualitative and non-mathematical for purposes of clarity, but reference is made to a quantitative treatment of the effects of masking.

Active Control of Discrete-Frequency Turbomachinery Noise Using a Rotary-Valve Actuator

Turbomachine discrete-frequency tones, a significant environmental concern, are generated by rotor-stator interactions. Specific spatial modes are generated, but not all generated modes propagate to the far field. It is the propagating spatial modes generated by rotor-stator interaction that represent the community-disturbing far field discrete-frequency noise. Fundamental active airfoil source discrete-frequency noise control experiments are performed, with the active airfoil source control optimized to generate propagating spatial modes to cancel the propagating spatial modes generated by the rotor-stator interaction. These experiments demonstrate the viability and effectiveness of rotary-valve discrete-frequency noise control. Specifically, the active noise control system combines the active airfoil source control with in-duct spatial mode measurement. In this unique design, the active airfoils of the stator vane row are driven by a remote centerbody-mounted rotary-valve system. Propagating spatial mode reductions of 5 dB and 9 dB upstream and downstream with negligible modal spillover are demonstrated. [S0742-4795(00)02702-2]

Analysis and reduction of acoustic noise from a brushless DC drive

It has been found that stator excitation in brushless DC motor drives can excite mechanical resonances in the motor and, thus, produce acoustic noise in a similar way to switched reluctance drives. A technique has been developed to alter the commutation pattern of a brushless DC motor to avoid exciting and reinforcing these mechanical resonances producing a significant reduction in pure tone acoustic noise which is emitted during mechanical resonance. The origin of the vibration and acoustic noise is explained and the theory of discrete frequency noise reduction described in this paper. The method proposed for quieter control of the brushless DC motor could be achieved at a very low cost.

Discrete Frequency Noise Reduction of the Cross-Flow Fan of the Split Type Room Air-Conditioners Using the Skewed Stabilizers.

The cross-flow fan system of the split type room air-conditioners often generates the discrete noise component that is called the Blade Passing Frequency tone. It has been known that the BPF tone is generated due to the fluctuating pressure field of the aerodynamic interaction between the rotating fan blades and the stationary stabilizer (or cut-off). Several methods have been invented to reduce this BPF noise. One of the promising schemes is to skew the stabilizer with respect to the fan blade span, which shifts the phase of the pressure fluctuations along the fan span. In this study some experiments have been carried out to investigate influences on the performance and the noise characteristics of the various skew shapes of the stabilizer with the experimental rig which simulates the indoor unit of the split type room air-conditioners. Also a guide is proposed to develop an optimal skew shape of the stabilizer, which dose not significantly impede the aerodynamic performance and has a better noise characteristics.

On the Generation of Discrete Frequency Tones by the Flow around an Aerofoil

Tonal noise, the self-induced discrete frequency noise generated by aerofoils, is investigated. It is heard from an aerofoil placed in streams at low Mach number flows when inclined at a small angle to the stream. It is a piercing whistle, commonly up to 30 dB above the background noise level. Previous authors have attributed tonal noise to a feedback loop consisting of a coupling between laminar boundary-layer instability waves and sound waves propagating in the free stream. Boundary-layer measurements have shown the presence of tonal noise is closely related to the existence of a region of separated flow close to the trailing edge of the aerofoil. An analysis of the linear stability of the boundary-layer flow over the aerofoil is presented. The amplification of the instability waves is shown to be controlled almost entirely by the region of separated flow close to the trailing edge. In light of these new experimental and theoretical results the suitability of the aero-acoustic feedback model is discussed.

A cut-off attenuator with perforated plate in a centrifugal fan and its aeroacoustic experiments

A new type of low-noise centrifugal fan with a special cut-off attenuator with a perforated plate has been invented and designed. The principle of noise suppression with this cut-off attenuator and its structure characteristics is introduced in this paper. A series of experimental comparisons of this type of centrifugal fan with a prototype centrifugal fan have been carried out. The experimental results show that the A-weighted noise level of a prototype fan without a cut-off attenuator with a perforated plate is 93 dB. As a result of using this special attenuator, the noise level of a centrifugal fan is decreased to 88.3 dB. Thus, sound attenuation due to this attenuator is 4.7 dB. In addition, decreasing the airflow and keeping the rotating velocity of the fan unchangeable, noise reduction would be 3.5 to 5.0 dB. It can be shown from the spectrum measurements that by using this cut-off attenuator with a perforated plate, not only the discrete frequency noise components of blade passing frequency and its harmonics, which predominate the sound level of a centrigual fan, can be attenuated, but also the broadband noise can be suppressed. Meanwhile, the aerodynamic performance of a centrifugal fan has not been greatly changed. [Work suported by NSFC.]

An Investigation of the Flow Field Around a Flat Plate Aerofoil Emitting Discrete Frequency Noise and the Generation Mechanism of the Noise.

Coherent flow fields around a two-dimensional flat plate aerofoil emitting discrete frequency noise in a uniform flow have been determined using phase-locked averaging technique, from which the generation mechanism of the noise has been discussed. The frequency and the noise level were artificially modified using a small circular cylinder inserted in the near wake region. The organized flow details such as velocity fluctuations in the boundary layer, pressure fluctuations on the aerofoil surface as well as the detailed structures of the shedding vortices in the turbulent wake have been presented. The generation mechanism of the discrete frequency noise has been examined in relation to the organized structures in the turbulent wake. It is confirmed that the source of the discrete frequency noise is generated by the organized fluctuating pressures on the aerofoil due to the fluctuating velocities induced by the organized turbulent shedding vortices in the wake.

Relation between Structure of Pseudo-Shock Waves and Discrete Frequency Noise. Analysis by Binary Image Method.

The effect of the structure and behavior of pseudo shock wave on jet noise containing discrete frequency has been studied by digital image processing. The shock waves were visualized by Schlieren method or a high-speed camera, and the motion of the over-compression regions was studied by the digital image processing. It was clarified that the behavior of over compression regions was periodical, and these periods were closely related to the generation of discrete frequency noise.

Dipole Active Control of Wake-Blade Row Interaction Noise

In high-bypass turbofan engines, discrete-frequency tones are generated at blade-pass frequency and its harmonics by the unsteady interaction between the rotor blades and stationary strut and vane rows. Specific circumferential acoustic modes are generated, determined by the number of rotor blades and stator vanes. However, only certain modes propagate to the far field and result in the discrete-frequency noise received by an observer. This paper is directed at active control of this discrete-frequency noise through cancellation of the propagating acoustic waves. A series of experiments is performed in the Purdue Annular Cascade Research Facility configured with a 16-bladed rotor and either one or three airfoil stator vane rows. The active discrete noise control is accomplished utilizing a speaker-dipole arrangement around each stator to generate additional control, propagating acoustic waves that interact and cancel those generated by the rotor-stator interaction, with the control parameter being the phase angle between the rotor wake and the speaker-dipole. In both one- and three-airfoil stator vane row configurations, the active noise control system simultaneously attenuated all upstream- and downstreampropagating modes. The system was also demonstrated in a monopole configuration that also provided significant noise reductions in the three-airfoil stator vane row.