Tonal Noise Generation Research Articles

Surface curvature effects on the tonal noise of a wall-mounted finite airfoil.

This paper is concerned with the influence of camber on the noise of a wall-mounted finite airfoil with natural boundary layer transition. Tonal noise measurements taken in an aeroacoustic wind tunnel are presented for airfoils with aspect ratio of 2, NACAxx12 profile and camber between 0 and 6% at 40% chord. The results show camber is an important parameter that determines the operating conditions for which acoustic tone generation occurs and the number and intensity of the tones produced. Airfoils with 0%-2% camber have an acoustic signature that is dominated by a high amplitude primary tone, whereas the spectra of airfoils with higher camber of 4%-6% feature a more pronounced side tone structure. Tonal noise production does not collapse with lift coefficient, demonstrating that the local flow conditions influence the noise source. Tonal noise production is explained in terms of changes to mean flow topology, namely the location of flow separation, which is linked to tonal noise generation. Scaling of airfoil tonal noise is found to vary with angle of attack and pressure gradient. Empirical scaling laws for the primary tone frequency dependence on velocity are also derived for the cambered airfoils.

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

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

Numerical Prediction of the Tonal Airborne Noise for a NACA 0012 Aerofoil at Moderate Reynolds Number Using a Transitional URANS Approach

Abstract Tonal airborne noise of aerofoils appears in a limited range of moderate Reynolds numbers and angles of attack. In these specific conditions, the aerofoil is characterised by a large region of laminar flow over the aerodynamic surface, typically resulting in two-dimensional laminar instabilities in the boundary layer, generating one or more acoustic tones. The numerical simulation of such phenomenon requires, beside an accurate prediction of the unsteady flow field, a proper modelling of the laminar to turbulent transition of the boundary layer, which generally imposes the use of highly CPU demanding approaches such as large eddy simulation (LES) or direct numerical simulation (DNS). This paper aims at presenting the results of numerical experiments for evaluating the capability of capturing the tonal airborne noise by using an advanced, yet low computationally demanding, unsteady Reynolds-averaged Navier-Stokes (URANS) turbulence model augmented with a transitional model to account for the laminar to turbulent transition. This approach, coupled with the Ffowcs Williams and Hawkings (FW-H) acoustic analogy, is adopted for predicting the far-field acoustic sound pressure of a NACA 0012 aerofoil with Reynolds number ranging from 0.39 · 106 to 1.09 · 106. The results show a main tone located approximately at 1.6-1.8 kHz for a Reynolds number equal to 0.62 · 106, increasing to 2.4 kHz at Reynolds number equal to 0.85 · 106 and 3.4 kHz at 1.09 · 106, while no main tones are observed at 0.39 · 106. The computed spectra confirm that the acoustic emission of the aerofoil is dominated by tonal structures and that the frequency of the main tone depends on the Reynolds number consistently with the ladder-like tonal structure suggested by Paterson et al. Moreover, in specific conditions, the acoustic spectra exhibit a multi-tonal structure visible in narrowband spectra, in line with the findings of Arbey and Bataille. The presented results demonstrate the capability of the numerical model of predicting the physics of the tonal airborne noise generation.

Frequency-domain method for multistage turbomachine tone noise calculation

A method of frequency-domain calculation of the multistage turbomachinery tone noise is presented. The method is based on the kinematic relations featuring dependence of flow fields in a turbomachine on time and circumferential angle. It solves the flow in several blade passages inside each row and can be used in conjunction with nonlinear equations. The method is developed at Central Institute of Aviation Motor and implemented in the Three Dimensional Acoustics Solver in-house solver. The multi-passage method is verified on two numerical problems. One is the tone noise generation by a 2D two stage turbine. The other is the problem of nonlinear interaction of circumferential modes in a 2D cylindrical channel.

Tonal noise prediction in a small high speed centrifugal fan and experimental validation

This paper presents the work done to establish a methodology that is capable of predicting tonal noise generation in high speed centrifugal fans. The deliverables from this work emphasize on identifying and characterizing the source of tonal noise in the centrifugal fan. Computational fluid dynamics (CFD) modeling was performed using 3-D Detached Eddy Simulation (DES) to compute the unsteady flow field in the fan. The calculated time history of surface data from the CFD is then used in Ffowcs Williams-Hawkings (FW-H) solver to predict the far field noise levels. The predicted aerodynamics and aeroacoustics results are in good agreement with the experimental data acquired from the flow testing facility and the anechoic chamber. The study conducted on the centrifugal fan shows that the aerodynamic interaction between the non-uniform impeller outflow and the leading edge of the diffuser vane is the source of tonal noise generation. The impingement of the jet-wake flow structure from the impeller outflow causes periodical pressure fluctuation on the leading edge of the diffuser vanes which leads to the tonal noise generation at the blade passing frequency (BPF).

The influence of booster design features on aicraft engine fan tonal noise

Fan and booster tonal noise makes significant contribution to the overall noise level of an aero engine, particularly in subsonic modes of operation. This noise is caused by the interaction not only of the fan rotor and stator, but also by the interaction of elements of the fan and booster. Tonal components at these frequencies are caused by nonlinear interaction of rotating and stationary blade rows. Therefore, fan noise spectra contain not only common fan tonal components at the blade passing frequency and its harmonics, but sum and difference tones of interaction between the fan and booster elements. In testing acoustic characteristics of a bypass wide-chord fan it was revealed that the sum and difference components of the fan and booster interaction noise appeared in subsonic operating modes and propagated basically in the forward hemisphere. Labyrinth seal is one of the features of the actual fan design. These seals are opened and let some airflow pass in low operating modes of the fan. As a result the flow pattern upstream the fan rotor is distorted. It was experimentally determined that the seal significantly affects the generation of the fan tonal noise in subsonic modes. So far, the impact of this fan design feature is not taken into account in fan noise prediction methods. However, we experimentally determined strong influence of labyrinth seals on the noise of the aero engine simulator. Experimental assessment of the influence of the labyrinth seal operating in off-design modes on UPS noise spectra is the purpose of this work.

The generation of tonal noise from sawtooth trailing-edge serrations at low Reynolds numbers

ABSTRACTThe flow and noise created by sawtooth trailing-edge serrations has been studied experimentally at a low Reynolds number. Experiments have been performed on a flat-plate model with an elliptical leading edge and an asymmetrically bevelled trailing edge at Reynolds numbers of Rec = 1 × 105–1.3 × 105, based on chord. Wide serrations with a wavelength (λs) to amplitude (2h) ratio of λs/h = 0.6 were found to reduce the overall sound pressure level by up to 11dB. In contrast, narrower serrations with λs/h = 0.2 produce tonal noise and increase the overall noise level by up to 4dB. Intense vortices across the span of the trailing edge with narrow serrations are shown to be the source of tonal noise. Wide serrations reduce turbulent velocity fluctuations at low frequencies which explains the lower radiated noise. The narrow serrations that produce low Reynolds number tonal noise were shown previously to be effective at higher Reynolds numbers (Rec > 2 × 105), demonstrating that care is needed to fully understand the flow field over serrations for all intended operating conditions.

Direct aeroacoustic simulation of acoustic feedback phenomena on a side-view mirror

The flow around a side-view mirror and its noise generation are investigated using large eddy simulation and direct acoustic simulation. To this end, we use the high order discontinuous Galerkin spectral element method on non-conforming curved elements. Tonal noise is observed, which originates at the trailing edge downstream of laminar separation, coinciding with experimental results. In order to determine the nature of the tonal noise generation mechanism, we perform a linear stability analysis and employ a global perturbation approach in combination with dynamic mode decomposition. The perturbation analysis based on the whole flow field demonstrates the existence of a global instability involving convective disturbance growth, acoustic scattering at the trailing edge and acoustic receptivity at a rounded edge slightly upstream of separation. The results clearly show the tonal noise to be caused by the so-called acoustic feedback loop known from airfoil aeroacoustics. This phenomenon has been simulated here for the first time for a complex geometry.

Regimes of tonal noise on an airfoil at moderate Reynolds number

Tonal noise generated by airfoils at low to moderate Reynolds number is relevant for applications in, for example, small-scale wind turbines, fans and unmanned aerial vehicles. Coherent and convected vortical structures scattering at the trailing edge from the pressure or suction sides of the airfoil have been identified to be responsible for such tonal noise generation. Controversy remains on the respective significance of pressure- and suction-side events, along with their interaction for tonal noise generation. The present study surveys the regimes of tonal noise generation for low to moderate chord-based Reynolds number between $\mathit{Re}_{c}=0.3\times 10^{5}$ and $2.3\times 10^{5}$ and effective angle of attack between $0^{\circ }$ and $6.3^{\circ }$ for the NACA 0012 airfoil profile. Extensive acoustic measurements with smooth surface and with transition to turbulence forced by boundary layer tripping are presented. Results show that, at non-zero angle of attack, tonal noise generation is dominated by suction-side events at low Reynolds number and by pressure-side events at high Reynolds number. At smaller angle of attack, interaction between events on the two sides becomes increasingly important. Particle image velocimetry measurements complete the information on the flow field structure in the source region around the trailing edge. The influences of both angle of attack and Reynolds number on tonal noise generation are explained by changes in the mean flow topology, namely the presence and location of reverse flow regions on the two sides. Data gathered from experimental and numerical studies in the literature are reviewed and interpreted in view of the different regimes.

Suppression of tonal noise in a centrifugal fan using guide vanes

This paper presents the work aiming for tonal noise reduction in a centrifugal fan. In previous studies, it is well documented that tonal noise is the dominant noise source generated in centrifugal fans. Tonal noise is generated due to the aerodynamic interaction between the rotating impeller and stationary diffuser vanes. The generation of tonal noise is related to the pressure fluctuation at the leading edge of the stationary vane. The tonal noise is periodic in time which occurs at the blade passing frequency (BPF) and its harmonics. Much of previous studies, have shown that the stationary vane causes the tonal noise and generation of non-rotational turbulent noise. However, omitting stationary vanes will lead to the increase of non-rotational turbulent noise resulted from the high velocity of the flow leaving the impeller. Hence in order to reduce the tonal noise and the non-rotational noise, guide vanes were designed as part of this study to replace the diffuser vanes, which were originally used in the chosen centrifugal fan. The leading edge of the guide vane is tapered. This modification reduces the strength of pressure fluctuation resulting from the interaction between the impeller outflow and stationary vane. The sound pressure level at blade passing frequency (BPF) is reduced by 6.8dB, the 2nd BPF is reduced by 4.1dB and the 3rd BPF reduced by about 17.5dB. The overall reduction was 0.9dB. The centrifugal fan with tapered guide vanes radiates lower tonal noise compared to the existing diffuser vanes. These reductions are achieved without compromising the performance of the centrifugal fan. The behavior of the fluid flow was studied using computational fluid dynamics (CFD) tools and the acoustics characteristics were determined through experiments in an anechoic chamber.

Numerical and Experimental Study on the Tonal Noise Generation of a Radial Fan

The main focus of this work is on the geometrical modifications can be made to the fan wheel and to the volute tongue of a radial fan to reduce the tonal noise. The experimental measurements are performed by using the in-duct method in accordance with ISO 5136. In addition to the experimental measurements, CFD (computational fluid dynamics) and CAA (computational aeroacoustics) simulations are carried out to investigate the effects of different modifications on noise and performance of the fan. It is shown that by modifying the blade outlet angle, the tonal noise of the fan can be reduced without impairing its aerodynamic performance. Moreover, it is indicated that increasing the number of blades leads to a significant reduction in the tonal noise and also an improvement in the aerodynamic performance. However, this trend is only valid up to a certain number of blades, and a further increment might reduce the aerodynamic performance of the fan. Besides modifying the impeller geometry, new volute tongues are designed and tested on the rig. It is demonstrated that the shape of the volute tongue plays an important role in the tonal noise generation of the fan. Moreover, in order to find out whether or not it is possible to reduce the tonal noise level through a destructive phase-shift generation, stepped tongues are comprehensively investigated by means of numerical simulations and experimental measurements.

Effects of trailing edge bluntness on airfoil tonal noise at low Reynolds numbers

The aim of this study is to analyze the effects of trailing edge bluntness on airfoil tonal noise generation and propagation at low Reynolds numbers. Several simulations are conducted for a NACA0012 airfoil at four different free-stream Mach numbers, $$M_{\infty }$$ = 0.2–0.5. The angle of incidence is set equal to 3°, and the Reynolds number based on the airfoil chord is $$Re_{c}$$ = 5,000. The effects of compressibility on sound generation and propagation are analyzed along with the effects of scattering by blunt trailing edges with four different radii of curvature. Sound generation by vortex shedding is computed by a hybrid method and an accurate two-dimensional direct calculation, and results are compared. The hybrid approach uses direct calculation for near-field source computations and the Ffowcs Williams–Hawkings equation as the acoustic analogy formulation. Numerical results show that the airfoil emits an intense “narrow-band” tone and that a thicker trailing edge emits higher noise levels than a thinner one since the magnitude of quadrupole sources is larger for the thicker configuration. Moreover, the spatial distribution of quadrupole sources shows that the peak quadrupole values are closer to the surface when the trailing edge is thicker, which, again, increases the scattered far-field noise.

Mechanism of tonal noise generation from circular cylinder with spiral fin

The pitch of the spiral finned tube influences seriously to the acoustic resonance in the heat exchanger. In this research, the flow characteristics in relating to the aeolian tone from the finned cylinder are studied by the numerical simulation. It is observed that the tonal noise generated from the finned tube at two pitch spaces. The ratio of the fin pitch to the cylinder diameter is changed at 0.11 and 0.27. The tone level increases and the frequency decreases with the pitch shorter. The separation flow from the cylinder generates the span-wise vortices, Karman vortices, and the separation flow from the fin generates the stream-wise vortices. When the fin pitch ratio is small, the stream-wise vortices line up to span-wise and become weak rapidly. Only the Karman vortices are remained and integrate in span. So the Karman vortex became large. This causes the low frequency and the large aeolian tone.

A review of flow-induced noise from finite wall-mounted cylinders

This paper reviews previous studies on the flow around and flow-induced noise produced by a finite wall-mounted cylinder (FWMC) of circular and square cross section. The flow around a square FWMC is somewhat understood. Conversely, the flow around a circular FWMC is not well understood, particularly because of the phenomenon of spanwise cellular shedding. The effect of the aspect ratio (the ratio of cylinder span to diameter) on the near wake structure has been most extensively studied in the literature. However, the effect of the height of the incoming boundary layer on the near wake structure has not been comprehensively investigated and is therefore unclear, despite it being identified as a major influencing parameter on the development of the wake. Similarly, only a few studies have been conducted on the flow-induced noise of FWMCs and there is a general lack of data on the flow-induced noise characteristics and mechanisms of an FWMCs in cross flow. Nevertheless, the amalgamation of the limited data set has led to several noise generation hypotheses: (1) lower frequency tonal noise generation is due to the development of separate structures along the span of both circular and square FWMCs and (2) that increasing the boundary layer thickness will promote tonal noise generation.

Tandem cylinder aerodynamic sound control using porous coating

This study is concerned with the application of porous coatings as a passive flow control method for reducing the aerodynamic sound from tandem cylinders. The aim here is to perform a parametric proof-of-concept study to investigate the effectiveness of porous treatment on bare tandem cylinders to control and regularize the vortex shedding and flow within the gap region between the two bluff bodies, and thereby control the aerodynamic sound generation mechanism. The aerodynamic simulations are performed using 2D transient RANS approach with k−ω turbulence model, and the acoustic computations are carried out using the standard Ffowcs Williams–Hawkings (FW–H) acoustic analogy. Numerical flow and acoustic results are presented for bare tandem cylinders and porous-covered cylinders, with different porosities and thicknesses. Experimental flow and acoustic data are also provided for comparison. Results show that the proper use of porous coatings can lead to stabilization of the vortex shedding within the gap region, reduction of the vortex shedding interaction with the downstream body, and therefore the generation of tonal and broadband noise. It has also been observed that the magnitude and the frequency of the primary tone reduce significantly as a result of the flow regularization. The proposed passive flow-induced noise and vibration control method can potentially be used for other problems involving flow interaction with bluff bodies.

Multiple pure tone noise prediction

This paper presents a fully numerical method for predicting multiple pure tones, also known as “Buzzsaw” noise. It consists of three steps that account for noise source generation, nonlinear acoustic propagation with hard as well as lined walls inside the nacelle, and linear acoustic propagation outside the engine. Noise generation is modeled by steady, part-annulus computational fluid dynamics (CFD) simulations. A linear superposition algorithm is used to construct full-annulus shock/pressure pattern just upstream of the fan from part-annulus CFD results. Nonlinear wave propagation is carried out inside the duct using a pseudo-two-dimensional solution of Burgers׳ equation. Scattering from nacelle lip as well as radiation to farfield is performed using the commercial solver ACTRAN/TM. The proposed prediction process is verified by comparing against full-annulus CFD simulations as well as against static engine test data for a typical high bypass ratio aircraft engine with hardwall as well as lined inlets. Comparisons are drawn against nacelle unsteady pressure transducer measurements at two axial locations as well as against near- and far-field microphone array measurements outside the duct.This is the first fully numerical approach (no experimental or empirical input is required) to predict multiple pure tone noise generation, in-duct propagation and far-field radiation. It uses measured blade coordinates to calculate MPT noise.

Numerical Investigations of Tonal Noise of Counter-Rotating Open Rotors

In the work are presented the results of a three dimensional computational analysis of tonal noise of two counter-rotating open rotors. The analysis was performed using 3DAS (3 Dimensional Acoustics Solver) CIAM in-house solver, designed for three dimensional calculation of turbomachinery tone noise generation, propagation and radiation in the near and the far acoustic fields. The results of the computations were compared with the results of experiments performed in the wind tunnel of Central Aerohydrodynamic Institute (TsAGI), Russia, within the project DREAM (Validation of radical engine architecture systems) of European Seventh Framework Programme (FP7). The comparison showed satisfactory correspondence between directivity diagrams for the tone noise on combinational harmonics.

Experimental investigation of aerofoil tonal noise generation

AbstractThe present study investigates the mechanisms associated with tonal noise emission from a NACA 0012 aerofoil at moderate incidence ($0^{\circ },1^{\circ },2^{\circ }$ and $4^{\circ }$ angle of attack) and with Reynolds numbers ranging from 100 000 to 270 000. Simultaneous time-resolved particle image velocimetry (PIV) of the aeroacoustic source region near the trailing edge and acoustic measurements in the far field are performed in order to establish the correspondence between the flow structure and acoustic emissions. Results of these experiments are presented and analysed in view of past research for a number of selected cases. Characteristics of the acoustic emission and principal features of the average flow field agree with data presented in previous studies on the topic. Time-resolved analysis shows that downstream convecting vortical structures, resulting from growing shear layer instabilities, coherently pass the trailing edge at a frequency equal to that of the dominant tone. Therefore, the scattering of the vortical structures and their associated wall pressure fluctuations are identified as tone generating mechanisms for the cases investigated here. Moreover, wavelet analysis of the acoustic pressure and velocity signals near the trailing edge show a similar periodic amplitude modulation which is associated with multiple tonal peaks in the acoustic spectrum. Periodic amplitude modulation of the acoustic pressure and velocity fluctuations on the pressure side are also observed when transition is forced on the suction side, showing that pressure-side events alone can be the cause.

Low noise blower fan for heating and cooling applications

A new low noise fan called foam blower fan is presented here for low noise heating, ventilation, and air conditioning (HVAC) applications. Traditional HVAC blower fans are made of discrete metal or plastic fan blades, which generate blade tonal noise when each blade interacts with blower cut-off or other housing structures. It is proposed here that foam wedge (porous media) is inserted between blades at strategic locations, to reduce the tonal noise generation. It is analytically and experimentally demonstrated in this paper that the proposed new foam fan can reduce discrete tonal noise at various operating speeds.

Advanced Numerical Methods for the Prediction of Tonal Noise in Turbomachinery—Part I: Implicit Runge–Kutta Schemes

This is the first part of a series of two papers on unsteady computational fluid dynamics (CFD) methods for the numerical simulation of aerodynamic noise generation and propagation. In this part, the stability, accuracy, and efficiency of implicit Runge–Kutta schemes for the temporal integration of the compressible Navier–Stokes equations are investigated in the context of a CFD code for turbomachinery applications. Using two model academic problems, the properties of two explicit first stage, singly diagonally implicit Runge–Kutta (ESDIRK) schemes of second- and third-order accuracy are quantified and compared with more conventional second-order multistep methods. Finally, to assess the ESDIRK schemes in the context of an industrially relevant configuration, the schemes are applied to predict the tonal noise generation and transmission in a modern high bypass ratio fan stage and comparisons with the corresponding experimental data are provided.