Tonal Noise Generation Research Articles

Experimental analysis of eigenmode frequency of an hydrofoil on trailing edge hydroelastic coupling generating tonal noise

The majority of anthropogenic noise in the oceans is due to commercial, military or tourist motorized navigation. Lifting surfaces such as hydrofoils, propeller blades, rudders and stabilizers can be prone to flow induceed vibrations and, under certain conditions, to produce tonal noise. Reducing vibration and noise generated by this type of structures, is therefore an important issue to limit noise pollution and to improve the acoustic discretion of ships. This study, carried out at the French Naval Academy Research Institute, highlights the hydroelastic coupling mechanism responsible of trailing edge vibrations and of lifting surface tonal noise evolving in laminar to turbulent transition regime. This experimental research was carried out in a hydrodynamic tunnel with a NACA0015 hydrofoil, for angles of incidence ranging from 0° to 12° and Reynolds numbers varying from 200,000 to 1,200,000. Laser vibrometry was used to specify the vibrational behavior of two identical hydrofoil made of aluminum and brass in order to estimate the influence of material and eigenfrequency on tonal noise generation. The results show that, contrary to the same mechanism in air, tonal noise of hydrofoil was strongly dependent of the natural frequency of the profile.

Effect of rotation speed fluctuation on rotor noise generation: A numerical and experimental study

The practical operations of rotors for unmanned aircraft systems can undergo time-varying rotation speeds due to various uncertainty factors. This work investigates the aeroacoustic effect of the rotor’s rotation speed fluctuation using both numerical and experimental approaches. The measured real-time angular speed signal reveals that a hovering rotor experiences highly deterministic rotation speed variations in one revolution, which are closely related to the motor property. Delayed detached eddy simulations are performed, and the fluctuating rotation speed is realized by controlling the mesh displacement at each time step. The results suggest that significant additional tones are produced at the excitation frequency and neighbour frequencies, even though the amplitudes of the rotation speed fluctuations at the dominant frequencies are much smaller than the mean rotation speed. By comparing the full signal to individual harmonic components, it is found that the phase difference between each mode is unimportant. Therefore, the total aeroacoustic effect can be regarded as a linear superposition of different harmonic components. Aeroacoustic source analysis is also performed to isolate the contribution of various source types and correlate the far-field noise with the near-field source distribution. The results show that the extra tonal noise is mostly dominated by the loading noise, with a weaker contribution from the thickness noise. The agreement between the experiments and the simulations indicates that the rotation speed fluctuation is likely responsible for the generation of middle-frequency tonal noise at harmonics of the blade passing frequency.

On the multiple tones in trailing edge noise of an aerofoil at low-to-moderate Reynolds number flows

In this work, the aeroacoustics of a NACA 0012 airfoil are studied at the Reynolds number ranges from 3.2×10^4 to 3.2×10^5, in which region there are controversies of tonal noise generation, especially for multiple tones. Particle image velocimetry and acoustic measurements are simultaneously performed in an anechoic wind tunnel. A dynamic acoustic feedback loop in different flow regimes is proposed to explain the noise generation mechanism. At the low Reynolds number, the existing tonal noise acts as an external force, trigging the upstream flow structures that will interact with the shedding vortices aft the trailing edge. Consequently, this vortex interaction at the hydrodynamic region can modulate the tonal noise amplitude, resulting in the observed secondary tonal noise. Intermittency of vortex dynamics occurs due to tonal noise amplitudes variation. With the increase of Reynolds number, the boundary-layer transition on the suction side leads to a weak tonal noise. If the Reynolds number is higher, the boundary-layer flow on the suction side is turbulent, leading to the dominance of broadband noise. However, flow instability on the pressure side is triggered by the acoustic waves such that the modulation process takes place again. The quantitative measurements of the boundary layer flow and acoustic signals provide insight into understanding the aeroacoustics of airfoils at low-to-moderate Reynolds number.

Acoustic and psychoacoustic characterisation of small-scale contra-rotating propellers

This paper reports an experimental investigation of noise radiation and psychoacoustic evaluation of contra-rotating propellers with different blade counts over multiple emission angles and thrust settings. A series of aeroacoustic and psychoacoustic tests were performed with a contra-rotating propeller rig at the University of Salford, UK, where both propellers’ blade count and their axial separation z were varied at static conditions and constant thrust. The sounds were analysed using conventional acoustic metrics, such as Sound Pressure Level (SPL) and Sound Power Level for tonal, broadband, and overall noise, and using the psychoacoustic Sound Quality Metrics (SQMs) Loudness, Fluctuation Strength, Roughness, Tonality, and Impulsiveness. Configurations with axial separation-to-diameter ratio z/D between 0.2–0.4 exhibited a balance between tonal and broadband noise generation mechanisms, resulting in reduced noise levels and reduced annoyance for on-axis observers. The number of blades played a secondary role on determining noise levels: higher blade counts resulted in lower overall noise levels at larger axial separations, although at shorter distances (z/D<0.2) increasing the blade count led to higher noise levels. The SQMs Loudness, Fluctuation Strength, and Tonality were found to also reach a minimum at z/D=0.3, although increasing the number of propeller blades led to a decrease in Loudness but not in Fluctuation Strength and Tonality. The dominant noise generation mechanisms observed here present a directivity null perpendicular to the propellers’ axis, and no clear trends were observed for acoustic and psychoacoustic metrics at this emission angle. A listening experiment was set up to validate the results of the acoustic and psychoacoustic analysis, and its results suggest that the reported noise annoyance is driven by Loudness, Fluctuation Strength and Tonality metrics (p<0.01) at on-axis radiation, while at 90° emission angle, Loudness (p<0.01) and Tonality (p<0.05) are the main contributors to noise annoyance.

Suppression of deep cavity aeroacoustics at low Mach number by localized surface compliance

A unique concept of utilizing localized surface compliance is proposed to suppress deep cavity aeroacoustics at a low Mach number. The core idea is to provide local absorption of the energy of aeroacoustic processes supporting cavity flow self-sustained feedback loop responsible for tonal noise generation. The concept is studied with a flow past cavity of length-to-depth ratio of 0.4 at freestream Mach number 0.09 and Reynolds number based on cavity length 4 × 104 using high-fidelity, two-dimensional direct aeroacoustic simulation. Having confirmed the replication of key aeroacoustic processes in the numerical solution through careful validation, localized surface compliance in the form of an elastic panel is strategically introduced to modify every process for cavity noise suppression. The panel natural frequency is set equal to the feedback loop characteristic frequency to facilitate its flow-induced structural resonance for energy absorption. Suppression of cavity noise pressure and power levels by 3.8 and 4.8 dB, respectively, is successfully achieved, together with an unforeseen cavity drag reduction by almost 19%. Comprehensive wavenumber–frequency analyses of the coupled aeroacoustics and flow-induced panel vibration are conducted to uncover the physical mechanism of noise suppression. The results show that the same type of aeroacoustic feedback loop occurs, but its efficacy is significantly reduced due to the exhaustion of aeroacoustic process energy to the flow-induced vibrating panel. The proposed concept is confirmed to be feasible in terms of giving remarkable cavity noise and drag suppression, yet it retains the basic problem geometry intact, which are considered important in many practical applications.

Inlet gap effect on aerodynamics and tonal noise generation of a voluteless centrifugal fan

In this paper, the gap effects on the aerodynamics and tonal noise generation of voluteless centrifugal fans are studied based on different gap geometries. The study is motivated by the state of the art of this type of fan, for which the tonal noise generation due to the gap turbulence has not been addressed concerning the gap geometry, while a recent study reported that there is tonal noise at the blade passing frequency (BPF) from the gap turbulence. We simulate the configurations using a hybrid method coupling the improved delayed detached eddy simulation (IDDES) with Formulation 1A of Farassat. Our simulation shows regions with high vorticity magnitudes in the channel between two blades near the trailing edges close to the shroud. The turbulence renders a uniform pressure rise. By changing the gap design, the turbulent regions can be reduced. The configurations show a similar trend of the root mean square (RMS) pressure on the blade leading edge (BLE), largest at the shroud, and decays when the distance to the gap increases. The gap designs affect the amplitude of the RMS pressure, which is connected to the BPF. Spectral analysis is performed for the surface pressure fluctuations and the sound pressure upstream of the fan. The surface pressure fluctuations show that, for all cases, the regions with high energy are identical to the locations where the gap turbulence evolves and accounts for the impingement on the BLE. The amplitude of the tonal noise at the BPF differs between the cases.

Effect of Mach number on the aeroacoustic feedback loop generating airfoil tonal noise

Airfoil tonal noise emission at low-to-moderate Reynolds number and flow conditions featuring a laminar separation bubble close to the trailing edge is often related to an aeroacoustic feedback mechanism and, therefore, the Mach number is a primary parameter for the flow field and noise generation. This study experimentally explores the effect of the Mach number on airfoil tonal noise generation in the nominally incompressible flow regime. Using airfoil profiles of different chord lengths, the Mach number is varied for a constant Reynolds number. Acoustic and flow field measurements for a range of combinations of Reynolds and Mach numbers were conducted. At zero incidence, the tonal noise regime in the Reynolds number domain is found to be sensitive to the Mach number. At non-zero angle of attack (2°), the noise generation is found to be dominated by vortex shedding over a separation bubble on the pressure side. The details of the separation bubble and shedding process depend on the Mach number. The frequencies of the dominant tones and the frequency intervals between tones increase with the Mach number. Moreover, the measured frequency interval can be collapsed using a relation based on the aeroacoustic feedback loop model. The relation is rewritten to separate the effects of the Reynolds and Mach numbers. As a result, the dependence on the Mach number is identified and tested. In contrast, the tonal noise level shows a more complex dependence on details of the laminar separation bubble and the vortex shedding process.

Switch of tonal noise generation mechanisms in airfoil transitional flows

Large eddy simulations are performed to study tonal noise generation by a NACA0012 airfoil at an angle of attack ${\alpha = 3}$ deg. and freestream Mach number of ${M_{\infty} = 0.3}$. Different Reynolds numbers are analyzed spanning ${0.5 \times 10^5 \le Re \le 4 \times 10^5}$. Results show that the flow patterns responsible for noise generation appear from different laminar separation bubbles, including one observed over the airfoil suction side and another near the trailing edge, on the pressure side. For lower Reynolds numbers, intermittent vortex dynamics on the suction side results in either coherent structures or turbulent packets advected towards the trailing edge. Such flow dynamics also affects the separation bubbles on the pressure side, which become intermittent. Despite the irregular occurrence of laminar-turbulent transition, the noise spectrum depicts a main tone with multiple equidistant secondary tones. Increasing the Reynolds number leads to a permanent turbulent regime on the suction side that reduces the coherence level causing only small scale turbulent eddies to be observed. Furthermore, the laminar separation bubble on the suction side almost vanishes while that on the pressure side becomes more pronounced and permanent. As a consequence, the dominant noise generation mechanism becomes the vortex shedding along the wake.

Coupled structural resonance of elastic panels for suppression of airfoil tonal noise

In this paper the aeroacoustic characteristics of coupled flow-induced vibration of elastic panels mounted on an airfoil is numerically studied to uncover its potential for suppression of airfoil tonal noise. The proposed approach utilizes the inter-dynamical structural coupling between two elastic panels which maintain a synchronized flow-induced vibration pattern to weaken the flow fluctuations responsible for tonal noise generation. In the study, a NACA0012 airfoil at a low Reynolds number of 5×104 and an angle of attack of 5° is taken as the basic structure on which various panel configurations are attempted. The results of preliminary analysis of aeroacoustic-structural interactions of various panel configurations, using a reduced-order modeling approach, reveal that an airfoil-panel configuration with two elastic panels separated by approximately one convective wavelength and mounted in a region exposed to high boundary layer flow instabilities yields optimal noise suppression. Comprehensive aerodynamic and acoustic analyzes of airfoil-panel configurations, using the results of high fidelity direct aeroacoustic simulation, reveal that an average and maximum noise suppression up to 7.6 dB and 7.9 dB respectively can be achieved with negligible sacrifice on overall airfoil aerodynamics when strongly coupled structural resonance between the panels prevails. The magnitude of noise suppression is higher than twice of that from the configuration mounted with a single panel. This fact firmly illustrates the synergy of coupled flow-induced structural resonance of the panels prevailing in noise suppression. Nonlinear fluid–structure interactions of coupled resonant panels in airfoil-panel configuration are characterized by extensive spectral analyzes in frequency and wavenumber domains as well as correlation analyzes. The panels exhibit highly synchronized dynamic behaviors and run into limit cycle oscillations. Their strongly coherent structural responses are proven able to provide a more effective suppression of the boundary layer instabilities to be scattered at airfoil trailing edge as noise. The upstream propagating component of the generated noise does not interfere the coupled structural resonance of the panels as revealed from their weak correlations. Key characteristics required for the design of strongly coupled structural resonant panels for effective absorption of flow fluctuation energy are discerned and discussed.

Experimental investigation of aerofoil tonal noise at low Mach number

This paper presents an experimental investigation of aerofoil tones emitted by a controlled-diffusion aerofoil at low Mach number ( $0.05$ ), moderate Reynolds number based on the chord length ( $1.4 \times 10^{5}$ ) and moderate incidence ( $5^{\circ }$ angle of attack). Wall-pressure measurements have been performed along the suction side of the aerofoil to reveal the acoustic source mechanisms. In particular, a feedback loop is found to extend from the aerofoil trailing edge to the regions near the leading edge where the flow encounters a mean favourable pressure gradient, and consists of acoustic disturbances travelling upstream. Simultaneous wall-pressure, velocity and far-field acoustic measurements have been performed to identify the boundary-layer instability responsible for tonal noise generation. Causality correlation between far-field acoustic pressure and wall-normal velocity fluctuations has been performed, which reveals the presence of a Kelvin–Helmholtz-type modal shape within the velocity disturbance field. Tomographic particle image velocimetry measurements have been performed to understand the three-dimensional aspects of this flow instability. These measurements confirm the presence of large two-dimensional rollers that undergo three-dimensional breakdown just upstream of the trailing edge. Finally, modal decomposition of the flow has been carried out using proper orthogonal decomposition, which demonstrates that the normal modes are responsible for aerofoil tonal noise. The higher normal modes are found to undergo regular modulations in the spanwise direction. Based on the observed modal shape, an explanation of aerofoil tonal noise amplitude reduction is given, which has been previously reported in modular or serrated trailing-edge aerofoils.

Effect of dual vortex shedding on airfoil tonal noise generation

To improve understanding of dual vortex shedding over a trailing edge with regard to airfoil tonal noise generation, synchronized velocity and noise measurements are conducted on a NACA (National Advisory Committee for Aeronautics) 0012 airfoil at angle of attack near zero and a Reynolds number of 220 000. Instantaneous flow fields obtained by particle image velocimetry show the development of separated shear layer, vortex roll-up, and vortex breakup near the airfoil trailing edge. The time-averaged flow fields feature separation bubbles on both sides, and the root mean square values of streamwise velocity fluctuations show triple peak structures. The velocity spectra agree well with the noise spectra in terms of broadband humps and discrete tones. Through proper orthogonal decomposition (POD) analysis, the most energetic modes are identified, which represent global structures in the flow field. The trailing edge noise being an integral effect of the velocity fluctuation near the trailing edge, the POD analysis provides an alternative view for understanding the noise generation mechanism. The first and second modes are dominated by out-of-phase vortex shedding along the airfoil surface and in the near wake, which dominates the high amplitude noise emission process. The third and fourth modes represent in-phase vortex shedding, which dominates the low amplitude noise emission process. The noise source region is determined by the correlation between the velocity and sound pressure, which shows approximately the same periodic pattern as the first and second POD modes.

Tonal noise of voluteless centrifugal fan generated by turbulence stemming from upstream inlet gap

Volutes for radial-flow turbomachines (e.g., centrifugal fans and pumps) are spiral funnel-shaped casings that house rotors. Their function is to guide the flow from rotors to outlets and maintain constant flow speeds. Under specific conditions, however, volutes are removed (termed voluteless) to reduce flow losses and noise. In this paper, a generic voluteless centrifugal fan is investigated for the tonal noise generation at an off-design operation point. In contrast to typical tonal noise sources induced by the fan blades, we find out that another predominant source is the turbulence stemming from the clearance gap between the fan front shroud and the inlet duct. The turbulence evolves along with the front shroud and is swept downstream to interact with the top side of the blade leading edge. An obvious additional tone is observed at 273 Hz other than the blade passing frequency (BPF0) and relevant harmonic frequencies. By coarsening the mesh resolution near the inlet gap and front shroud in the simulations, we artificially deactivate the gap turbulence. Consequently, the tone at 273 Hz disappears completely. The finding indicates that the interaction between the gap turbulence and blades accounts for the tone. As the gap turbulence exists near the front shroud, this rotating wall introduces rotational momentum into the turbulence due to skin friction. Hence, this tonal interaction frequency is smaller than BPF0 with a decrement of the fan rotation frequency. To the authors' knowledge, this is the first time that voluteless centrifugal fans are studied for the gap-turbulence noise generation.

A study of the effect of airfoil thickness on the tonal noise generation of finite, wall-mounted airfoils

The noise generated by finite, wall-mounted airfoils strongly affects the total noise from aerodynamic applications such as small unmanned aerial vehicles, micro wind turbines and cooling fans. In an attempt to understand the effect of airfoil thickness on the trailing edge tonal noise generated by such airfoils, detailed wind tunnel experiments have been performed on two symmetric NACA-type airfoils with aspect ratio of 1.2 and nominal thicknesses of 12% and 18% of the chord length. The measurements included microphone array measurements, hot-wire measurements in the near wake of the airfoils and surface flow visualizations. It was found that the thickness has a strong impact on the tonal noise generation. At non-lifting conditions, the thinner airfoil generates tonal noise, while the thicker airfoil radiates broadband noise. At a slightly higher angle of attack (6∘), both airfoils generate tonal noise, but with different frequency and side-tone characteristics. Further increasing the angle (to 12∘) resulted in the generation of tonal noise from the thicker airfoil, while the far field spectrum obtained for the thinner airfoil has broadband character. This is attributed to the tip flow having stronger influence over the airfoil span in the case of the thinner airfoil leading to suppression of the tonal noise mechanism. Maps of the sound source locations, flow visualizations as well as detailed hot-wire measurements also provide further insight into the sound source mechanisms.

Numerical noise prediction and source identification of a realistic landing gear

Noise predictions of realistic landing gear configurations are obtained combining high-fidelity CFD simulations and the Ffowcs Williams–Hawkings (FWH) acoustic analogy. The aeroacoustic prediction of such a complex aeronautical system depends on geometry fidelity and the ability of the mesh and numerical method to resolve important flow features responsible for noise generation. To understand the role of small components in the far-field noise predictions, different setups are analyzed including a detailed full complexity landing gear and a simplified, yet realistic, configuration. For the latter case, two different setups in terms of mesh resolution and topology are analyzed. For the finer mesh, refinement is applied in regions with strong pressure fluctuations and also close to edges where shear layers develop flow instabilities. We also compare noise predictions obtained by a landing gear installed on a full aircraft with those computed only for a setup with the bottom half of the fuselage. An assessment of the solutions obtained from solid and permeable FWH surfaces is presented for different configurations of permeable surfaces. One objective of this work consists in the identification and analysis of noise sources in the landing gear. For this task, we first employ acoustic analogy to individual components of the landing gear to identify potential sources of tonal noise. Then, proper orthogonal decomposition is applied to identify mechanisms of noise generation at specific frequencies. It is shown that turbulent coherent structures are responsible for tonal noise generation by the towing link, wheel cavities and landing gear compartment. For the external cavity of the wheel, a Rossiter mode is excited and leads to resonance. We demonstrate that removing small-scale components of the full complexity geometry affects the flow characteristics inside the landing gear compartment and, consequently, its noise emission. In this case, the presence of small scale components leads to finer turbulent eddies that generate more noise at higher frequencies. These finer scales also reduce the coherence of larger turbulent structures, reducing amplitude of the low frequency band of the spectrum.

Aeroacoustic analysis of aerodynamically optimized joined-blade propeller for future electric aircraft at cruise and take-off

A novel propeller with the blade tips joined in pairs, named Boxprop, is designed and optimized for a conceptual electric aircraft using an efficient optimization platform. According to the thrust requirement of the electric aircraft at cruise, the Boxprop with optimal efficiency is down-selected from the Pareto front of thrust coefficient and propeller efficiency. Furthermore, the blade pitch angle is adjusted to meet the thrust requirement at take-off. It is found that the Boxprop is capable of suppressing tip vortices and inducing a wider wake behind blade tip in comparison to a conventional propeller usually shedding a concentrated tip vortex, which could potentially improve the propulsive efficiency. Afterwards, the aeroacoustic analysis performed by the hybrid integral method of Reynolds-Averaged Navier Stokes equations (RANS) and convected Ffowcs Williams and Hawkings (FW-H) equation shows that the tonal noise from the Boxprop with three joined blades operating at cruise is similar to a conventional three-bladed propeller, though being stronger than a conventional six-bladed propeller. Although the tip vortices have been suppressed by the joined-blade tips of the Boxprop, the corresponding tonal noise reduction is not prominent. Next, the Boxprop noise at take-off is studied. Unsteady RANS is used to resolve varying flow structures that become dominant under the take-off condition. Angle of attack (AOA) is found as an important factor influencing the noise generation. The radiated noise upstream and downstream of the propeller significantly intensifies due to increasing AOA. The AOA effects of the Boxprop follow a similar trend to a conventional propeller. The findings for the Boxprop aeroacoustics have enhanced the understanding of tip-vortex suppression techniques in connection with the tonal noise generation, which will be greatly helpful to the aeroacoustic design of Boxprop applied to electric aircraft in the future.

Passive airfoil tonal noise reduction by localized flow-induced vibration of an elastic panel

In this paper a novel passive control method is explored for airfoil tonal noise reduction using localized flow-induced vibration of a short elastic panel flush mounted on suction surface of a NACA 0012 airfoil at low Reynolds number. The key idea is to provide local absorption of energy of natural instabilities evolving in the laminar boundary layer by self-sustained flow-induced vibration of a properly designed panel. The panel ultimately acts to weaken the aeroacoustic feedback loop responsible for airfoil tonal noise radiation. Perturbation evolution method with acoustic broadband excitation is utilized to assess the tonal noise reduction potential with various combinations of elastic panel design parameters and optimal resonating and non-resonating elastic panel configurations are determined. Subsequently, direct aeroacoustic simulation of airfoil flow with optimal panel configurations are carried out to uncover the mechanism of tonal noise reduction using localized flow-induced vibration in a quantitative manner. Extensive analysis of numerical results reveals that a resonating panel located just ahead of the sharp growth of boundary layer instability within the airfoil separation bubble provides the strongest reduction of flow instabilities and an overall tonal noise reduction up to 3 dB. Such significant noise reduction is achieved without any sacrifice in the original aerodynamic characteristics of the airfoil. The outcomes of the study evidently suggest that the proposed passive control method with a localized flow-induced panel vibration is effective in suppressing the fundamental mechanism responsible for tonal noise generation of airfoil flow, making it a promising approach for modifying the acoustics of existing aerodynamic or wing profile operating at low Reynolds numbers.

Experimental evaluation of cylinder vortex shedding noise reduction using porous material

The tonal noise generation of a circular cylinder in a uniform flow is an important source of aerodynamic noise. It can be found at parts of the landing gear of airplanes, at pantographs of trains, at antennas and basically all other protruding parts of vehicles. This noise is due to the periodic shedding of vortices along the cylinder span. One method to reduce this noise is the use of flow permeable covers around the cylinders. In the present study, measurements were performed in an aeroacoustic wind tunnel on a large set of porous covered cylinders. In addition to varying the porous material, which is characterized by its air flow resistivity and its porosity, the thickness of the porous layer was varied as well. The measurements were performed at Reynolds numbers between 14,000 and 103,000 using microphones located in the acoustic far field. It was found that the porous covers lead to a notable narrowing of the vortex shedding tonal peak in the sound pressure level spectra, an effect that increases with increasing porosity and thickness and decreasing air flow resistivity of the porous layer. Based on the large set of experimental data, basic trends were derived for the estimation of the vortex shedding Strouhal number and the reduction in the energy in the vortex shedding peak using the method of linear regression. Constant temperature anemometry measurements in the wake of selected cylinders basically showed a similar narrowing of the vortex shedding peak in the spectra of the turbulent velocity fluctuations. In addition, the measurement of wake profiles showed a reduction in the mean velocity and the turbulence in the wake as well as a widening of the wake region, while an analysis of the spanwise coherence revealed that the cause of the overall noise reduction is not a breakup of spanwise turbulent structures. Rather, the results imply that viscous damping of turbulent flow pressure amplitudes by the porous material strongly contributes to the noise reduction.Graphic abstract

Experimental and Analytical Investigation of the Tonal Trailing-Edge Noise Radiated by Low Reynolds Number Aerofoils

An experimental and analytical study of the tonal trailing-edge noise of a symmetric NACA-0012 aerofoil and of a cambered SD7003 aerofoil has been achieved. It provides a complete experimental database for both aerofoils and improves the understanding of the underlying mechanisms. The analysis stresses the high sensitivity of the tonal noise phenomenon to the flow velocity and the angle of attack. Several regimes of the noise emission are observed depending on the aforementioned parameters. The contributions of the pressure and the suction sides are found to vary with the flow parameters too. A special attention has been paid to the role of the separation bubble in the tonal noise generation. Hot-wire measurements and flow visualization prove that the separation bubble is a necessary condition for the tonal noise production. Moreover, the bubble must be located close enough to the trailing edge. Several tests with small-scale upstream turbulence confirm the existence of the feedback loop. Analytical predictions with a classical trailing-edge noise model show a good agreement with the experimental data; they confirm the cause-to-effect relationship between the wall-pressure fluctuations and the radiated sound. Finally, previously reported works on fans and propellers are shortly re-addressed to show that the tonal noise associated with laminar-boundary-layer instabilities can take place in rotating blade technology.

Wavelet analysis of multiple tonal phenomena generated from a simplified nose landing gear and a ring cavity

Discrete tones exist in the airframe noise spectra. For landing gear components, some cavities may generate multiple tones. The tonal frequencies can be accurately predicted and two different tonal noise generation mechanisms have been already proposed, namely fluid-acoustic feedback loop and acoustic resonance. However, the inherent temporal features and the relationships between multiple tones are rarely researched. This paper introduces the continuous wavelet transform method to analyze the acoustic signals from a simplified nose landing gear model and a ring cavity model, aiming at revealing the temporal features and excitation rules of these multiple tones. For the landing gear model, the wavelet analysis results show the mid-high frequency tones generated from the acoustic resonance phenomenon are randomly excited in time and the excitation states are independent of one another. For the ring cavity model, the low frequency tones generated from fluid-acoustic feedback loop are excited alternately and the primary acoustic energy switches from one to another in time, namely the mode switching mechanism. While among the high frequency tones generated from acoustic resonance, the second to the fourth tones satisfy the amplitude modulation mechanism and the other tones are randomly excited.

Influence of a weak adverse pressure gradient on the generation of tonal protuberance noise in a laminar boundary layer

Abstract When a two-dimensional protuberance is placed in an unstable laminar boundary layer at moderate Reynolds numbers, tonal noise can be generated by an acoustic feedback-loop mechanism, not unlike that in case of the trailing-edge noise of an airfoil. In this paper, the generation of tonal protuberance noise was examined experimentally, focusing on the influence of the weak adverse pressure gradient on the critical protuberance height for the onset of tonal noise. It was shown that the weak adverse pressure gradient could cause the critical protuberance height to decrease, while the frequencies of tonal noise were little changed in the present experimental condition. At the onset condition for the tonal noise, the separation bubble formed just upstream of the protuberance was found to be almost the same size in both the zero- and weak adverse-pressure-gradient boundary layers, despite of the difference in the instability nature in the absence of the protuberance. A linear stability analysis revealed that the separation bubble could amplify disturbances by a factor of one hundred or more within a short distance (several times the boundary-layer thickness) at the onset condition. Such rapid disturbance growth was requisite for the sustenance of the feedback-loop mechanism even for the adverse-pressure-gradient case at least in the present Reynolds number range.