Vortex Shedding Noise Research Articles

Aerodynamic noise reduction by plasma actuators for a flat plate with blunt trailing edge

An experimental study of active control of the vortex shedding narrowband tonal noise radiated near the blunt trailing edge of a flat plate with elliptical leading edge was performed using three different configurations of the single dielectric barrier discharge (DBD) plasma actuators. These devices can produce electric winds in the tangential, downward and spanwise directions, respectively, near the blunt trailing edge. Acoustics and flow measurements were carried out simultaneously at Reynolds numbers between 0.75 × 105 and 4 × 105, based on the flat plate chord length, inside an aeroacoustic facility. The range of alternating-current (AC) input voltages to these plasma actuators was relatively low at < 5 kV. The “tangential” plasma actuator is not very effective in the suppression of vortex shedding tonal noise (maximum 1–2 dB reduction), although the spatial distribution of the wake coherent modes calculated from the proper orthogonal decomposition becomes more compact than that produced by the baseline plasma off case, resulting in a shift of the tone frequency to a higher value. The “downward” plasma actuator can suppress the vortex shedding noise almost completely at the tone frequency (about 15 dB reduction at input voltage of 4.2 kV). The mechanism is related to the induced plasma jet acting as a virtual barrier to inhibit the interaction between the upper and lower separating shear layers, and to delay the formation of the vortex shedding. The “spanwise” plasma actuator, which can project array of streamwise vortices into the wake and compartmentalise the vortex shedding across the span, demonstrated a more superior tonal noise reduction capability at low input voltage (about 12 dB reduction at 3.0 kV). It is found that the plasma-induced jet magnitudes between 9 and 10% and 7% of the freestream velocity for the downward and spanwise plasma actuators, respectively, are already sufficient to achieve an effective reduction of the vortex shedding tonal noise.

A generalized sound extrapolation method for turbulent flows

Sound extrapolation methods are often used to compute acoustic far-field directivities using near-field flow data in aeroacoustics applications. The results may be erroneous if the volume integrals are neglected (to save computational cost), while non-acoustic fluctuations are collected on the integration surfaces. In this work, we develop a new sound extrapolation method based on an acoustic analogy using Taylor’s hypothesis (Taylor 1938 Proc. R. Soc. Lon. A 164 , 476–490. ( doi:10.1098/rspa.1938.0032 )). Typically, a convection operator is used to filter out the acoustically inefficient components in the turbulent flows, and an acoustics dominant indirect variable D c p ′ is solved. The sound pressure p ′ at the far field is computed from D c p ′ based on the asymptotic properties of the Green’s function. Validations results for benchmark problems with well-defined sources match well with the exact solutions. For aeroacoustics applications: the sound predictions by the aerofoil–gust interaction are close to those by an earlier method specially developed to remove the effect of vortical fluctuations (Zhong &amp; Zhang 2017 J. Fluid Mech. 820 , 424–450. ( doi:10.1017/jfm.2017.219 )); for the case of vortex shedding noise from a cylinder, the off-body predictions by the proposed method match well with the on-body Ffowcs-Williams and Hawkings result; different integration surfaces yield close predictions (of both spectra and far-field directivities) for a co-flowing jet case using an established direct numerical simulation database. The results suggest that the method may be a potential candidate for sound projection in aeroacoustics applications.

Prediction of vortex-shedding noise from the blunt trailing edge of a flat plate

A time-domain hybrid approach for aerodynamic noise prediction is developed based on a discrete vortex model (DVM) for the unsteady incompressible flow simulation and the acoustic perturbation equations (APE) for the acoustical field computation. The aim is to assess the applicability of the present DVM-APE method to the problems where sound is generated by the large-scale coherent flow structures. The hybrid DVM-APE approach is employed to predict the vortex-shedding noise from the blunt trailing edge of a flat plate. Simulations are implemented on flat plates with different thicknesses in a certain range of low Mach numbers, in order to identify the scaling dependence of the vortex-shedding noise on the freestream speed as well as the plate thickness. Acoustical directivity patterns at different Helmholtz numbers are presented, and agreements are achieved when compared with previous studies. A comparison of the sound pressure level spectrum between the present DVM-APE simulation and the published experimental results is also presented, showing good agreements for both the peak frequencies and the sound pressure levels.

Numerical Analysis of Flatback Trailing Edge Airfoil to Reduce Noise in Power Generation Cycle

Turbo machinery is an essential part in the power generation cycle. However, it is the main source of noise that annoys workers and users, and contributes to environmental problems. Thus, it is important to reduce this noise when operating the power generation cycle. This noise is created by a flow instability on the trailing edge of the rotor blade—an airfoil that becomes a section of the rotor blade of the rotating machine—manufactured as a blunt trailing edge (T.E.), with a round or flatback shape, rather than the ideal sharp T.E. shape, for the purposes of production and durability. This increases the tonal noise and flow-induced vibrations at a low frequency, owing to vortex shedding behind T.E. when compared with a sharp T.E. In order to overcome this problem, the present study investigates the oblique T.E. shape using numerical simulations. In order to do so, flow was simulated using large eddy simulation (LES) and the noise was analyzed by acoustic analogy coupled with the LES result. Once the simulation results were verified using the flatback airfoil measurements of the Sandia National Laboratories, numerical prediction was performed to analyze the flow and the noise characteristics for the airfoils, which were modified to have oblique trailing edge angles of 60°, 45°, and 30°. From the simulation results of the oblique T.E. airfoil, it could be seen that the vortex shedding frequency moves in accordance with the oblique angle and that the vortex shedding noise characteristics change according to the angle, when compared to the flatback T.E. airfoil. Therefore, it is considered that modifying the flatback T.E. airfoil with an appropriate oblique angle can reduce noise and change the tonal frequency to a bandwidth that is suitable for mechanical systems.

Establishment of a Beamforming Dataset on Basic Models of Low-Speed Axial Fan Blade Sections

The paper presents wind tunnel experiments, supplemented with phased array microphone measurements, on 2D basic models of low-speed axial fan blade sections: a flat plate, a cambered plate, and a RAF6-E airfoil. It aims at documenting the establishment of an acoustic beamforming dataset for the three profiles. The phased array microphone measurements offer spatially resolved information on the generated noise. The measurement setup enables the correlation of the streamwise evolution of the blade boundary layer with the associated noise characteristics. The dataset incorporates a wide range of incidence and Reynolds-numbers investigated. The present paper is confined to reporting on experimental results for arbitrarily selected representative incidences, Reynolds numbers, frequency bands, and profiles. The paper outlines a methodology for the evaluation and representation of the beamforming data in the following forms: source strength level based third-octave spectra obtained using background noise subtraction; maps presenting the loci of source strength level maxima; noise source maps for frequency bands of anticipated vortex shedding noise.

Numerical study on reduction of aerodynamic noise around an airfoil with biomimetic structures

A biomimetic airfoil featuring leading edge waves, trailing edge serrations and surface ridges is proposed in this study, based on flow control with each section meeting the NACA 0012 airfoil profile. Numerical simulations have been conducted to compare aerodynamic and acoustic performances between the NACA 0012 and biomimetic airfoils. These simulations utilize the large eddy simulation (LES) method and aeroacoustic analogy at an angle of attack of 0° and a Reynolds number of 1.0×105, based on using the airfoil chord as the characteristic length. The simulation results reveal the overall sound pressure levels (OASPLs) for all frequencies and at the seven observer points around the biomimetic airfoil, and a decrease of 13.1–13.9dB is observed, whereas the drag coefficient is almost unchanged. The biomimetic structures can transform the shedding vortices in laminar mode for the NACA 0012 airfoil to regular horseshoe-type vortices in the wake, and reduce the spanwise correlation of the large-scale vortices, thereby restrain the vortex shedding noise around the biomimetic airfoil.

An improved prediction model of vortex shedding noise from blades of fans

The main source of the noise of an axial flow fan is the fluctuating pressure field on blade surfaces caused by the shedding of vortices at the trailing edge of blades. An analytical model to predict the vortex shedding noise generated at the trailing edge of blades of axial flow fans was proposed by Lee in 1993. In this model, for mathematical convenience, an idealized vortex street is considered. However, the agreement between the analytical results and the experimental data needs to be improved because of the simplification about the Karman vortex street in the wake of blade. In the present study, a modified model is proposed based on the prediction model by Lee. The boundary layer theory is used to analyze and calculate the boundary layer development on both the pressure and the suction sides of blades. Considering the effect of boundary layer separation on the location of noise source, the predicted overall sound pressure level compares favorably with the experimental data of an axial fan. In the calculation of A-weighted sound pressure level (LA), considering the effect of static pressure on radiate energy, the predicted broadband noise with the modified model compares favorably with the experimental data of a multiblade centrifugal fan.

Vortex shedding noise of a cylinder with hairy flaps

This study describes the modification of acoustic noise emitted from cylinders in a stationary subsonic flow for a cylinder equipped with flexible hairy flaps at the aft part as a passive way to manipulate the flow and acoustics. The study was motivated by the results from previous water tunnel measurements, which demonstrated that hairy flaps can modify the shedding cycle behind the cylinder and can reduce the wake deficit. In the present study, wind tunnel experiments were conducted on such a modified cylinder and the results were compared to the reference case of a plain cylinder. The acoustic spectrum was measured using two microphones while simultaneously recording the flap motion. To further examine the flow structures in the downstream vicinity of the cylinder, constant temperature anemometry measurements as well as flow visualizations were also performed. The results show that, above a certain Reynolds number, the hairy flaps lead to a jump in the vortex shedding frequency. This phenomenon is similarly observed in the water flow experiments as a jump in the non-dimensional Strouhal number that is related to the change of the shedding cycle. This jump appears to be coupled to a resonant excitation of the flaps. The specific Reynolds number at which the jump occurs is higher in the present case, which is attributed to the lower added mass in air as compared with the one in water. The flow visualizations confirmed that such action of the flaps lead to a more slender elongated shape of the time-averaged separation bubble. In addition, the hairy flaps induce a noticeable reduction of the tonal noise as well as broadband noise as long as the flaps do not touch each other.

Vortex shedding noise from a beveled trailing edge

Coherent vortex shedding from blunt and beveled trailing edges generates tonal noise, which is usually undesired. To obtain a better understanding of the noise generation under such conditions, the flow field around a beveled trailing edge was characterized for Reynolds numbers based on the bluntness ranging from 2.5 × 104 to 5.1 × 104. Flow field statistics were obtained by means of planar high-speed two-component and stereoscopic particle image velocimetry measurements. The development of the shear layers and vortex roll-up is described in the present study. Related length scales, the vortex formation length, and wake thickness parameter were derived from the measurements. Noise emission due to vortex shedding was predicted from an analytic solution, derived from diffraction theory and the reversed Sears’ problem, and compared to acoustic phased array measurements. This approach has previously been shown to provide accurate results for sharply truncated edges, but questions with regard to the applicability with different trailing edge geometries remained open. The prediction required the auto-spectral density, correlation length, and convective velocity of the upwash velocity component in the vortex formation region. Direct application with data obtained from particle image velocimetry measurements showed an overestimation of about 20 dB when compared to the acoustic measurements. The results thus showed that the prediction of vortex shedding noise based on the simplified wake model and diffraction theory is not generally applicable.

Circular cylinders with soft porous cover for flow noise reduction

The use of porous materials is one of several approaches to passively control or minimize the generation of flow noise. In order to investigate the possible reduction of noise from struts and other protruding parts (for example components of the landing gear or pantographs), acoustic measurements were taken in a small aeroacoustic wind tunnel on a set of circular cylinders with a soft porous cover. The aim of this study was to identify those materials that result in the best noise reduction, which refers to both tonal noise and broadband noise. The porous covers were characterized by their air flow resistivity, a parameter describing the permeability of an open-porous material. The results show that materials with low air flow resistivities lead to a noticeable flow noise reduction. Thereby, the main effect of the porous cylinder covers is that the spectral peak of the aeolian tone due to vortex shedding appears much narrower, but is not suppressed completely. Based on the measurement results, a basic model for the estimation of the total peak level of the aeolian tone was derived. In addition to the minimization of the vortex shedding noise, a reduction of broadband noise can be observed, especially at higher Reynolds numbers. The noise reduction increases with decreasing air flow resistivity of the porous covers, which means that materials that are highly permeable to air result in the best noise reduction.

Numerical investigation of aeroacoustic characteristics of a circular cylinder in cross-flow and in yaw

It is well known that aerodynamic noise becomes dominant when the speed of high-speed trains exceeds about 300 km/h. The pantograph is one of the main aerodynamic noise sources, particularly in the presence of noise barriers which shield the sources on the lower part of the train more effectively. The pantograph consists of a number of slender bodies with different cross-sections. In the current research, the aerodynamic characteristics of a circular cylinder have been investigated through computational fluid dynamics simulations using a Delayed Detached-Eddy Simulation model. Then the aeroacoustic behaviour has been predicted by using Ffowcs Williams-Hawkings equation. Simulations have been carried out for various speeds, resulting in a wide range of Reynolds number, which includes subcritical, critical, and supercritical flow states. The results have been compared with experiments and give good agreement. As the pantograph arms are inclined to the flow, the effect of yaw angle is analyzed in this paper and the effect on vortex-shedding frequency and noise level is determined. Moreover, it is demonstrated that the critical Reynolds number, which determines the beginning of the critical flow state, is affected by the yaw angle. In addition, considering the high turbulence intensity on the train roof, a turbulent inflow with different levels of intensity has been considered and the results will be presented.

Numerical investigation of tandem-cylinder noise reduction using plasma-based flow control

AbstractThe noise of flow over tandem cylinders at $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}{\mathit{Re}}_D= 22\, 000$ and its reduction using single dielectric barrier discharge (SDBD) plasma actuators are simulated numerically both to confirm and extend experimental results. The numerical approach is based on large-eddy simulation (LES) for the turbulent flow field, a semi-empirical plasma actuation model, and Lighthill’s theory for acoustic calculation. Excellent agreement between LES and experimental results is obtained for both the baseline flow and flow with plasma control in terms of wake velocity profiles, turbulence intensity, and frequency spectra of pressure fluctuations on the downstream cylinder. The validated flow-field results allow an accurate acoustic analysis based on Lighthill’s equation, which is solved using a boundary-element method. The effectiveness of plasma actuators for reducing noise is clearly demonstrated. In the baseline flow, the acoustic field is dominated by the interaction between the downstream cylinder and the upstream wake. Through suppression of vortex shedding from the upstream cylinder, the interaction noise is reduced drastically by the plasma flow control, and the vortex-shedding noise from the downstream cylinder becomes equally important. At a free-stream Mach number of 0.2, the peak sound pressure level is reduced by approximately 16 dB. This suggests the viability of plasma actuation for active aeroacoustic control of airframe noise.

Prediction and control of trailing edge noise based on bionic airfoil

As a promising means, the passive porosity technology is used for the trailing-edge noise reduction of a bionic airfoil. The detailed two-dimensional Large Eddy Simulation is achieved to gain a better understanding of the prediction and passive control of trailing-edge noise source with the non-porous and porous treatment, respectively. The flow fields around the bionic airfoil indicate that the leading-edge separation causes both the noise contributors, i.e., the turbulent boundary layer and the vortex shedding. In addition, the effect of the porous trailing edge is substantiated in the distribution of the static pressure. The relevant noise also suggests a pronounced noise reduction potential in excess of 10 dB, but it has dependence on the flow resistivities. The two trailing-edge noise reduction mechanisms are characterized: (1) the suppression of the tonal vortex shedding noise; (2) the reduction of broadband turbulent boundary layer scattering noise. The findings may be used as reference in the design of silent aircraft.

J0910302 流れ中のフィン付き円柱から発生する渦放出音に及ぼすフィンピッチの影響([J091-03]流体関連の騒音と振動(3))

J0910302 流れ中のフィン付き円柱から発生する渦放出音に及ぼすフィンピッチの影響([J091-03]流体関連の騒音と振動(3))

Self-Noise Produced by an Airfoil with Nonflat Plate Trailing-Edge Serrations

This paper represents the results of an experimental study aimed at reducing the airfoil self-noise by the trailing-edge serration of four different sawtooth geometries (defined in the serration angle and length). These serrations have a common feature: all of the sawtooth patterns are cut directly into the trailing edge of a realistic airfoil. This configuration offers better structural strength and integrity. For the sawtooth trailing edges investigated here, the radiation of the extraneous vortex shedding noise in a narrowband frequency due to the partial bluntness at the serration roots is unavoidable. However, this narrowband component tends to be less significant provided that the serration angle is large and the serration length is moderate. Sound power was measured, and some of the sawtooth geometries have been shown to afford significant boundary-layer instability tonal noise and moderate turbulent broadband noise reductions across a fairly large velocity range. This paper demonstrates that a nonflat plate serrated trailing edge can also be effective in the self-noise reduction. Some experimental results are also presented in order to explain the self-noise mechanisms.

Airfoil self noise reduction by non-flat plate type trailing edge serrations

This paper represents the results of a preliminary study which aims to reduce the airfoil trailing edge self noise by employing non-flat plate type trailing edge serrations. This configuration offers better structural strength and integrity, as well as a more straightforward manufacturing process compared to the conventional flat plate type serrations. We found that the non-flat plate serration not only reduces the broadband self noise significantly, but also eliminates the high-frequency noise that was observed by others who used flat plate type serration. However, due to the presence of certain bluntness at the sawtooth root, vortex shedding noise in a narrow frequency bandwidth is also produced. This extraneous noise is found to be less significant if a wider-angle serration is used. To increase the effectiveness of the proposed serration geometry a hybrid configuration composed of a non-flat plate type trailing edge serration with woven-wire mesh screen is employed for the reduction of the narrowband vortex shedding noise.

Trailing-Edge Noise Predictions Using Compressible Large-Eddy Simulation and Acoustic Analogy

The present investigation of airfoil trailing-edge noise generation and propagation concerns the broadband noise that arises from the interaction of turbulent boundary layers with the airfoil trailing edge and the tonal noise that arises from vortex shedding generated by laminar boundary layers. Large-eddy simulations are conducted for a NACA0012 airfoil with rounded trailing edge for two flow configurations with different freestream Mach numbers (M1 � 0:115 and 0.4) for an angle of incidence of 5 deg. The Reynolds number based on the airfoil chord is fixed at Rec � 408;000. The acoustic predictions are performed by the Ffowcs Williams and Hawkings acoustic analogy formulation and incorporate convective effects. Surface and volume integrations of dipole and quadrupole source termsappearingintheFfowcsWilliamsandHawkingsequationareperformedusingathree-dimensionalwideband multileveladaptivefastmultipolemethodtoacceleratethecalculationsofaeroacousticintegrals.TheeffectsofMach numberandnonlinearquadrupolesources areassessed.Nonlinear quadrupolenoise sourcesplay animportant role infar-fieldsoundradiationforthehigherMachnumber flowconfiguration,M1 � 0:4.Ascalingstudyisperformed and, although surface pressure spectra scale with hydrodynamic scaling, far-field acoustics has a more complex behavior. The broad vortex-shedding tone generated by the laminar boundary layer vortex shedding noise causes a departure from the classical fifth power-law scaling, a behavior also seen in previous experiments.

Noise Radiation from Single and Multiple Rod Configurations

Acoustic measurements were performed on single and multiple rod configurations to study the effect of Reynolds number, surface roughness, freestream turbulence, proximity and wake interference on the radiated noise. The Reynolds number ranged from 3.8 × 103to 105. Directivity measurements were performed to determine how well the dipole assumption for the radiation of vortex shedding noise holds for the different model configurations tested. The dependence of the peak Sound Pressure Level on velocity was also examined. Several concepts for the reduction of the noise radiating from cylindrical rods were tested. It was shown that wire wraps and collar distributions could be used to significantly reduce the noise radiating from rods in tandem configurations.

Extensions and limitations of analytical airfoil broadband noise models

The present paper is a state-of-the-art of a special class of analytical models to predict the broadband noise generated by thin airfoils in a flow, either clean or disturbed. Three generating mechanisms are addressed, namely the noise from the impingement of upstream turbulence called turbulence-interaction noise, the noise due to the scattering of boundary-layer turbulence as sound at the trailing edge for an attached flow called trailing-edge noise, and the noise generated due to the formation of a coherent vortex shedding in the near wake of a thick trailing edge, called vortex-shedding noise. Different analytical models previously proposed for each mechanism are reviewed, as declinations of the same basic approach inherited from the pioneer work performed by Amiet in the seventies and based on an extensive use of Schwarzschild's technique. This choice is only an alternative to other models available in the literature and is made here for the sake of a unified approach. Issues dealing with the input data and related to the practical applications to fan noise predictions are rapidly outlined. The validity of the models is ckeched against dedicated experiments with thin airfoils and the limitations as the real configurations depart from the model assumptions are pointed out.

Identification of an aeroacoustic source using the inverse boundary element method

The use of the inverse boundary element method (BEM) to identify aeroacoustic noise sources is explored. In the suggested approach, sound pressure at locations out of the flow field are measured, followed by the reconstruction of acoustic particle velocity on the surface where the noise is generated. Using this reconstructed acoustic particle velocity, the acoustic response elsewhere in the field can be predicted. This paper demonstrates that good results can be obtained using a manageable number of sound pressure measurements that need not be in the flow field or conformal to the source. The first example is an open-ended duct without flow where measurements made around the periphery of the duct are used to reconstruct the particle velocity on the opening. The second example applies the approach to predicting vortex shedding noise resulting from airflow over a cylindrical rod. The reconstructed particle velocity is used to predict sound pressure in the field. The predicted sound pressure agreed with measured results