Airfoil Self-noise Research Articles

Design optimization of wind turbine blades for reduction of airfoil self-noise

To reduce airfoil self-noise from a 10 kW wind turbine, we modified the airfoil shape and planform of a wind turbine blade. To obtain the optimal blade design, we used optimization techniques based on genetic algorithms. The optimized airfoil was first determined based on a section of the rotor blade, and then the optimized blade was designed with this airfoil. The airfoil self-noise from the rotor blades was predicted by using a semi-empirical model. The numerical analysis indicates that the level of the airfoil self-noise from the optimized blade is 2.3 dB lower than that from the baseline blade at the rated wind speed. A wind tunnel experiment was also performed to validate the design optimization. The baseline and optimized rotors were scaled down by a factor of 5.71 for the wind tunnel test. The experimental results showed that airfoil self-noise is reduced by up to 2.6 dB.

Airfoil optimization for noise emission problem and aerodynamic performance criterion on small scale wind turbines

Noise emission is one of the major concerns in wind turbine industry and especially for small scale wind turbines, which are mostly erected to the urban areas; the concern is turning into a problem. This paper focuses on the optimization of six airfoils which are widely used on small scale wind turbines in terms of the noise emission and performance criteria and the numerical computations are performed for a typical 10 kW wind turbine. The main purpose of this optimization process is to decrease the noise emission levels while increasing the aerodynamic performance of a small scale wind turbine by adjusting the shape of the airfoil. The sources of the broadband noise emission are defined and their dominancy is investigated with respect to the operating conditions. While redesigning, together with the principals of reducing the airfoil self-noise, the aerodynamic prospects of increasing the performance have been taken into account. The codes which are based on aero-acoustic empirical models and a collection of well-known aerodynamic functions are used in this study. The results obtained from the numerical analysis of the optimization process have shown that, the considered commercial airfoils for small scale wind turbines are improved in terms of aero-acoustics and aerodynamics. The pressure sides of the baseline airfoils have been manipulated together with the trailing edge and redesigned airfoils have lower levels of noise emission and higher lift to drag ratios.

A case study of localization and identification of noise sources from a pitch and a stall regulated wind turbine

We have experimentally identified the noise-generation mechanisms of large modern upwind wind turbines (WTs). First, the sound measurement procedures of IEC 61400-11 were used in the field test, and noise emissions from two WTs were evaluated: a stall-controlled WT with powers of 1.5MW and a pitch-regulated WT with powers of 660kW. One-third octave band levels were normalized using the scale law for the velocity dependence of the inflow broadband noise and airfoil self-noise. The results showed that for the 1.5MW WT, inflow turbulence noise was dominant over the whole frequency range. For the 660kW WT, the inflow broadband noise did not contribute across the whole audible frequency range. The distribution of noise sources in the rotor plane was visualized using a beam-forming measurement system (B&K 7768, 7752, and WA0890) consisting of 48 microphones. The array results for the 660kW WT indicated that all noise was produced during the downward movement of the blades. This finding was in good agreement with theoretical results obtained using an empirical formula that includes the effects of the convective amplification, directivity, and flow-speed dependence of the turbulence boundary-layer trailing edge noise. This agreement implies that this trailing edge noise is dominant over the whole frequency range in the case of the 660kW WT.

Wall pressure sources near an airfoil trailing edge under turbulent boundary layers

In the present work the wall pressure sources under turbulent boundary layers developing on the surface of an airfoil have been investigated in the region near the trailing edge. A trip wire placed on the surface of the airfoil close to the leading edge ensured a turbulent boundary layer for the two Reynolds numbers investigated. When the angle of attack was different than zero, for the lower Reynolds number, a laminar/transitional boundary layer was observed on the airfoil pressure surface due to relaminarisation of the flow. This study aimed at improving the understanding of the relationship between the vortical velocity field and its surface pressure signature which is a matter of relevance for the so-called airfoil self-noise and for the vibration phenomena observed in ship hulls. A NACA0012 airfoil of 30cm chord and aspect ratio of 1 placed at the exit of an open-circuit blower type wind tunnel was used in the investigation. Two different flow conditions corresponding to Reynolds numbers based on the chord of the airfoil of Rec=2×105 and 4×105 and three different angles of attack have been analysed. Simultaneous measurements of the unsteady surface pressure fluctuations and of the velocity field in the boundary layer and wake of the airfoil were performed, allowing the calculation of cross-correlations and cross-spectra between the two components of the velocity and the surface pressure. This has permitted the analysis of the surface pressure generating flow structures and revealed information about the sources of wall pressure fluctuations, which are closely related to those of the radiated noise.

Airfoil Trailing-Edge Blowing: Broadband Noise Prediction from Large-Eddy Simulation

Trailing-edge blowing has in recent studies found to be a potential technique for broadband noise reduction in turbomachinery applications with rotor-stator arrangement. The key-idea is to inject fluid into the wake of the rotor in such a manner that the wake will become momentumless and the turbulence structure will be modified by the enhanced mixing process. Ultimately, this wake manipulation should lead to a reduced aeroacoustic response of the downstream stator-vane with the modified turbulent wake of the rotor. The study presented within this paper investigates the trailing edge blowing mechanism by numerical means in a simplified configuration. A large-eddy simulation on a NACA 6512-63 airfoil without and with trailing-edge blowing is undertaken to investigate the effect of blowing on wall-pressure statistics of the blowing airfoil and also its wake turbulence structure and characteristics. The broadband self-noise of the airfoil is predicted by Amiet’s trailing edge noise theory. It appears that two competing mechanisms exist: a potentially increased airfoil self-noise due to the blowing jet interacting with the jet slot-lip and the trailing edge, and a reduction in interaction noise through manipulation of the wake turbulence and early wake-turbulence decay. Overall this study provides a first insight into some relevant parameters concerning the potential for broadband noise reduction through trailing-edge blowing.

Numerical simulation on the NACA0018 airfoil self-noise generation

Airfoil self-noise is a common phenomenon for many engineering applications. Aiming to study the underlying mechanism of airfoil self-noise at low Mach number and moderate Reynolds number flow, a numerical investigation is presented on noise generation by flow past NACA0018 airfoil. Based on a high-order accurate numerical method, both the near-field hydrodynamics and the far-field acoustics are computed simultaneously by performing direct numerical simulation. The mean flow properties agree well with the experimental measurements. The characteristics of aerodynamic noise are investigated at various angles of attack. The obtained results show that inclining the airfoil could enlarge turbulent intensity and produce larger scale of vortices. The sound radiation is mainly towards the upper and lower directions of the airfoil surface. At higher angle of attack, the tonal noise tends to disappear and the noise spectrum displays broad-band features.

Research on Design Methods and Aerodynamics Performance of CQU-DTU-B21 Airfoil

This paper presents the design methods of CQU-DTU-B21 airfoil for wind turbine. Compared with the traditional method of inverse design, the new method is described directly by a compound objective function to balance several conflicting requirements for design wind turbine airfoils, which based on design theory of airfoil profiles, blade element momentum (BEM) theory and airfoil Self-Noise prediction model. And then an optimization model with the target of maximum power performance on a 2D airfoil and low noise emission of design ranges for angle of attack has been developed for designing CQU-DTU-B21 airfoil. To validate the optimization results, the comparison of the aerodynamics performance by XFOIL and wind tunnels test respectively at Re=3×106 is made between the CQU-DTU-B21 and DU93-W-210 which is widely used in wind turbines.

Stochastic Method for Airfoil Self-Noise Computation in Frequency Domain

A stochastic Fourier method for the computation of synthetic solenoidal velocity uctuations is revised and applied to the problem of noise radiation from the trailing edge of a NACA 0012 airfoil. The method is used to compute the source term of the Howe’s acoustic analogy equation for an isentropic ow that is solved in frequency-domain using a FEM discretization. For each spectral component of the source term, a prescribed number of realizations of the synthetic source eld is generated by seeding the stochastic generator. The corresponding sound elds are then computed and nally averaged to track statistically converged noise spectra. For the present study, two-dimensional CFD/CAA computations are carried out and the linear system is solved using a direct method with multi-right-handside capabilities. Therefore, for each frequency, the computation of the dierent stochastic realizations is managed in a very ecient way. The self-noise spectra computed for an airfoil at dierent values of the free-stream velocity are compared with literature experimental data and semi-analytical predictions performed using the same RANS solutions. It is shown that the the CAA results are in good agreement with the semi-analytical results: both the free-stream velocity noise power law and the spectral content at each velocity compare quite well. Furthermore, a very good agreement between the CAA noise spectra and the measured ones is obtained for higher values of the free-stream velocity. Conversely, for lower velocities, the predicted noise levels, using both CAA and semi-analytical methods, are signicantly underestimated, in particular, in the high frequency regime. The cause of the disagreement can be inferred to the low-Reynolds behaviour of the employed RANS model.

Aerodynamic noise analysis of large horizontal axis wind turbines considering fluid–structure interaction

Aerodynamic noise is one of the most serious barriers in wind energy development. To develop technologies for wind turbine noise reduction and assessment, noise needs to be predicted precisely with special consideration given to blade flexibility. The numerical tool, WINFAS, which can simulate fluid–structure interaction, consists of three parts: the first part, the Unsteady Vortex Lattice Method, analyzes aerodynamics; the second part, the Nonlinear Composite Beam Theory, analyzes structure; and the third part uses a semi-empirical formula to analyze airfoil self-noise and the Lowson’s formula to analyze turbulence ingestion noise. In this study, using this numerical tool, the change in the noise strength due to blade flexibility was examined. This research showed that elastic blades decreased broadband noise because pitching motion reduced the angle of attack.

Numerical analysis of tonal airfoil self-noise and acoustic feedback-loops

In this study the role of acoustic feedback instabilities in the tonal airfoil self-noise phenomenon is investigated. First, direct numerical simulations are conducted of the flow around a NACA-0012 airfoil at Re = 1 × 10 5 and four angles of attack. At the two lowest angles of attack considered the airfoil self-noise exhibits a clear tonal contribution, whereas at the two higher angles of attack the tonal contribution becomes less significant in comparison to the broadband noise. Classical linear stability analysis of time-averaged boundary layer profiles shows that the tonal noise occurs at a frequency significantly lower than that of the most convectively amplified instability wave. Two-dimensional linear stability analysis of the time-averaged flowfield is then performed, illustrating the presence of an acoustic feedback loop involving the airfoil trailing edge. The feedback loop is found to be unstable only for the cases where tonal self-noise is prominent, and is found to self-select a frequency almost identical to that of the tonal self-noise. The constituent mechanisms of the acoustic feedback loop are considered, which appear to explain why the preferred frequency is lower than that of the most convectively amplified instability wave.

Direct numerical simulations of airfoil self-noise

Direct numerical simulations (DNS) of airfoil self-noise were conducted. For the low Reynolds number airfoil flows accessible by DNS, the occurrence of laminar separation bubbles involving laminar-turbulent transition and turbulent reattachment leads to additional noise sources other than the traditionally studied trailing-edge noise. Cross-correlations of acoustic and hydrodynamic quantities in conjunction with rayacoustic theory are used to identify the main source locations for a NACA-0006 airfoil. It is found that the contribution of trailing edge noise dominates at low frequencies while for the high frequencies the radiated noise is mainly due to flow events in the reattachment region on the suction side. DNS have also been conducted of NACA-0012 airfoils with serrated and straight flat-plate trailing-edge extensions using a purposely developed immersed boundary method. Noise reduction for higher frequencies is shown and the effect of the trailing edge serrations on the acoustic feedback loop observed in previous simulations and the subsequent effect on the laminar separation bubble is studied.

Reprint of: Direct Numerical Simulations of Airfoil Self-Noise

Direct numerical simulations (DNS) of airfoil self-noise were conducted. For the low Reynolds number airfoil flows accessible by DNS, the occurrence of laminar separation bubbles involving laminar-turbulent transition and turbulent reattachment leads to additional noise sources other than the traditionally studied trailing-edge noise. Cross-correlations of acoustic and hydrodynamic quantities in conjunction with rayacoustic theory are used to identify the main source locations for a NACA-0006 airfoil. It is found that the contribution of trailing edge noise dominates at low frequencies while for the high frequencies the radiated noise is mainly due to flow events in the reattachment region on the suction side. DNS have also been conducted of NACA-0012 airfoils with serrated and straight flat-plate trailing-edge extensions using a purposely developed immersed boundary method. Noise reduction for higher frequencies is shown and the effect of the trailing edge serrations on the acoustic feedback loop observed in previous simulations and the subsequent effect on the laminar separation bubble is studied.

Direct numerical simulations of tonal noise generated by laminar flow past airfoils

A numerical investigation is presented of noise generated by flow past symmetric NACA airfoils with different thickness and at various angles of attack at M = 0.4 and a Reynolds number based on chord of Re = 50 , 000 . Direct numerical simulations (DNS) are employed to directly compute both the near-field hydrodynamics and the far-field sound. The DNS data are then used to investigate whether the approach of determining tonal noise radiation based on the surface pressure difference, as done in the classical trailing-edge theory of Amiet, yields satisfactory results for finite thickness airfoils subject to mean loading effects. In addition, the accuracy of Amiet's surface pressure jump function is evaluated. Overall, the modified theory of Amiet appears to be suitable for finite thickness airfoils up to moderate incidence. However, when increasing the airfoil thickness to 12% chord, which corresponds to a trailing-edge angle of 16 . 8 ∘ , an unexpected phase change between the incident and scattered pressure is found at the frequency of the forced instability waves. This phase change is attributed to the flow oscillating around the trailing edge at a separate wake frequency. For the largest incidence investigated, Amiet's response function does not predict the total surface pressure difference as accurately as for zero or small incidence at the vortex shedding frequency, resulting in a poor prediction of the directivity and amplitude of the acoustic pressure. Moreover, predicting the airfoil self-noise based on the surface pressure difference appears not to be generally applicable at higher angles of attack because the radiated sound is only partly due to classical trailing-edge noise mechanisms. In these cases, it appears as if volume sources in the flow cannot be neglected.

Design and Wind-Tunnel Verification of Low-Noise Airfoils for Wind Turbines

A method for the prediction of the airfoil trailing-edge far-field noise is presented. The model employs the airfoil analysis code XFOIL to determine the initial and boundary conditions for a subsequent boundary-layer analysis using the finite-difference code EDDYBL featuring a Reynolds stress turbulence model that finally provides the input data for the noise prediction by a modified TNO Institute of Applied Physics model. The prediction scheme was applied in the European silent rotors by acoustic optimization project to design new, quieter airfoils for the outer blade region of three different wind turbines in the megawatt class. The objective was to reduce the airfoil self-noise without loss in aerodynamic performance

Analysis of Flow Conditions in Freejet Experiments for Studying Airfoil Self-Noise

A new set of mean wall pressure data has been collected on a controlled diffusion airfoil at a chord Reynolds number of 1.2 £ £105 in a freejet anechoic wind tunnel. Comparisons of the experimental data with Reynoldsaveraged Navier‐ Stokes (RANS) simulationsin freeairshow signie cant e owe eld and pressure loading differences, indicatingsubstantialjetinterferenceeffects.Toanalyzetheseeffects,asystematicRANS-basedcomputationale uid dynamicsstudyoftheexperimentale owconditionshasbeencarriedout,whichquantie esthestrongine uenceofthe e nite jet (nozzle) width on the aerodynamic loading and e ow characteristics. When the jet width is not sufe ciently large compared to the frontal wetted area of the airfoil, the airfoil pressure distribution is found to be closer to the distribution on a cascade than that of an isolated proe le. The airfoil lift is signie cantly reduced. Accounting for the actual wind-tunnel setup recovers the wall pressure distribution on the airfoil without further empirical angle-of-attack corrections. These jet interference effects could be responsible for the discrepancies among some earlier experimental and computational studies of airfoil self-noise. They should be accounted for in future noise computations to ensure that the experimental e ow conditions are simulated accurately.

Airfoil Self-Noise Generated in a Cascade

Self-noise from fans and rotors is generated by blade boundary-layer turbulence interacting with the trailing edges of the blades. Previous theories describing this mechanism have considered only an isolated airfoil and have shown that acoustic scattering from the sharp trailing edge is responsible for the sound that propagates to the far e eld. In aeroengine applications, fans or stators have relatively high solidity, and there will be acoustic scattering from adjacent blades as well as the blade locally excited by the e ow. The theory is given for self-noise from a fan modeled as a linear cascade of semi-ine nite e at plates. The magnitude of the self-noise mechanism is compared with the e eld that would be generated by a single airfoil without adjacent blade scattering.

Airfoil self-noise generated in a cascade

Self-noise from fans and rotors is generated by blade boundary-layer turbulence interacting with the trailing edges of the blades. Previous theories describing this mechanism have considered only an isolated airfoil and have shown that acoustic scattering from the sharp trailing edge is responsible for the sound that propagates to the far field. In aeroengine applications, fans or stators have relatively high solidity, and there will be acoustic scattering from adjacent hlades as well as the blade locally excited by the flow. The theory is given for self-noise from a fan modeled as a linear cascade of semi-infinite flat plates. The magnitude of the self-noise mechanism is compared with the field that would he generated by a single airfoil without adjacent blade scattering.

Scaling of airfoil self-noise using measured flow parameters

Data from an airfoil broadband self-noise study are reported. Attention here is restricted to two-dimensional models at zero angle of attack to the flow. The models include seven NACA 0012 airfoil sections and five flat plate sections with chordlengths ranging from 2.54 to 60.96 cm. Testing parameters include flow velocity to 71.3 m/s and boundary-layer turbulence through natural transition and by tripping. Detailed aerodynamic measurements are conducted in the near-wake of the sharp trailing edges. The noise spectra of the self-noise sources are determined by the use of a cross-spectral technique. The acoustic data are normalized using the measured aerodynamic parameters in order to evaluate a commonly used scaling law. An examination of the Reynolds number dependence of the normalized overall levels has revealed a useful scaling result. This result appears to quantify the transition between turbulent boundary-layer trailing-edge noise and laminar boundary-layer vortex shedding noise.