347 research outputs found

    "Ladder" structure in tonal noise generated by laminar flow around an airfoil

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    The presence of a “ladder” structure in the airfoil tonal noise was discovered in the 1970s, but its mechanism hitherto remains a subject of continual investigation in the research community. Based on the measured noise results and some numerical analysis presented in this letter, the variations of four types of airfoil tonal noise frequencies with the flow velocity were analyzed individually. The ladder structure is proposed to be caused by the acoustic/hydrodynamic frequency lag between the scattering of the boundary layer instability noise and the discrete noise produced by an aeroacoustic feedback loop

    Poro-Serrated trailing edge devices for airfoil self-noise reduction

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    This paper represents the continuation of the works previously published in Chong et al. (“Self-Noise Produced by an Airfoil with Nonflat Plate Trailing-Edge Serrations,” AIAA Journal, Vol. 51, No. 11, 2013, pp. 2665–2677), who used several nonflat plate serrated trailing edges for the reduction of airfoil self-noise. The poro-serrated concept developed in the current work improves substantially the overall noise performance of the nonflat plate trailing-edge serration type. The use of porous metal, synthetic foams, or thin brush bundles to fill the gaps between adjacent members of the sawtooth can completely suppress the bluntness-induced vortex shedding tonal noise. Most important, up to 7 dB turbulent boundary layer–trailing-edge broadband noise reduction can simultaneously be achieved without compromising the aerodynamic performances in lift and drag. The poro-serrated trailing edges do not cause any noise increase throughout the frequency range investigated here. The reduction of the turbulent broadband noise is primarily caused by the serration effect, but under a condition that the sawtooth surface must be solid and nonporous. The primary role of the porous metal foams in a poro-serrated trailing edge is to suppress the vortex shedding tonal noise. However, an optimum selection of the porous material is also found to be able to further reduce the broadband noise level. The new serrated trailing-edge concept developed here has the potential to improve the industrial worthiness of the serration technology in achieving low noise radiation in fan and turbine blades.The EPSRC Doctoral Training Grants in the United Kingdo

    Self-noise produced by an airfoil with nonflat plate trailing-edge serrations

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    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.This work is partly supported by the Brunel Research Initiative and Enterprise fun

    Design of serrate-semi-circular riblets with application to skin friction reduction on engineering surface

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    Drag reduction in wall-bounded flows can be achieved by the passive flow control technique through the application of bio-inspired riblet surfaces. This paper presents the innovative design of Serrate-Semi-Circular riblet surfaces particularly focusing on the intrinsic relationship between the riblet features and the turbulent boundary layer structure resulting from these surfaces in engineering applications. The available experimental facilities, instrumentation (i.e. hotwire) and measurement techniques (i.e. velocity spectra) have been employed to investigate the boundary layer velocity profiles and skin friction for flat plate and Serrate-Semi-Circular riblet surfaces. Both the simulation and experimental wind tunnel testing results show that the Serrate-Semi-Circular riblet surface can provide 7% drag reduction, which is better than other riblet configurations, such as V and U shaped ones

    Aeroacoustic and aerodynamic performances of an aerofoil subjected to sinusoidal leading edges

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    This paper presents the preliminary results on the aeroacoustic and aerodynamic performances of a NACA65-(12)10 aerofoil subjected to 12 sinusoidal leading edges. The serration patterns of these leading edges are formed by cutting into the main body of the aerofoil, instead of extending the leading edges. Any of the leading edges, when attached to the main body of the aerofoil, will always result in the same overall chord length. The experiment was mainly performed in an aeroacoustic wind tunnel facility, although a separate aerodynamic type wind tunnel was also used for the force measurements. These sinusoidal leading edges were investigated for their effectiveness in suppressing the laminar instability tonal noise (trailing edge self-noise) and turbulence–leading edge interaction noise. The largest reduction in aerofoil noise tends to associate with the sinusoidal leading edge of the largest amplitude, and smallest wavelength. However, noticeable noise increase at high frequency is also observed for this combination of serration. In terms of the aerodynamic performance, increasing the serration wavelength tends to improve the stall angles, but the lift coefficient at the pre-stall regime is generally lower than that produced by the baseline leading edge. For a sinusoidal leading edge with large serration amplitude, the effect of the reduction in “lift-generating” surface is manifested in the significant reduction of the lift coefficients and lift curve slope. The sinusoidal leading edge that produces the best performance in the post-stall regime belongs to the largest wavelength and smallest amplitude, where the lift coefficients are shown to be better than the baseline leading edge. In conclusion, large amplitude and small wavelength is beneficial for noise reduction, whilst to maintain the aerodynamic lift a small amplitude and large wavelength is preferred

    Statistical-Empirical Modelling of Aerofoil Noise Subjected to Leading Edge Serrations and Aerodynamic Identification of Noise Reduction Mechanisms

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    With the objective of reducing the broadband noise, emitted from the interaction of highly turbulent flow and aerofoil leading edge, sinusoidal leading edge serrations were analysed as an effective passive treatment. An extensive aeroacoustic study was performed in order to determine the main influences and interdependencies of factors, such as the Reynolds number (Re), turbulence intensity (Tu), serration amplitude (A/C) and wavelength (λ/C) as well as the angle of attack (AoA) on the noise reduction capability. A statistical-empirical model was developed to predict the overall sound pressure level and noise reduction of a NACA65(12)- 10 aerofoil with and without leading edge serrations in the analysed range of chord-based Reynolds numbers of 2.5·105 ≤ Re ≤ 6·105 and a geometrical angle of attack -10 deg ≤ α ≤ +10 deg. The observed main influencing factors match current research results to a high degree, and were quantified in a systematic order for the first time. Moreover, significant interdependencies of the turbulence intensity and the serration wavelength (Tu·λ/C), as well as the serration wavelength and the angle of attack (λ/C·AoA) were observed, validated and quantified. In order to study the noise reduction mechanisms, Particle Image Velocimetry (PIV) measurements were conducted upstream of the aerofoil leading edge and along the interstices of the leading edge serrations. Velocity, turbulence intensity and vorticity in the plane perpendicular to the main flow direction (y/z plane) were analysed and linked to the acoustic findings. It was observed that a noise reduction is accompanied by a reduction of the turbulence intensity within the serration interstices. The reduction in turbulence intensity is more pronounced with large serration amplitudes. However, the impact of the serration wavelength was found to be no function of the turbulence. It is more likely to be affected acoustically by spanwise de-correlation effects as a response to the incoming gusts

    On the leading edge noise and aerodynamics of thin aerofoil subjected to the straight and curved serrations

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    This paper presents the results of an experimental study into the effect of add-on type leading edge serrations on the aeroacoustic and aerodynamic performances of a symmetrical NACA0008 aerofoil. The aeroacoustic part of this paper studies the reduction of interaction broadband noise in the presence of elevated freestream turbulence by employing serrated leading edges. For non-dimensional frequencies f0 < 1, the resulting sound pressure level reduction (DSPL) was found to be a linear function of f0 and the DSPL depends only on the serration amplitude, serration wavelength and freestream velocity. Leading edge with a large serration amplitude was found to be very effective in the reduction of broadband noise where up to DSPL z 8 dB is achievable. It is generally more beneficial to choose a leading edge with a smaller serration wavelength, although the most effective configuration actually combines the largest serration amplitude and the largest serration wavelength. Interestingly, for a curved-serration, the most optimised configuration (with small serration wavelength, large serration amplitude, small inclination angle and large curvature radius) was found to outperform its straight-serration counterpart by a further 5 dB reduction of broadband noise at the same frequency. Concerning the aerodynamic part of the study, to effectively suppress boundary layer stall without incurring severe drag penalties for low freestream turbulence intensity, the most effective leading edge serration should possess a large serration wavelength and small serration amplitude. Hence, the serration geometry that works very well for a low noise aerofoil is usually inferior in the aerodynamic performance, and vice versa. The best compromise for the serration geometry that can still harness good performances in both the aeroacoustic and aerodynamic should possess the largest serration amplitude and the largest serration wavelength. This paper demonstrates that, when optimised properly, the add-on type leading edge serration can be very effective in both the reduction of the interaction broadband noise, and the suppression of the boundary layer separation at high angle of attack.Engineering and Physical Sciences Research Counci
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