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

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

Non-Engine Order Blade Vibration in a High Pressure Compressor

International audienceHigh amplitude levels of blade vibration have occurred on the first rotor of a multi stage high pressure compressor. The frequencies are not in resonance with harmonics of the rotor speed. The excitation is aerodynamically caused and associated with a rotating flow instability in the blade tip region of the first compressor stage. A vortex shedding mechanism can be interpreted as a rotating source to generate pressure waves. The source moves relative to the blade row at a fraction of the rotor speed, similar to the 'well known mechanism' of rotating stall. To investigate the unsteady flow field in the tip region of the rotor and its relation to the blade vibration, measurements of the pressure and velocity fluctuations in the vicinity of the blade tips are compared with blade vibrations. A calculation model of the spectral characteristics of the pressure fluctuations confirms the measured data. The effect is sensitive to aerodynamic blade loading so that a modification in the design could reduce the mechanism of the rotating excitation

Statistical-Empirical Modelling of Airfoil Noise Subjected to Leading Edge Serrations

Quiet Aerofoils of the Next Generatio

Polyoptimisation of the aerodynamic and aeroacoustic performance of aerofoils with serrated leading edges

Recent research confirmed leading edge serrations to be an effective passive noise reduction treatment for aerofoil broadband noise at high-turbulent inflow conditions. Therefore, reducing leading edge broadband noise while maintaining acceptable aerodynamic aerofoil performance represents a pressing task for future applications. In this context, an extensive aeroacoustic study, analysing a NACA65(12)-10 aerofoil, was continued towards defining an aeroacoustic optimum between aerofoil noise radiation and noise reduction due to serrated leading edges in order to provide ideal design parameters for low-noise serrations. On this basis, part of the aeroacoustically analysed experimental space was extracted and analysed in terms of aerodynamic performance parameters, defined by lift and drag coefficients. This was carried out both, numerically and experimentally. The main parameters of interest were a variation of the Reynolds number, the angle of attack and the serration design parameters, namely the serration amplitude and the serration wavelength. The aerodynamic study showed a good match between experimental and numerical results in the pre-stall regime. Slight deviations occurred in a precise determination of the stall-angle and the maximum lift coefficients which mainly could be assigned to differing boundary conditions. However, for the serrations slight improvements of the maximum pre-stall angles as well as high post-stall lift coefficients were observed, which could be linked to specific separation pattern on the aerofoil suction side. An increase of the serration wavelength showed an increased lift performance, which could not be linked solely to a change in the aerofoils surface. Combining aeroacoustic and aerodynamic results showed that the aerodynamic trends towards a maximum lift performance compete aeroacoustic maximum-performance findings. Finally, defining a polyoptimum of the multi-parameter system in terms of maximum noise reduction effects while maintaining an acceptable aerodynamic performance provides a deepened insight into the relations between aerodynamics and aeroacoustics, where the presented data pool might give assistance for future design processes

Influence of tip clearance on flow behavior and noise generation of centrifugal compressors in near-surge conditions

CFD has become an essential tool for researchers to analyze centrifugal compressors. Tip leakage flow is usually considered one of the main mechanisms that dictate compressor flow field and stability. However, it is a common practice to rely on CAD tip clearance, even though the gap between blades and shroud changes when compressor is running. In this paper, sensitivity of centrifugal compressor flow field and noise prediction to tip clearance ratio is investigated. 3D CFD simulations are performed with three different tip clearance ratios in accordance to expected operating values, extracted from shaft motion measurements and FEM predictions of temperature and rotational deformation. Near-surge operating conditions are simulated with URANS and DES. DES shows superior performance for acoustic predictions. Cases with reduced tip clearance present higher pressure ratio and isentropic efficiency, but no significant changes in compressor acoustic signature are found when varying clearance. In this working point, tip clearance is immersed in a region of strongly swirling backflow. Therefore, tip leakage cannot establish any coherent noise source mechanism.The equipment used in this work has been partially supported by FEDER project funds "Dotacion de infraestructuras cientifico tecnicas para el Centro Integral de Mejora Energetica y Medioambiental de Sistemas de Transporte (CiMeT), (FEDER-ICTS-2012-06)", framed in the operational program of unique scientific and technical infrastructure of the Ministry of Science and Innovation of Spain. The authors wish to thank Mr. Pau Raga for his worthy assistance during the meshing process.Galindo, J.; Tiseira Izaguirre, AO.; Navarro García, R.; López Hidalgo, MA. (2015). Influence of tip clearance on flow behavior and noise generation of centrifugal compressors in near-surge conditions. International Journal of Heat and Fluid Flow. 52:129-139. https://doi.org/10.1016/j.ijheatfluidflow.2014.12.004S1291395