Elias Arcondoulis

Airfoil Trailing Edge Noise Source Location at Low to Moderate Reynolds Number

A study of the effects of Reynolds number and angle of attack on the dominant acoustic source location relative to the trailing edge for airfoils at low-to-moderate Reynolds number is presented. This study, which was performed using acoustic beamforming in an anechoic wind tunnel, helps assess the influence of each test parameter on the distance of the acoustic source from the trailing edge thus improving our knowledge of how airfoils produce tonal noise. The results show that the acoustic source location of a NACA0012 airfoil varies along the chord for the frequency range 500 Hz to 3000 Hz at low-to-moderate Reynolds number. Changing the angle of attack and Reynolds number resulted in small changes to the acoustic source location within this frequency range. A modified flat plate, which only radiated broadband noise under similar flow conditions, presented a scattered distribution of source location between the range 300 Hz to 1200 Hz.

On the generation of airfoil tonal noise at zero angle of attack and low to moderate Reynolds number

The generation of tones from a NACA 0012 airfoil at zero angle of attack and Reynolds numbers of 50,000 to 150,000 was investigated using acoustic beamforming and surface flow visualization techniques in an anechoic wind tunnel. Boundary layer separation lengths were obtained using surface flow visualization techniques and XFOIL software and were input into a feedback loop equation to determine if the measured tones were generated by a feedback mechanism. Using the maximum point of velocity on the airfoil surface as a feedback length provided the best agreement with the measured tones. Boundary layer tripping on one airfoil surface resulted in a decrease in the primary tone amplitude and some of the secondary tone amplitudes. Applying a trip to both surfaces resulted in a significant noise decrease and no tones were observed.

A Modification to Logarithmic Spiral Beamforming Arrays for Aeroacoustic Applications

Acoustic beamforming is an experimental tool that can be used to locate and quantify aeroacoustic noise sources. Much of the available aeroacoustic beamforming literature presents beamforming results of noise at relatively high frequencies. There are few experimental acoustic beamforming results for acoustic frequencies between 1 kHz and 5 kHz, although much of the literature for airfoil self noise at low to moderate Reynolds number fits in this frequency range. One difficulty with acoustic beamforming of relatively low frequency noise is the large size of the main lobe in the beamformer output, resulting in a potential inability to resolve acoustic sources within close proximity to each other. This paper provides a detailed comparison between grid, randomized, logarithmic spiral and modified logarithmic spiral arrays and a discussion of the performance of each array type over a range of low frequencies (1 kHz-5 kHz). The spiral arrays were found to have lower sidelobe levels over a wider frequency range than the grid and random array. For frequencies less than 4.84 kHz and greater than 12.36 kHz, the modified logarithmic spiral exhibited smaller sidelobe magnitudes than the unmodified logarithmic spiral. An experimental verification of a modified logarithmic spiral using a small headphone in an anechoic environment is also provided. This showed that the error of the measured noise source locations placed 600 mm from the array plane is within 20 mm for the series of locations studied. The estimation error was shown to be dependent on the source location and direction from the center of the array.

Aeroacoustic beamforming for airfoil trailing edge tonal noise sources.

Airfoils produce tonal and broadband noise at low to moderate Reynolds number flow conditions. The effect of variation in flow Reynolds number and airfoil angle of attack on the quantification and localization of the acoustic sources responsible for the observed tonal noise component is still uncertain. An investigation of the effect of Reynolds number, angle of attack, and acoustic frequency on the tonal noise sources will be presented. This study was performed using aeroacoustic beamforming in a small anechoic wind tunnel. Accurately locating the tonal noise source locations of an airfoil will improve knowledge of how airfoils produce tonal noise in this flow regime which will help the implementation of noise reduction techniques. In addition, some of the problems associated with beamforming low frequency noise sources and close proximity coherent tonal noise sources using conventional beamforming techniques and deconvolution algorithms will be addressed. [The authors would like to thank the School of M...