Acoustic measurements of a DU96-W-180 airfoil with flow-misaligned serrations at a high Reynolds number in a closed-section wind tunnel

Trailing-edge serrations are passive noise reduction add-ons widely used in wind-turbine applications. This study presents acoustic beamforming results of microphone array measurements of a cambered airfoil (DU96-W-180) in a closed-section wind-tunnel at a Reynolds number of industrial interest. Two different serration geometries with different lengths were tested and compared with the straight-edge baseline airfoil. The serrations were set at a flap angle of 6 degrees. Several flow velocities and angles of attack were tested at three chord-based Reynolds numbers ranging from 5105 to 1.5106. A phased microphone array was used to obtain source maps of the trailing-edge noise; Particle Image Velocimetry (PIV) was employed to obtain information about the turbulent boundary layer approaching the trailing edge; further, a numerical simulation using the Lattice Boltzmann Method (LBM) was performed for comparison. Far-field noise from the experimental data and computations shows a satisfactory agreement. Noise reductions of several dB were obtained, especially at lower frequencies. An increase in high-frequency noise is observed after a crossover frequency, which is assumed to be due to the set flap angle.

[1]  D. Palumbo,et al.  Determining correlation and coherence lengths in turbulent boundary layer flight data , 2012 .

[2]  Thierry Maeder,et al.  Reduction of Wind Turbine Noise using Optimized Airfoils and Trailing-Edge Serrations , 2008 .

[3]  Mirjam Snellen,et al.  Boundary layer characterization and acoustic measurements of flow-aligned trailing edge serrations , 2016 .

[4]  Pieter Sijtsma,et al.  On the use of global optimization methods for acoustic source mapping. , 2017, The Journal of the Acoustical Society of America.

[5]  Stefan Oerlemans.  Reduction of wind turbine noise using blade trailing edge devices , 2016 .

[6]  Pieter Sijtsma,et al.  Acoustic Array Measurements of a 1:10.6 Scaled Airbus A340 Model , 2004 .

[7]  Luc Mongeau,et al.  Prediction of the Sound radiated from Low-Mach Internal Mixing Nozzles with Forced Mixers using the Lattice Boltzmann Method , 2013 .

[8]  Thomas F. Brooks,et al.  A Deconvolution Approach for the Mapping of Acoustic Sources (DAMAS) Determined from Phased Microphone Arrays , 2004 .

[9]  Scott C. Morris,et al.  Numerical and experimental investigation of a beveled trailing edge flow and noise field , 2015 .

[10]  Hugo E. Camargo,et al.  Aeroacoustic Testing of Wind Turbine Airfoils: February 20, 2004 - February 19, 2008 , 2010 .

[11]  J. Westerweel.  Fundamentals of digital particle image velocimetry , 1997 .

[12]  Jian Li,et al.  A covariance fitting approach for correlated acoustic source mapping. , 2010, The Journal of the Acoustical Society of America.

[13]  P. Bhatnagar,et al.  A Model for Collision Processes in Gases. I. Small Amplitude Processes in Charged and Neutral One-Component Systems , 1954 .

[14]  Pieter Sijtsma,et al.  Suppression of spurious noise sources in airfoil self-noise measurements , 2015 .

[15]  C. C. Pagani,et al.  Slat Noise: Aeroacoustic Beamforming in Closed-Section Wind Tunnel with Numerical Comparison , 2016 .

[16]  Jesper Madsen,et al.  Effect of trailing edge serration-flow misalignment on airfoil noise emissions , 2017 .

[17]  J. Boon.  The Lattice Boltzmann Equation for Fluid Dynamics and Beyond , 2003 .

[18]  Gert Herold,et al.  A Microphone Array Method Benchmarking Exercise using Synthesized Input Data , 2017 .

[19]  G. P. Succi,et al.  A review of propeller discrete frequency noise prediction technology with emphasis on two current methods for time domain calculations , 1980 .

[20]  Phillip Joseph,et al.  On the mechanisms of serrated airfoil trailing edge noise reduction , 2011 .

[21]  Daniele Ragni,et al.  Three-dimensional flow field over a trailing-edge serration and implications on broadband noise , 2016 .

[22]  Roberto Merino-Martínez,et al.  Functional Beamforming Applied to Imaging of Flyover Noise on Landing Aircraft , 2016 .

[23]  Don H. Johnson,et al.  Array Signal Processing: Concepts and Techniques , 1993 .

[24]  David Freed,et al.  A Ffowcs Williams-Hawkings Solver for Lattice-Boltzmann based Computational Aeroacoustics , 2010 .

[25]  Pieter Sijtsma,et al.  Phased Array Beamforming Applied to Wind Tunnel And Fly-Over Tests , 2010 .

[26]  S. Orszag,et al.  Extended Boltzmann Kinetic Equation for Turbulent Flows , 2003, Science.

[27]  D. L. Hawkings,et al.  Sound generation by turbulence and surfaces in arbitrary motion , 1969, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[28]  Stefan Oerlemans,et al.  Numerical analysis of noise reduction mechanisms on improved trailing edge serrations using the Lattice Boltzmann method , 2017 .