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LOCALIZATION OF WIND TURBINE NOISE USING A COMPACT MICROPHONE ARRAY WITH ADVANCED BEAMFORMING ALGORITHMS
Title
LOCALIZATION OF WIND TURBINE NOISE USING A COMPACT MICROPHONE ARRAY WITH ADVANCED BEAMFORMING ALGORITHMS
Creator
Ramachandran, Rakesh Chandran
Date
2014, 2014-12
Description
The knowledge of noise source location on a wind turbine is crucial for de- signing low noise wind turbines. Even though there has been...
Show moreThe knowledge of noise source location on a wind turbine is crucial for de- signing low noise wind turbines. Even though there has been considerable e ort to theoretically predict the noise from wind turbines in the past, very few experimental investigations have been performed to validate these models. Currently, compliance standards for wind turbine noise are based on single microphone measurements at various designated locations around a wind turbine. This only provides amplitude and frequency data of the overall wind turbine noise. However, it is impossible to locate noise sources, and to understand the dominant noise generation mechanism, using single microphone measurements. A sophisticated method, namely, the use of a microphone phased array with beamforming is necessary to locate the noise sources. Even though microphone arrays have been widely used to study aircraft yover and jet noise, it has just recently found application in locating wind turbine noise. Typical microphone arrays that have been used in the past for monitoring aircraft noise and wind turbine noise are very large ranging up to 270 m2 in area with about 148 mi- crophones. The setup is xed for a particular wind turbine and takes a considerable amount of time. In this thesis it is shown that a compact microphone array (with 24 microphones spread over 1.5 m2) is su cient to locate and separate wind turbine noise sources successfully. It is also shown that in order to use a compact micro- phone array, advanced deconvolution based beamforming methods such as DAMAS, CLEAN-SC, LP, and TIDY are necessary. However, before attempting to validate our idea, it is necessary to understand the concept of array resolution and di erent methods to improve it. We start with an in-depth study on the microphone array resolution and the e ect of di erent beamforming algorithms on it. This study has resulted in several interesting results: (i) The resolution limits that govern the resolution of the imaging system in optics also govern the resolution of microphone array. (ii) The resolution is a function of source frequency, diameter of the array, separation distance between sources, and the distance between the array and the sources. (iii) The resolution lost due to the compact microphone array can be recovered by using deconvolution algorithms. (iv) Di erent algorithms are useful for di erent scenarios; CLEAN-SC is most e cient in extracting the dominant noise source from the source map, LP is e cient in locating multiple or extended noise sources, and TIDY is e cient in locating moving sources. (v) LP and DAMAS show superresolution. (vi) The dynamic range of source maps could be improved by eliminating the diagonal elements of CSM or CCM but care should be take as this could also eliminate weak sources. Following this study, we discuss the results from the full scale large wind tur- bine noise measurements. Our study focuses on the ability of a compact microphone array to successfully locate both mechanical and aerodynamic noise sources on the wind turbine. Several interesting results have emerged from this study: (i) A compact microphone array is su cient to perform a detailed study on wind turbine noise if advanced deconvolution methods are applied. (ii) Noise sources on the blade and on the nacelle can clearly be separated. (iii) Noise of the blades is dominated by trailing edge noise which is frequency dependent and is distributed along the length of the blade with the dominant noise source closer to the tip of the blade. (iv) The LP and DAMAS algorithms represent the distributed trailing edge noise source better than CLEAN-SC and conventional beamforming. (v) Additional tonal noise produced dur- ing yawing operation is believed to be radiating from the tower of the wind turbine that acts like a resonator. (vi) Ground re ection is not believed to have a signi cant e ect on noise source location estimates in this study. (vii) The asymmetry in the aerodynamic noise is attributed to the Doppler ampli cation e ect. Finally, the noise measurements made on the small scale wind turbine showed that the compact array was successfully able to separate out the mechanical and aerodynamic noise. The mechanical noise from the nacelle was found to be the dominant noise source below 3000 Hz and the aerodynamic noise from the blades was found to be dominant above 3000 Hz. The asymmetry in the aerodynamic noise was also present in this case. The study on mechanical noise found that gear box was the dominant noise source. It is hoped that the work presented in this thesis will serve as a guide to researchers who intend to use compact microphone arrays with advanced beamforming algorithms in the future and also to those who intend to validate theoretical models of wind turbine noise.
Ph.D. in Mechanical and Aerospace Engineering, December 2014
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MINIATURE SHOCK TUBE ACTUATORS FOR HIGH SPEED FLOW CONTROL APPLICATIONS
Title
MINIATURE SHOCK TUBE ACTUATORS FOR HIGH SPEED FLOW CONTROL APPLICATIONS
Creator
Ramachandran, Rakesh Chandran
Date
2011-04-11, 2011-05
Description
In the field of aero-acoustics, one of the primary areas of interest has always been centered around the resonant flows. The fluid structure...
Show moreIn the field of aero-acoustics, one of the primary areas of interest has always been centered around the resonant flows. The fluid structure interactions gives rise to a reverberant field, which results in high amplitude fluctuating pressures associated with the resonant tones that could lead to sonic fatigue failure of sensitive components in the vicinity of such flows. One such fluid structure interaction is the cavity flow problem. Several flow control technologies exist to suppress cavity tones but most of them lose efficacy at off-design conditions and particularly at higher subsonic and supersonic flows. It is due to this fact that there is a high demand for high control authority flow control actuators. In order for an actuator to have higher control authority, one must be able to use a mechanism that has sufficient energy to disrupt the cavity tone generation mechanism at higher subsonic Mach numbers. One such mechanism is the shock tube generating high strength shock waves. To the best of our knowledge, there does not appear to be any use of shock waves to suppress noise emitted by cavity in aircrafts. The main challenge lied in developing the miniature shock tubes that could produce high intensity shock waves. In the present work, we designed, developed and tested these miniature shock tube flow control actuators. The initial part of this work involved a detailed study of the theory behind producing shock waves using a shock tube. The designing and development part included a lot of trial and error adjustments to produce shock waves as predicted by the 1D shock wave theory. Two shock tube actuators were developed, one having a single shock tube and another with three shock tubes but with the same exit area as the previous one. We carried out various characterization experiments measuring the unsteady pressure at the exit of these shock tubes and also the walls of the shock tube. The experimental investigation of the shock tubes revealed that miniature version of the shock tubes indeed produced high intensity shock waves as predicted by the shock wave theory. Apart from the shock tube actuators, fast acting solenoid valves which does not produce any shock were also tested, to compare the efficacy of both types of actuators. The later part of the work focuses on the actual application of these actuators as flow control devices. First, we delve into the acoustic suppression results which show the amount of tonal noise suppression achieved by using these actuators. For the M = 0.6 case the valves produced very good suppression up to 20 dB but the mass flow rate of these valves were about 26% of the main jet mass flow rate. In the M = 0.8 flows the valves produced negligible suppression. The multiple shock tube performed better than the single shock tube in both cases with a tonal noise suppression of up to 12 dB and 10 dB for M = 0.6 and M = 0.8 flows, respectively. The effect of the shock wave on the cavity tone was evident from the results and had a memory effect on the cavity tone suppression. Due to this the mass flux and the momentum coefficient for the shock tube actuators were considerably low. In order to better understand the mechanism through which the actuators suppress cavity tones, mean velocity measurements and phase averaged pressure measurements were carried out. Based on these results, it was observed that the lifting of the shear layer was the dominant mechanism behind steady and pulsed mass injection. In addition to this beamforming, used for locating the noise sources, was also used to study the cavity tones. Detailed discussion of the results are presented in this report.
M.S. in Mechanical and Aerospace Engineering, May 2011
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