TY - BOOK

T1 - Experimental characterization of airfoil boundary layers for improvement of aeroacoustic and aerodynamic modeling

A1 - Fischer,Andreas

AU - Fischer,Andreas

A2 - Aagaard Madsen ,Helge

A2 - Bertagnolio,Franck

A2 - Shen,Wen Zhong

ED - Aagaard Madsen ,Helge

ED - Bertagnolio,Franck

ED - Shen,Wen Zhong

PY - 2011/11

Y1 - 2011/11

N2 - The present work aims at the characterization of aerodynamic noise from wind <br/>turbines. There is a consensus among scientists that the dominant aerodynamic <br/>noise mechanism is turbulent boundary trailing edge noise. In almost all operational <br/>conditions the boundary layer flow over the wind turbine blades makes a <br/>transition from laminar to turbulent. In the turbulent boundary layer eddies are <br/>created which are a potential noise sources. They are ineffective as noise source <br/>on the airfoil surface or in free flow, but when convecting past the trailing edge <br/>of the airfoil their efficiency is much increased and audible sound is radiated. <br/>We performed measurements of the boundary layer velocity fluctuations and the <br/>fluctuating surface pressure field in two different wind tunnels and on three different <br/>airfoils. The first wind tunnel is the one of LM Wind Power A/S following <br/>the classic concept for aerodynamic wind tunnels with a hard wall test section. <br/>Acoustic far field sound measurements are not possible in this tunnel due to <br/>the high background noise. The second wind tunnel is owned by Virginia Tech <br/>University. The test section has Kevlar walls which are acoustically transparent <br/>and it is surrounded by an anechoic chamber. In this experiment the far field <br/>sound was measured with a microphone array placed in the anechoic chamber. <br/>The measurements were compared to predictions with an analytical model for <br/>trailing edge noise. The analytical model is divided into two steps. First the <br/>fluctuating velocity field is related to the fluctuating surface pressure field, then <br/>the far field trailing edge noise is related to the surface pressure field close to the <br/>trailing edge of the airfoil. The data base of measurements was used to evaluate <br/>the different parts of the original analytical trailing edge noise model and to <br/>improve it, because the predictions gave in general too low far field noise levels. <br/>Our main finding is that the acoustic formulations to relate the fluctuating surface <br/>pressure field close to the trailing edge of airfoil to the radiated far field <br/>sound give excellent results when compared to far field sound measurements <br/>with a microphone array and measured surface pressure statistics as input up <br/>to a frequency of about 2000-3000Hz. The fluctuating surface pressure field <br/>can be measured in a wind tunnel with high background noise due to the high <br/>level of the fluctuating surface pressure field. Hence, trailing edge noise can be <br/>evaluated by means of measured surface pressure field, even in cases where a <br/>direct measurement of trailing edge noise is not possible. This opens up great <br/>new vistas, i.e. by testing new airfoils in a standard industrial wind tunnel or <br/>by testing new wind turbine rotors in the field. <br/>The main difficulty for trailing edge noise modeling is to predict the fluctuating <br/>surface pressure field correctly and one uncertainty of the original model was the <br/>assumption of isotropic turbulence. This was investigated in the present work <br/>and a new model to relate the boundary layer velocity field to the surface pressure <br/>field accounting for an anisotropic turbulence spectrum was proposed. The <br/>results were very similar compared to the original model and underestimated <br/>the measured one point surface pressure spectrum, even though the prediction <br/>of the one point velocity spectra was improved.

AB - The present work aims at the characterization of aerodynamic noise from wind <br/>turbines. There is a consensus among scientists that the dominant aerodynamic <br/>noise mechanism is turbulent boundary trailing edge noise. In almost all operational <br/>conditions the boundary layer flow over the wind turbine blades makes a <br/>transition from laminar to turbulent. In the turbulent boundary layer eddies are <br/>created which are a potential noise sources. They are ineffective as noise source <br/>on the airfoil surface or in free flow, but when convecting past the trailing edge <br/>of the airfoil their efficiency is much increased and audible sound is radiated. <br/>We performed measurements of the boundary layer velocity fluctuations and the <br/>fluctuating surface pressure field in two different wind tunnels and on three different <br/>airfoils. The first wind tunnel is the one of LM Wind Power A/S following <br/>the classic concept for aerodynamic wind tunnels with a hard wall test section. <br/>Acoustic far field sound measurements are not possible in this tunnel due to <br/>the high background noise. The second wind tunnel is owned by Virginia Tech <br/>University. The test section has Kevlar walls which are acoustically transparent <br/>and it is surrounded by an anechoic chamber. In this experiment the far field <br/>sound was measured with a microphone array placed in the anechoic chamber. <br/>The measurements were compared to predictions with an analytical model for <br/>trailing edge noise. The analytical model is divided into two steps. First the <br/>fluctuating velocity field is related to the fluctuating surface pressure field, then <br/>the far field trailing edge noise is related to the surface pressure field close to the <br/>trailing edge of the airfoil. The data base of measurements was used to evaluate <br/>the different parts of the original analytical trailing edge noise model and to <br/>improve it, because the predictions gave in general too low far field noise levels. <br/>Our main finding is that the acoustic formulations to relate the fluctuating surface <br/>pressure field close to the trailing edge of airfoil to the radiated far field <br/>sound give excellent results when compared to far field sound measurements <br/>with a microphone array and measured surface pressure statistics as input up <br/>to a frequency of about 2000-3000Hz. The fluctuating surface pressure field <br/>can be measured in a wind tunnel with high background noise due to the high <br/>level of the fluctuating surface pressure field. Hence, trailing edge noise can be <br/>evaluated by means of measured surface pressure field, even in cases where a <br/>direct measurement of trailing edge noise is not possible. This opens up great <br/>new vistas, i.e. by testing new airfoils in a standard industrial wind tunnel or <br/>by testing new wind turbine rotors in the field. <br/>The main difficulty for trailing edge noise modeling is to predict the fluctuating <br/>surface pressure field correctly and one uncertainty of the original model was the <br/>assumption of isotropic turbulence. This was investigated in the present work <br/>and a new model to relate the boundary layer velocity field to the surface pressure <br/>field accounting for an anisotropic turbulence spectrum was proposed. The <br/>results were very similar compared to the original model and underestimated <br/>the measured one point surface pressure spectrum, even though the prediction <br/>of the one point velocity spectra was improved.

BT - Experimental characterization of airfoil boundary layers for improvement of aeroacoustic and aerodynamic modeling

ER -