**Table 2.**

*Sound power level, LwA for utility scale commercial wind turbine models.*

rating of the machine, rotor diameter and blade tip speed only. Sound power predictions from [22, 23] agree well for rotor diameters that range between 10 m and 100 m and thought to be less conservative compared to actual or measured data. Similarly, Lowson's empirical equation make use of only nominal power rating of machine, which implies that sound power level varies with size of machine. Hagg (1992) also developed a slightly more sophisticated model which can predict sound pressure level based on the axial thrust force coefficient, rotor swept area and the number of blades in machine along with empirical constants given in [21]. However, the model does not predict sound power levels for broadband frequency range of noise spectra. Some advanced noise prediction simulation software's developed by Siemens XNoise, NREL's NAFNoise are useful tools which can predict the noise levels for utility scale wind turbines. **Table 2** shows the measured sound power level (PWL) for some of commercial wind turbine models taken from SoundPLAN software.

*Trailing Edge Bluntness Noise Characterization for Horizontal Axis Wind Turbines [HAWT]… DOI: http://dx.doi.org/10.5772/intechopen.99880*

#### **Figure 6.**

*Shape function, G5, computed for different trailing edge bluntness thickness, h/δ\* avg using (a) original BPM [6] (b) modified BPM by [9].*

**Figure 6(a)** depicts the results for shape function, G5 obtained from original BPM model. As the average boundary layer displacement thickness is reduced, the frequency of vortex shedding increased despite a change in the angle of attack and flow Mach number, *M* along the blade span. This difference can be attributed to solid angle inclusion in the original BPM model which considered trailing edge sloping angle, ψ as essential condition to vortex shedding phenomenon in addition to the trailing edge height, flow Mach number and Reynolds number.

From **Figure 6(b)** the shape function G5 modified by [9] has been computed for trailing edge height to average boundary layer displacement thickness ratios, h/δ\* avg between 0.51 and 1.01. The function showed a linear change in amplitude, dB for all Strouhal numbers of ratios between 0 and 1. As the peak Strouhal number, Stpeak is increased, one can notice that tonal peak for trailing edge bluntness was found to be increasing. This effect was also observed with numerical CAA results obtained by [9] in their study for NACA 0012 and NACA 63–418 airfoil which have 3% camber and maximum thickness of 18%. It is important to note that CAA computations such as large eddy simulation (LES) can predict the acoustic radiation from airfoils by solving for the largest scales of turbulent flows and approximating the small scale motions. In contrast to the semi-empirical BPM model, the sound pressure level near the surface can be computed by solving the 2D-Navier–Stokes (N-S) equations that are coupled to advanced turbulence models and high accuracy computational grid schemes suitable for acoustic pressure computations [14]. Similarly, the A-weighted 1/3rd octave band tonal noise spectra has been computed at wind speed of 6 m/s, 14 RPM having a blade pitch of 3.5<sup>o</sup> .

**Figure 7(a)–(d)** demonstrates the contour plot of peak Strouhal number, plotted along the blade span for various blade azimuth angles in rotor plane and for different trailing edge thicknesses computed at wind speed of 8 m/s [19]. The maximum values can be observed between 0.1 r/R and 0.75 r/R along the blade span where the thickness to chord ratio is high when the blade azimuth angle is at 300<sup>o</sup> . With increasing trailing edge thicknesses, the peak Strouhal number kept increasing from 0.13 to 0.2. This also signifies shape function, G5 have high tonal peaks demonstrating influence of trailing edge vortex shedding from blade caused due to change in the trailing edge thicknesses.

On the other hand, the original BPM model showed a strong tonal peak effect in noise spectrum at 12% r/R where the thickness to chord ratio is found increasing. **Figure 8** shows the computed values for overall A-weighted 1/3rd octave band sound power level for 2 MW turbine, turbulent boundary layer trailing edge noise

**Figure 7.**

*Peak Strouhal number, Stpeak"', along normalized blade span and blade azimuth angles at U = 8 m/s for TE thicknesses (a) 0.1% chord (b) 0.5% chord (c) 1% chord (d) 1.5% chord.*

#### **Figure 8.**

*Comparison of trailing edge bluntness noise using present approach to those predicted by BPM original, BPM modified [9]) and its validation with experimental data from Siemens SWT 2.3 MW and GE 1.5sle turbines at wind speed of 8 m/s for trailing edge thickness of 0.1% chord.*

*Trailing Edge Bluntness Noise Characterization for Horizontal Axis Wind Turbines [HAWT]… DOI: http://dx.doi.org/10.5772/intechopen.99880*

(TBL-TE) as well as trailing edge bluntness noise using original BPM model. All the computations were done in MATLAB 2020b software, for a wind speed of 8 m/s at blade pitch angle of 3.5<sup>o</sup> and trailing edge thickness taken 0.1% chord length. Further, modified BPM model by [9] for trailing edge bluntness noise has also been computed to compare the actual results with present method. The present method focused on the regression approach for thickness correction along the blade span. It can be noted that present results produced similar trailing edge noise characteristics except for the noise peak change found at 10 kHz in the noise spectra. Also, one can observe that current approach for thickness correction leads to better agreement of the trailing edge bluntness peak with experiment data obtained from GE 1.5sle rather than Siemens 2.3 MW-101, Siemens 2.3 MW-95 and Siemens 2.3 MW-93 turbines. On the contrary, the trailing edge bluntness peak from original BPM model showed a broad hump which do not agree well with experiment validation data for turbines. For frequencies below 1 kHz, the turbulent boundary layer trailing edge noise dominates with a peak value of 96dBA. The trailing edge bluntness noise tonal peak computed from the original BPM model was found to be 89 dBA. The peak trailing edge bluntness noise level for modified BPM by [9] was found to be 78 dBA near 8 kHz which agreed well with experiment data. The present computations for modified BPM showed an increase of 2 dBA for frequency range of 20 Hz and 6 kHz, but reached almost same values for frequencies, f > 6 kHz.

In this section we present results for the turbulent boundary layer vortex shedding noise from a 2 MW horizontal axis wind turbine blade using original BPM model predictions and compare them with OSPL (overall sound power level) experiment data obtained for GE 1.5sle, Siemens SWT 2.3 MW machines. All the computations were done in MATLAB 2020b software, for a wind speed of 7 m/s and 10 m/s at blade pitch angle of 3.5<sup>o</sup> and trailing edge thickness taken 0.1% chord length. **Figure 9** shows the Strouhal number, *St*"' computed in terms of displacement thickness, *δ*\*, for wind speeds of 7 m/s and 10 m/s respectively. The maximum value for *St*"' was found to be 2.2 and 4.16 for wind speeds of 7 m/s and 10 m/s at frequency f 10 kHz, where the turbulent boundary layer trailing edge bluntness noise produces peak tonal amplitude.

**Figure 10(a)** and **(b)** shows the trailing edge bluntness peak from original BPM model as a broad band hump that agrees well within 5% of experiment validation data for GE 1.5sle, Siemens 2.3 MW turbines for wind speed of 7 m/s and 10 m/s at

**Figure 9.** *Illustration of Strouhal number, St"' as function of displacement thickness, δ\* , at wind speeds of 7 m/s and 10 m/s.*

**Figure 10.**

*Validation of the computed BPM-turbulent boundary layer vortex shedding noise (TEB-VS), for a blade length of 38 m, 2 MW turbine having trailing edge thickness of 0.1%c with OSPL measured data of GE-1.5sle, Siemens 2.3 MW (Pieter), Siemens 2.3 MW (Pedersen) of blade length 47 m at two different wind speeds of (a) 7 m/s (b) 10 m/s.*

trailing edge thickness of 0.1% local blade chord length. For frequencies below 1 kHz, the turbulent boundary layer trailing edge noise dominates with a peak value of 96 dB that is obtained using measured data of experiment turbines. The tonal peak of trailing edge bluntness noise computed from the original BPM model was found to be 82 dB for wind speed of 7 m/s and 96 dB for wind speed of 10 m/s.

From **Figure 11(a)** and **(b)** one can notice the computed turbulent boundary layer trailing edge bluntness noise level using BPM model shows peaks that shift closer to frequencies, f 5 kHz and reach an amplitude values of 97 dB and 115 dB respectively. It must be noted that when the trailing edge thickness or heights are increased to 0.5% of local blade chord length, a difference of 15 dB was found for wind speed of 7 m/s while a difference of 10 dB was obtained for wind speed of 10 m/s. Further, from **Figure 12** it is evident that the difference in the sound power levels between 7 m/s and 10 m/s continued to increase by a maximum value of 15 dB for frequencies, f < 200 Hz when the trailing edge thicknesses are 0.1% and 0.5% of local blade chord length respectively. However, for frequencies, f > 200 Hz a noise reduction of 17 dB was observed when the trailing edge thickness was 0.5% local chord length.

**Figure 13** shows the measured and computed sound power level, LwA for wind speeds between 4 m/s and 10 m/s. The experiment data for Vestas V82 and GE 1.5sle turbines have source heights of 80 m and blade lengths of 40 m which are nearly same as present investigated 2 MW turbine. This data is obtained for one of Vestas V82 and GE 1.5sle turbines from Jericho Rise operating wind farm located in US state of New York [20]. The results demonstrate that for wind speeds lesser than 7 m/s both experiment noise data for Vestas 82 and GE 1.5sle agree closely with each other within 1%. However, from 7 m/s to 10 m/s the sound power level remained constant which implies that there is no influence of wind speeds on sound levels which contradicts the BPM model predictions as the model is strongly dependent on Mach number. This suggests that turbines are deliberately controlled above certain wind speeds in order to regulate power. Further, the model simulated values for the present case of 2 MW turbine also agree closely with experiment data of both turbines with a peak difference of 5dBA at wind speed of 6 m/s. This shows that model can predic the sound levels accurately and reliably be used for the noise assessment of wind turbines.

*Trailing Edge Bluntness Noise Characterization for Horizontal Axis Wind Turbines [HAWT]… DOI: http://dx.doi.org/10.5772/intechopen.99880*

#### **Figure 11.**

*Computed turbulent boundary layer vortex shedding noise (TEB-VS) for a blade length of 38 m, 2 MW turbine using trailing edge thickness of 0.5%c at two different wind speeds (a) 7 m/s (b) 10 m/s.*

#### **Figure 12.**

*Computed difference, Δ dB, of the turbulent boundary layer vortex shedding noise level (TEB-VS) between wind speeds 7 m/s and 10 m/s, at trailing edge thicknesses of 0.1%c and 0.5%c.*

#### **Figure 13.**

*Sound power levels computed for various wind speeds, for present 2 MW turbine, 38 m blade length compared to experiment data for Vestas V82, GE 1.5sle at same hub heights.*
