4.2.3.5. Model Verification
4.2.3.5.1. Reference Wind Turbine
The noise model of OpenFAST is exercised by simulating the aeroacoustics noise emissions of the IEA Wind Task 37 landbased reference wind turbine ([aaBTD+19]). The main characteristics of the reference wind turbine are presented in Table 4.2.
Data 
Value 
Data 
Value 

Wind class 
International
Electrotechnical
Commision 3A

Rated
electrical
power

3.37 megawatts 
Rated
aerodynamic
power

3.6 megawatts 
Drivetrain &
generator
efficiency

93.60% 
Rotor diameter 
130 meters 
Hub height 
110 meters 
Cutin wind speed 
4 meters/second 
Cutout wind speed 
25 meters/second 
Rotor cone angle 
3 degrees 
Nacelle tilt angle 
5 degrees 
Max blade tip speed 
80 meters/second 
Rated
tipspeed
ratio

8.16 
Maximum
aerodynamic Cp

0.481 
Rated rotor speed 
11.75
revolutions per
minute

The OpenFAST model of the wind turbine is available at https://github.com/OpenFAST/rtest and is optionally coupled to the Reference OpenSource Controller. 2
4.2.3.5.2. CodetoCode Comparison
A detailed codetocode comparison was conducted to verify the implementation of the noise models linked to OpenFAST with the implementation available at the Wind Energy Institute of the Technical University of Munich, Germany. The latter is described in Sucameli ([aaSBCB18]) and is implemented in the wind turbine design framework CpMax, which adopts the multibodybased aeroservoelastic solver CpLambda.
The comparison is conducted for the main noise sources—turbulent inflow and the TBLTE noise—for both the single airfoil profile and full turbine. This helped resolve a few implementation mistakes and small inconsistencies. The comparison is performed with a steady wind of 8 meters per second (m/s), no shear, a rated pitch angle of 1.17 degrees (deg), and a fixed rotor speed of 10.04 revolutions per minute (rpm). A fixed value of 0.1 is assumed for the incident turbulent intensity, \(I_{1}\).
Fig. 4.17 shows the predictions in terms of SPL for the Amiet model with the angleofattack correction from OpenFAST, the Simplified Guidati model generated by OpenFAST, and the Amiet model from CpMax.
The two implementations of the turbulent inflow Amiet model return a perfect match between OpenFAST and CpMax. The chosen scenario sees the blade operating at optimal angles of attack and, therefore, the effect of the angle of attack correction is negligible. The plots also show the great difference between the Amiet model and the Simplified Guidati model. It may be useful to keep in mind that the Simplified Guidati model has, in the past, been corrected with a factor of +10 dB, which is applied here.
For the same inflow and rotor conditions, the BPM and TNO TBLTE noise models are compared in Fig. 4.18. The match is again satisfactory, although slightly larger differences emerge that are attributed to differences in the angles of attack between the two aeroelastic solvers and in different integration schemes in the TNO formulations.
The last comparison looked at the directivity models and the overall sound pressure levels at various observer locations. Simulations are run distributing 200 observers in a horizontal square of 500 meters (m) by 500 m (see Fig. 4.19). The noise is computed from the Amiet and the BPM turbulent boundary layertrailing edge models. The codetocode comparison returns similar predictions between OpenFAST and CpMax. The comparison is shown in Fig. 4.20.
The main conclusion of this codetocode comparison is that, to the best of authors’ knowledge, the models are now implemented correctly and generate similar SPL and overall SPL levels for any arbitrary observer. Nonetheless, it is clear that all of the presented models are imperfect, and improvements could be made both at the theoretical implementation levels.
4.2.3.5.3. Model Usage
The aeroacoustics model of OpenFAST has four options for the outputs:
Overall sound pressure level (dB/Aweighted decibels [dBA])—one value per time step per observer is generated
Total sound pressure level spectra (dB/dBA)—one spectrum per time step per observer is generated between 10 Hz and 20 kHz
Mechanismdependent sound pressure level spectra (dB/dBA)—one spectrum per active noise mechanism per time step per observer is generated between 10 Hz and 20 kHz.
Overall sound pressure level (dB/Aweighted decibels [dBA])—one value per blade per node per time step per observer is generated
The overall SPL from the first option can be used to plot directivity maps of the noise. An example, which was generated using a Python script, 3 is shown in Fig. 4.21. The noise map, which shows the overall SPL averaged over 1 rotor revolution, is generated for a steady wind speed of 8 m/s, a fixed rotor speed of 10.04 rpm, and a 1.17deg pitch angle. In a horizontal circle of 500 m in diameter, 1681 observers are placed at a 2m height. Only the Simplified Guidati and the BPM TBLTE noise models are activated.
The second output can be used to generate SPL spectra. These spectra can be computed for various observers and optionally Aweighted to account for human hearing. Fig. 4.22 shows the total SPL spectra computed for the same rotor conditions of the previous example. The Aweight greatly reduces the curve at frequency below 1,000 Hz while slightly increasing those between 1 kHz and 8 kHz.
The third output distinguishes the SPL spectrum per mechanism. Fig. 4.23 shows the various SPL spectra estimated by each noise model for the same rotor conditions reported earlier. The total spectrum is visibly dominated by the turbulent inflow, TBLTE, and trailingedge bluntness noise mechanisms. Notably, the latter is extremely sensitive to its inputs, \(\Psi\) and \(h\). The reference wind turbine is a purely numerical model, and these quantities have been arbitrarily set. Users should pay attention to these inputs when calling the trailingedge bluntness model. Consistent with literature, the laminar boundary layervortex shedding and tip vortex noise mechanisms have negative dB values and are, therefore, not visible. Notably, these spectra are not Aweighted, but users can activate the flag and obtain Aweighted spectra.
Finally, the fourth output can be used to visualize the noise emission across the rotor. Fig. 4.24 shows the noise generation of the rotor as seen from an observer located 175 meters downwind at a height of 2 meters. The map is generated by plotting the overall SPL generated by one blade during one rotor revolution. The plot shows that higher noise is observed when the blade is descending (the rotor from behind is seen rotating counterclockwise). This effect, which matches the results shown in [aaMM03], is explained by the asymmetry of (4.60). Noise is indeed higher when the observer faces the leading edge of an airfoil (high \(\Theta_e\)), than when it faces the trailing edge (low \(\Theta_e\)).