The yaw state constitutes a typical operating condition for wind turbines. However, the widely used Blade Element Moment (BEM) theory, due to its adoption of planar disc assumptions, introduces certain computational inaccuracies in yaw conditions. This research aims to develop a new modified BEM method by replacing the momentum theory in traditional BEM with the Madsen analytical linear two-dimensional actuator disc model in order to enhance the accuracy in calculating the aerodynamic performance of yawed wind turbines. Two approaches are introduced to determine the variable parameters in the new modified model: one based on traditional BEM predictions in non-yaw conditions and the other using empirical values determined using experimental data. The new modified model is evaluated against experimental data, CENER FAST, and HAWC2 for the MEXICO rotor. From the comparisons, the new modified method demonstrates closer agreements with experimental values, particularly in the mid and outer parts of the blades. At a wind speed of 15 m/s and a yaw angle of 30°, the discrepancies between computation and measurement are reduced by at least 2.33, 1.22, and 3.25 times at spanwise locations of 60%Radius (R), 82%R, and 92%R, respectively, compared to CENER FAST or HAWC2, demonstrating the feasibility of the proposed methodology.

The ground surface near a wind turbine has a significant influence on the sound propagation from the turbine, and it is therefore important to determine the ground impedance, a quantity that characterizes the ground surface acoustically. Outdoor ground parameters required by a multi-parameter model used to calculate the ground acoustic-impedance are typically unknown, which brings inconvenience for the model use. This paper introduces a technique to determine the parameters of ground acoustic-impedance for use in a multi-parameter impedance model (for example, the Attenborough four-parameter model). The technique consists of three steps: first, the data for sound-pressure level measured at a distance from two different heights are collected, and the sound-pressure-level difference is calculated; second, in line with the experimental data and the sound-pressure-level calculation formula, the MATLAB optimization tool is used to find the optimal values of the parameters used in the impedance model; and finally, when the optimization is finished, the acoustic impedance of the ground is obtained by substituting the optimal values into the impedance model. To check the performance of the calculation, the calculated sound-pressure-level difference is compared to the experimental one. Compared with a traditional method, the technique can significantly reduce the calculation error.