Boundary Layer Development and Flow-Induced Noise of Airfoils: the effect of high inflow turbulence on trailing-edge noise generation
Laura Botero Bolivar is a PhD student in the department Engineering Fluid Dynamics. (Co)Promotors are prof.dr.ir. C.H. Venner from the faculty of Engineering Technolgy and dr. L.D. de Santana from DNW AERO.
Wind energy is one of the leading options for sustainable power generation. Consequently, human factors regarding wind turbines have come into play in recent years, primarily due to the noise emitted by wind turbines, which has significant implications for human well-being. Noise regulations have emerged to limit the noise around wind parks, making wind turbine noise a limitation for the further exploitation of wind energy resources. This situation has led to much research in understanding and modeling wind turbine noise to maximize wind power within acceptable noise thresholds and integrate noise emissions into design and optimization processes. A future expectation is to control wind turbines based on noise predictions, imitating the current practices in optimizing aerodynamic performance.
In uniform inflow conditions, trailing-edge noise is the minimum amount of noise a wind turbine produces. It emerges when the wall-pressure fluctuations generated by the turbulence inside the boundary layer scatter at the trailing edge of the blades. In cases of turbulent inflow (as is usual for wind turbines), leading-edge noise becomes an additional noise source. It is caused by the interaction of atmospheric turbulence with the leading edge of the blades. It has been considered that trailing-edge noise is unaffected by the inflow turbulence, which is valid for low turbulence intensities. However, when inflow turbulence is high, there is a strong interaction between the free-stream turbulence and the turbulent boundary layer, which would increase trailing-edge noise. This phenomenon has not been studied for airfoils before, nor has the effect of this interaction on the wall-pressure fluctuations and trailing-edge noise. This thesis aims to fill this gap by examining the impact of high inflow turbulence on the boundary layer, the wall-pressure fluctuations along the airfoil chord, and, ultimately, the trailing-edge noise of airfoils. To this end, dedicated experiments were conducted in the aeroacoustic wind tunnel at the University of Twente, measuring the boundary layer, the wall-pressure fluctuations, and the flow-induced far-field noise of airfoils both with and without inflow turbulence.
This thesis is divided into four parts. The first part focuses on the experimental investigation of trailing-edge noise generation for the case of uniform very low turbulent inflow. Trailing-edge noise generation is highly tied to the state of the boundary layer close to the trailing edge and the associated convective pressure pattern. Therefore, this part investigates experimentally the boundary layer development on airfoils, as well as the relationship between the boundary layer state and the wall-pressure spectrum and far-field noise. The remote microphone probe (RMP) technique is proven to be the best option for measuring wall-pressure fluctuations in small-scale models with a high spatial resolution. Therefore, this part also presents an extensive analysis of the technique, including calibration procedures and the impact of RMP installation on determining turbulent quantities. RMPs were used to instrument two airfoils, a NACA 63(3)018 (chosen as it is the symmetric version of typical wind turbine tip airfoils) and a NACA 0008 (chosen to generalize results to thinner airfoils). Each airfoil is instrumented with 82 RMPs distributed strategically along the chord and span. This instrumentation is key for studying the boundary layer development with and without inflow turbulence presented in this thesis.
The second part of the thesis extends the research about trailing-edge noise generation, considering the effect of turbulence on the noise source. Flow-induced noise from several distinct airfoils was measured under both turbulent and non-turbulent inflow conditions. The inflow turbulence used in this thesis (approximately 13% of intensity) increases trailing-edge noise across the entire frequency range. This increase is attributed to the penetration of free-stream turbulence (FST) into the turbulent boundary layer developed over the airfoils, resulting in higher velocity fluctuations and larger turbulent structures across the boundary layer. This, in turn, increases the wall-pressure spectrum level across all frequencies and the spanwise correlation length in the low-frequency range.
Consequently, the increase of trailing-edge noise is more pronounced in the low-frequency range, creating a strong competition with leading-edge noise for dominance. The findings further demonstrate that FST penetration on the boundary layer occurs differently for the large and small turbulent structures within the boundary layer. This directly affects how the effect of the FST is observed in the low- and high-frequency ranges of the wall-pressure spectrum and trailing-edge noise. These insights facilitate a comprehensive assessment of the impact of FST across the entire frequency spectrum for various airfoils at several conditions, only considering the turbulent boundary layer characteristics for the case of undisturbed inflow.
The third part of the thesis shows the prediction of leading- and trailing-edge noise of a full-scale wind turbine validated again measurements. The method proposed in this thesis couples relatively fast wind turbine computational fluid dynamic simulations using actuator lines with a fast turn-around noise prediction method. With this approach, the noise prediction method is of high fidelity since the wind turbine geometry is accounted for in both flow and acoustics predictions. Furthermore, when coupling the actuator line with the noise prediction method, it is possible to account for transient effects that can cause asymmetries in the rotor plane noise predictions. A key contribution of this part is the publication of a validated open-source code for wind turbine noise prediction.
The conclusions of this thesis are presented in the fourth part. What we achieved in this doctorate thesis contributes to a better understanding and evaluation of noise generation with and without inflow turbulence for airfoils, which is relevant for wind turbines and other applications. This would lead to reducing the environmental impact (for men and animals) of several aerodynamic applications and sustainable wind energy growth. Accounting for the influence of inflow turbulence on trailing-edge noise is pivotal for accurately assessing aerodynamic noise and for designing effective noise mitigation strategies for a particular frequency range. The results of this thesis for evaluating the boundary layer development and standard use of the RMP are very useful for experimentalists to plan their instrumentation or for new researchers in this area. Finally, providing validated open-source codes for assessing leading-edge noise, trailing-edge noise, and full-scale wind turbine noise significantly contributes to the aeroacoustics and wind energy communities to have standardized prediction methods and use them for optimization processes.
