Franck Bertagnolio

Trailing Edge Noise Model Validation and Application to Airfoil Optimization

The aim of this article is twofold. First, an existing trailing edge noise model is validated by comparing with airfoil surface pressure fluctuations and far field sound pressure levels measured in three different experiments. The agreement is satisfactory in one case but poor in two other cases. Nevertheless, the model reproduces the main tendencies observed in the measurements with respect to varying flow conditions. Second, the model is implemented into an airfoil design code that is originally used for aerodynamic optimization. An existing wind turbine airfoil is optimized in order to reduce its noise emission, trying at the same time to preserve some of its aerodynamic and geometric characteristics. The new designs are characterized by less cambered airfoils and flatter suction sides. The resulting noise reductions seem to be mainly achieved by a reduction in the turbulent kinetic energy across the boundary layer near the trailing edge and to a lesser extent by a smaller boundary layer displacement thickness.

Aerodynamic Noise Characterization of a Full-Scale Wind Turbine through High-Frequency Surface Pressure Measurements

The aim of this work is to investigate and characterize the high-frequency surface pressure fluctuations on a full-scale wind turbine blade and in particular the influence of the atmospheric turbulence. As these fluctuations are highly correlated to the sources of both turbulent inflow noise and trailing edge noise, recognized to be the two main sources of noise from wind turbines, this work contributes to a more detailed insight into noise from wind turbines. The study comprises analysis and interpretation of measurement data that were acquired during an experimental campaign involving a 2 MW wind turbine with a 80 m diameter rotor as well as measurements of an airfoil section tested in a wind tunnel. The turbine was extensively equipped in order to monitor the local inflow onto the rotating blades. Further a section of the 38 m long blade was instrumented with 50 microphones flush-mounted relative to the blade surface. The measurements of surface pressure spectra are compared with the results of two engineering models for trailing edge noise and for turbulent inflow noise. The measured pressure fluctuations are related to the local inflow angle and are also compared to measurements in a wind tunnel on a copy of the blade section of the full scale blade. Computational Fluid Dynamics calculations were conducted to investigate the influence of the inflow conditions on the airfoil and blade sections aerodynamics and aeroacoustics. Comparisons between measurement data and model results show the influence of atmospheric turbulence. The different noise generation mechanisms can be identified and the influence of various parameters can be consistently reproduced by the models.

Experimental Investigation of the Surface Pressure Field for Prediction of Trailing Edge Noise of Wind Turbine Aero Foils

This paper concerns the characterisation of turbulent boundary layer trailing edge noise by measuring the surface pressure field. Two aerofoils typically used at the outer blade section of modern MW wind turbines were tested in an anechoic wind tunnel for Reynolds numbers ranging from 1 million to 1.9 million and angles of attack ranging from −10° to 14°. The emitted trailing noise from the aerofoils was measured with a microphone array at a distance of 1.6 m away from the aerofoil. The two-dimensional surface pressure field, which is considered the source of the emitted trailing edge noise, was measured with pinhole microphones distributed in streamwise and spanwise direction on the surface of the aerofoil. Two acoustic formulations relating the fluctuating surface pressure field to far field trailing edge noise were investigated. The measurements of the fluctuating surface pressure field were used as input to the model. There was a factor of 2 as difference between the two models. The prediction of the far field trailing edge noise with one model was in excellent agreement with the microphone array measurements in a frequency range of 500-2000 Hz. This opens up the possibility of quantifying trailing edge noise with surface pressure measurements. In the high frequency range the models overpredicted the measured far field sound. For frequencies lower than 500 Hz more validation is necessary. The measurements of the surface pressure field revealed that the spanwise correlation length scale is only a function of the Reynolds number when properly normalised. This result can be used to reduce the number of surface pressure microphones necessary to characterise far field trailing edge noise.

Noise emission from wind turbines in wake - Measurement and modeling

The influence of the wake of an upstream turbine impinging another one located further downstream is studied focusing on the latter’s noise emission. Measurement data are investigated in the form of surface pressure fluctuations acquired using microphones flush-mounted in a wind turbine blade near its tip, characterizing the noise sources. Numerical results from a wind turbine noise model are also included in the analysis. The wind speed deficit and increased turbulence levels of the wake flow are clearly observed. Surface pressure measurements strongly support the fact that turbulent inflow noise is increased. However, numerical results show that the wake velocity deficit reduces noise in certain circumtances. This can compensate, or even sometime more than compensate, the additional noise emission expected as a result of the wake turbulence. Furthermore, noise amplitude modulation appears to increase when the turbine is impacted by the wake flow.

Aerodynamic effects of compressibility for wind turbines at high tip speeds

In the present work two dimensional airfoil computations are used to investigate the effects of compressibility in the tip region of large scale wind turbines of 20 MW+ size. In the past application of incompressible CFD solvers have been wide spread for wind turbine aerodynamics, due to their efficiency and robustness at the near incompressible conditions experienced near the rotor center. With the increasing size of modern wind turbines and the desire to approach high tip speeds, the incompressible assumption might be violated in the tip region of the turbine. To investigate the effects of compressibility and the possibility of correcting incompressible flow solutions using explicit compressibility corrections, a CFD study of 2D airfoil aerodynamics at conditions of a large scale wind turbine is performed. The present study show that classical compressibility corrections can be successfully applied as a post-processing step to incompressible solutions, reducing the error in the predicted lift and drag to within a few percent for attached flow conditions where viscous effects are limited at Mach numbers upto 0.3.

In search for a canonical design ABL stability class for wind farm turbines

Production as well as loading of wake exposed wind turbines is known to depend significantly on stability of the Atmospheric Boundary Layer (ABL), which adds a new dimension to design of wind farm turbines. Adding this new aspect in wind turbine design makes the number of design cycle computations to blow up with a factor equal to the number of representative stability bin classes. The research question to be answered in this paper is: Can an ABL stability probability distribution in a meaningful way be collapsed into a representative design stability class as based on a (predefined) confidence level.

A Stochastic Static Stall Model Applied to Wing Turbine Blade

The aim of this work is to improve aeroelastic simulation codes by accounting for the unsteady aerodynamic forces that a blade experiences in static stall. A model based on the spectral representation of the aerodynamic forces is defined. Some three-dimensional features of the actual flow are taken into account in the model. The input data for the model can be collected either from wind tunnel measurements or numerical results from Computational Fluid Dynamics calculations of airfoil sections at constant angles of attack. An analysis of these data is provided which helps to understand the characteristics of stall. The model is applied to the case of a wind turbine rotor and results are compared to experimental data.