University of Twente Student Theses


Airfoil self-noise predictions using DDES and the FWH analogy

Pindi Nataraj, P. (2022) Airfoil self-noise predictions using DDES and the FWH analogy.

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Abstract:Noise emissions from wind turbines are one of the hurdles to expanding existing wind energy infrastructure in the vicinity of urban environments. Airfoil self-noise is a fundamental contributor to wind turbine noise. Noise mitigation strategies in the form of blade add-ons have been the focus of contemporary research. A comprehensive understanding of the physics involved in noise generation is necessary to devise add-ons such as trailing edge serrations, metal foam trailing edges, and vortex generators; this can be accomplished by conducting wind tunnel experiments or high-fidelity simulations. However, wind tunnel experiments for certain flow regimes such as deep stall involve installation effects and jet interactions, which are cumbersome to decouple. Thus, it is common to resort to high-fidelity simulations for noise estimation in such cases. The scale disparity between the aerodynamic and acoustic phenomena renders high-fidelity approaches such as Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) impractical for direct noise predictions as they would require long simulation run times, complex numerical schemes, and very fine spatial and temporal resolution; these complications necessitate the use of lower fidelity approaches and acoustic analogies. The scope here is to explore the applicability of a CFD-CAA framework involving Delayed Detached-Eddy Simulation (DDES) and the Ffowcs-Williams and Hawkings (FWH) analogy to obtain far-field noise predictions for the significant airfoil self-noise mechanisms of turbulent boundary layer – trailing edge noise (TBL-TE) and separation-stall noise. The CFD-CAA framework is first composed and validated successfully for a quasi-2D laminar flow test case. Then, the case of the NACA0021 in deep stall is analyzed. The influence of dissipative convective schemes and sub-grid scale models on the flow resolution is investigated, following which the DDES approach is validated with existing experimental and numerical data. Furthermore, the far-field acoustic data obtained with the help of the FWH workflow agrees qualitatively with the trends observed in the literature. Finally, the case of the NACA0018 with attached flow is investigated with the DDES-FWH framework; this is an unconventional case for DDES as it does not involve flow separation. The boundary layer being resolved by RANS leads to an insufficient resolution of the length scales relevant for TBL-TE noise, producing a far-field noise signature that is not in line with the literature. It is then concluded that the DDES-FWH framework is only suitable for separation-stall noise and blunt trailing edge vortex shedding noise regimes.
Item Type:Essay (Master)
TNO Energy Transition, Petten, Netherlands
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
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