Witold Krusz, ZA, MEiL, PW, Passive methods of airfoil noise reduction

Passive methods for airfoil noise reduction

A mechanism of airfoil noise reduction by passive methods at low Reynolds number flows is investigated by means of numerical methods such as DNS (Direct Numerical Simulations) and LES (Large Eddy Simulations). This has been performed for the Reynolds number of 200,000. The low-Reynolds-number regime is typical for small and medium UAVs (Unmanned Aerial Vehicles), wind turbines and low-speed fans. A typical far field sound spectra of rotating machinery parts contain both tonal and broadband components. Discrete tonal components are usually the most audible, therefore means for cancelling this type of noise are crucial in the engineering practice. In order to better understand the mechanism of the tonal noise generation, it has been studied widely for airfoil flows at Reynolds numbers between 100,000 and 2,000,000.

Generation of airfoil tonal noise is due to the feedback loop between the boundary layer on an airfoil pressure side and a dipole type acoustic source placed near the trailing edge. A linear spatial stability analysis has shown that the frequencies of the tonal components agree well with those of the most amplified Tollmien-Schlichting (T-S) waves on the pressure side of an airfoil. Lowson et al showed that the presence of the laminar separation bubble is a necessary condition for airfoil laminar instability noise to occur, and the separation bubble acts as an amplifier for the T-S waves. Moreover, Desequesnes et al showed that instabilities on the suction side of an airfoil also play an important role in the generation of the tonal components. Therefore, it can be stated that the mechanism of airfoil instability noise generation consists of two feedback loops, i.e. the main loop on the airfoil pressure side and the secondary feedback loop on its suction side.

The validation of the LES (Large Eddy Simulations) against the DNS (Direct Numerical Simulations) and some comparisons to the experimental data for the clean NACA0012 airfoil have preliminarily proven that the LES approach is an efficient and sufficiently accurate method for detailed simulations of airfoil boundary layer instability noise. This has been confirmed at the aerodynamic and aeroacoustic levels by comparisons of velocity profiles, location of the laminar separation bubbles, aerodynamic coefficients and far-field acoustic solutions. However, more research is needed that focuses on the near-field acoustic solutions. Based on the preliminary investigation of the leading and trailing edge serrations, it can be concluded that the LES is also an efficient and sufficiently accurate approach for the serrated airfoil geometries. It has been shown that serrated configurations can indeed be used to reduce the tonal components of airfoil noise.

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