Leandro D. de Santana

Reduction of Airfoil Turbulence-Impingement Noise by Means of Leading-Edge Serrations and/or Porous Material

The paper is about the sound produced as turbulence impinges on the leading edge of an airfoil and its reduction by means of either leading-edge serrations (tubercles) or the use of porous materials. The first part describes a series of experiments performed on a NACA-12 airfoil in a low-speed open-jet anechoic wind tunnel. The airfoil is held between end-plates and the sound is measured in the far field in the mid-span plane The chord-based Reynolds number ranges from 1.3 105 to 2 105. Various versions of the airfoil are tested and compared to the baseline. Sound reduction is achieved by both serrations and porosity in a wide frequency range. The second part is devoted to dedicated prediction techniques. A new analytical model of the response of a serrated leading-edge is proposed, extending Amiets theory, in the limit of arbitrary large chord. Preliminary numerical modeling is also discussed for the response of a porous aifoil to incident disturbances, based on a panel method combined with a locally-reacting impedance model.

Panel method for turbulence-airfoil interaction noise prediction

This paper describes the theoretical formulation of an unsteady panel code for the prediction of turbulence-airfoil interaction noise, valid for incompressible flows. This panel code formulation adopts the vorticity as the elementary distributed singularity, in contrast with other approaches that use distributed sources and a constant intensity vortex sheet. The present analysis concentrates on the determination of the airfoil response to an unsteady periodic perturbation. Results shows the convergence of this code as the number of panels is increased and the time step is decreased. In addition it is presented the effect of the airfoil thickness, camber and angle of attack to its response function.

Leading-Edge Noise Prediction of General Airfoil Profiles with Spanwise-Varying Inflow Conditions

This paper presents a study of the leading-edge noise radiated by an airfoil undergoing a turbulent inflow. The noise prediction of generic airfoil profiles subjected to spanwise-varying inflow con...

Numerical computation of airfoil-gust lift response with applications to leading-edge noise generation

We propose a numerical framework to compute the airfoil-gust lift response and its subsequent leading-edge noise generation due to an incident compressible turbulent flow. This approach is valid for blades with large aspect ratios, general airfoil geometries, three-dimensional supercritical perturbations and compressible subsonic flows. The linearized equation for unsteady potential flow is rewritten as a Helmholtz equation in the transformed Prandtl-Glauert plane, leading to a boundary value problem prescribed by the linearized airfoil theory. The boundary element method is then employed iteratively to solve the Helmholtz equation for realistic airfoil configurations. Results show that non-zero thickness airfoils drastically reduce the pronounced acoustic radiation expected by oblique gusts. However, at moderate Mach numbers, the compressibility effects may increase the noise radiation in the upstream direction compared to Amiet’s analytical solution when thickness is addressed into the analysis.

Numerical computation of gust aerodynamic response for realistic airfoils: Application of Amiet’s theory

Current knowledge on the noise generation mechanisms of an airfoil subjected to a turbulent flow indicates that an increment to the airfoil thickness leads to a reduction of the leading-edge noise. This effect is generally attributed to the turbulence distortion occurring close upstream the airfoil leading-edge, combined to a reduction in the magnitude of the aerodynamic transfer function. However, current methodologies do not allow to clearly separate the role of those two distinct physical mechanisms. This paper proposes a technique to compute the aeroacoustic transfer function allowing the study of the leading-edge noise radiated by realistic airfoil geometries. This approach is able to account for trailing-edge aerodynamic back-scattering effects and is valid for blades with large spans, general airfoil geometries, high-frequency perturbations and subsonic compressible flows. The proposed technique deals with the possibility of rewriting the linearized potential flow equations as the Helmholtz formulation leading to a boundary value problem prescribed by the linearized airfoil theory. This problem is calculated by an iterative procedure, where the linearized airfoil theory is solved by a boundary element method (BEM). The proposed numerical methodology is verified against analytical results presented by Amiet’s theory. In this paper we show the importance to account for the effects of a realistic airfoil geometry in the calculation of the aeroacoustic transfer function to improve leading-edge airfoil noise prediction.

The UTwente aeroacoustic wind tunnel upgrade

This paper describes the UTwente Aeroacoustic Wind Tunnel including its recent anechoic chamber renovation and commissioning. This testing facilitywas originally built in the 1970’s as a closed circuit/closed test section aerodynamic wind tunnel and converted into an aeroacoustic open jet/closed circuit facility in 2001. In 2018 this aeroacoustic wind tunnel has been further upgraded with a special focus on reducing the anechoic chamber cut-off frequency. In the current configuration, the UTwente Aeroacoustic Wind Tunnel is an open test-section facility equipped with a 132 kW electric motor able to produce flow up to 60 m/s with turbulence intensity below 0.08%. The 0.7 x 0.9 m2 rectangular jet with Reynolds number of 324.000 based on the criteria of 0:1 √S. This test facility can be used with airfoil models with chord of up to 0.30 m, reaching a chord-based Reynolds number of up to 1.2 million. The anechoic chamber measures 6 x 6 x 4 m3 equipped with a combination of wedges and flat absorbers leading to a cut-off frequency of 160 Hz and is commissioned accordingly to the ISO 3745 norm.

Experimental characterization of vortex generators induced noise of wind turbines

The wind energy industry is progressively aiming towards larger wind turbine size as a means of increasing energy production efficiency. Wind turbine rotor blades are becoming significantly more flexible and have an increased risk of structural failure. Thicker airfoil profiles can be used to increase the structural strength. However, thicker airfoils are more prone to flow separation with adverse effect on power generation. Vortex generators (VGs) are an effective solution to delay flow separation. However, VGs significantly increase radiated noise levels. This presented research experimentally investigates the effect of VGs on blade noise production. Specifically, the influence of the VG geometric shape is evaluated while keeping the dimensions constant. Wind tunnel tests have been carried out on a NACA 0018 that is equipped with triangular vane-type VGs specifically designed for the test conditions. A microphone phased array is used to assess the radiated noise using the beamforming technique. The results show that the VGs significantly contribute to increase the blade noise production with having a predominant role at high-frequencies where the VG acts as a non-compact aeroacoustic source.

An experimental investigation of trailing-edge noise reduction due to elasticity

The proximity of the source and an edge can make the acoustic scattering by wings a significant source of aerodynamic sound. Theoretical results have shown that elastic edges lead to reductions of acoustic scattering; however, experimental confirmation of theoretical trends is difficult, since surface vibrations modify both the source structure and the scattering properties. A simplified, controlled setting for measurements of acoustic scattering, allowing the evaluation of fluid-structure interactions, would thus be desirable to study how elastic edges modify the radiated sound. We present an experimental procedure to isolate the scattered field using a loudspeaker in the vicinity of at plates. The methodology is applied to three different plates, made of steel, aluminum and carbon fiber, as a demonstration. The responses of these elastic plates are studied for a sound source of dipole type near the trailing edge. The method is based on the experimental determination of frequency response functions between source and radiated sound for experiments with and without the plate; subtraction of results, accounting for amplitude and phase, isolates the scattered field. Experimental results treated with the developed procedure were compared with predictions made by numerical simulations performed with a Boundary Element Method (BEM), coupling the acoustic problem with the plate vibration. The comparison between experimental and numerical results revealed that a two-dimensional model can predict satisfactorily the reductions in scattered field by elastic plates observed in the experiment. The present methods can be used to support the choice between different materials for edges focusing on their respective acoustic benefit.

Comparison between analog and digital microphone phased arrays for aeroacoustic measurements

Microphone arrays are useful measurement devices for estimating the location and strength of sound sources. Numerous comparative studies have been conducted regarding the performance of acoustic imaging methods in the past, but literature lacks of a systematic investigation on the role of the hardware on the measurements. This research focuses on the performance differences between two 63-microphone arrays: one with digital MEMS (Micro Electro Mechanical Systems) microphones and the other with analog condenser microphones. Both systems are used on an aeroacoustic experiment performed in an anechoic open-jet wind tunnel featuring two airfoils (NACA 0012 and NACA 0018) equipped with trailing-edge serrations. Whereas both arrays provided similar frequency spectra when analyzing trailing-edge noise emissions (which are in agreement with previous research), the analog array seems to offer source maps of higher quality with a higher dynamic range (lower sidelobe level). Moreover, the results of the digital array featuring trailing-edge serrations show a noise increase at the higher frequencies (4 kHz) with respect to the straight-edge case, which is not expected from the findings of previous experimental research. The results of the analog array do not present such behavior. This manuscript is the result of a collaboration project between the University of Twente (UTwente) and Delft University of Technology (TU Delft).

Application of amiet’s theory for noise prediction of general airfoil profiles subjected to spanwise-varying inflow conditions

In this paper, three different techniques are combined to provide a complete physics-based semi-anytical model for leading-edge noise prediction. The model is based on the classical theory of Amiet. Here, the two-dimensional turbulence spectrum is computed by a model based on the rapid distortion theory and the aeroacoustic transfer function is numerically evaluated by the boundary element method to account for the effects of the general airfoil profiles. The influence of spanwise inhomogeneities is also considered through the application of the inverse strip method. An assessment of each individual technique on the radiated noise is provided. This research shows that the turbulence distortion occurring at the leadingedge plays a significant role on the predicted noise levels. Compared with the von Karman model for isotropic turbulence, the rapid distortion theory predicts reduced noise levels at high-frequencies and increased levels at low-frequencies. This paper also shows that the spanwise-varying inflow, here represented by a linearly varying condition, contributes to raising the acoustic radiation when compared to the similar uniform inflow case. By considering modifications on the airfoil leading-edge radius and on the airfoil overall thickness, we show that the leading-edge bluntness plays a key role on reducing gust-airfoil interaction noise. This observation is more pronounced for microphones positioned downstream of the airfoil and for high frequencies.