W. Würz

Design and Wind-Tunnel Verification of Low-Noise Airfoils for Wind Turbines

A method for the prediction of the airfoil trailing-edge far-field noise is presented. The model employs the airfoil analysis code XFOIL to determine the initial and boundary conditions for a subsequent boundary-layer analysis using the finite-difference code EDDYBL featuring a Reynolds stress turbulence model that finally provides the input data for the noise prediction by a modified TNO Institute of Applied Physics model. The prediction scheme was applied in the European silent rotors by acoustic optimization project to design new, quieter airfoils for the outer blade region of three different wind turbines in the megawatt class. The objective was to reduce the airfoil self-noise without loss in aerodynamic performance

Comprehensive evaluation and assessment of trailing edge noise prediction based on dedicated measurements

An extensive assessment and step by step validation of turbulent boundary-layer trailing-edge interaction (TBL-TE) prediction was conducted based on the boundary layer properties calculated by three different aerodynamics methods, XFOIL, Wilcox EDDYBL and the RANS solver FLOWer. For this purpose detailed measurements of turbulent boundary-layer properties like two-point turbulent velocity correlations, the spectrum of the associated wall pressure fluctuations (WPFs) and the emitted trailing-edge noise have been performed in the Laminar Wind Tunnel (LWT). The measurements were performed for the NACA 0012 airfoil. Most of the investigated cases show that the numerical WPF and far-field radiated noise models capture the measured peak amplitude level as well as the peak position remarkably well, if the turbulence noise source parameters are estimated properly including turbulence anisotropy effects.

Three-dimensional acoustic-roughness receptivity of a boundary layer on an airfoil: experiment and direct numerical simulations

We describe an experimental and numerical investigation of the problem of excitation of three-dimensional Tollmien-Schlichting (TS) waves in a boundary layer on an airfoil owing to scattering of an acoustic wave on localized microscopic surface non-uniformities. The experiments were performed at controlled disturbance conditions on a symmetric airfoil section at zero angle of attack. In each set of measurements, the acoustic wave had a fixed frequency f ac , in the range of unstable TS-waves. The three-dimensional surface non-uniformity was positioned close to the neutral stability point at branch I for the two-dimensional perturbations. To avoid experimental difficulties in the distinction of the hot-wire signals measured at the same (acoustic) frequency but having a different physical nature, the surface roughness was simulated by a quasi-stationary surface non-uniformity (a vibrator) oscillating with a low frequency f v

Broadband Airfoil Trailing-Edge Noise Prediction from Measured Surface Pressures and Spanwise Length Scales

In this paper an effective approach to estimate airfoil Turbulent Boundary-Layer Trailing-Edge (TBL-TE) far-field noise from measured surface pressure fluctuations (SPF) is evaluated. Measurements of both SPF and TE noise were performed on a NACA 0012 airfoil of 0.4 m chord at Reynolds numbers of 1.0–1.9 millions for various angles of attack. A non-homogeneously spaced array of five Kulite-sensors near the TE at x/c = 0.989 is employed to measure point spectra and spanwise two-point correlations of surface pressure fluctuations. Finally spanwise SPF length-scales are derived as function of frequency. Comparisons to measured TE noise and semi-empirical predictions of surface pressures and far-field noise show very good agreement. It is found, that the proposed method can cover a larger frequency range than standard acoustic measurement techniques. Therefore it can provide valuable assistance in extending spectra obtained conventionally, mainly to low frequencies. Furthermore, pressure and suction side contributions to far-field noise can be obtained separately.

A Semi-Empirical Surface Pressure Spectrum Model for Airfoil Trailing-Edge Noise Prediction

A semi-empirical model to determine the wall pressure frequency spectrum beneath a two-dimensional, pressure gradient turbulent boundary-layer is presented. The model is derived based on the experimental wall pressure data of various research groups. The experimental database includes both the equilibrium flat plate and non-equilibrium airfoil boundary-layer flow cases and covers a large range of Reynolds numbers, 1.0 times 103 < Reδ2 < 3.0 times 104. The enhanced model is a combination of the modified Chase-Howe, Goody and Rozenberg models, and is a simple function of the ratio of pressure and timescales of the outer to inner part of the boundary-layer. The key advantage of the present model is that it incorporates the Reynolds number, the boundary-layer loading as well as pressure gradient effects through an amplitude scaling function and timescale ratio, and compares well to the experimental data. Spectral features of the detailed measurement data and various scaling behavior of the wall pressure spectrum are elaborately investigated. A summary of the results on the applicability and limitation of the model for various test cases is discussed. The enhanced model is further applied to develop an airfoil turbulent boundary-layer trailing-edge interaction (TBL-TE) far-field noise prediction scheme. Prediction results are compared with the well established experimental database and encouraging results are found. The enhanced Wall Pressure Fluctuation (WPF) as well as trailing-edge noise spectra models accuracy for the maximum noise level is in ±2dB range for the test cases examined. The model can be applied further for acoustic airfoil design and optimization and in various aeroacoustic applications.

Experimental Investigations of Tonal Noise on a Vehicle Side Mirror

Experimental investigations on an isolated vehicle side mirror model are performed in a low turbulence wind tunnel aiming at the identification of the acoustic source mechanism leading to tonal noise emission. A combination of acoustic measurements and Particle Image Velocimetry shows that the discrete frequency noise can be related to the presence of a laminar boundary layer separation extending up to the trailing edge on the mirror’s side surface and upper side. In the vicinity of the trailing edge, the shear layer develops into a vortical structure, which impinges on the trailing edge. By utilization of Proper Orthogonal Decomposition on the ensembles of instantaneous velocity fields and application of a phase sorting method, the length scales of the vortical structures could be extracted and related to the acoustic radiation. Order of magnitude analyses show parallels to the acoustic feedback mechanism known from the NACA 0012 airfoil.

Study of 3D Wall Roughness Acoustic Receptivity on an Airfoil

Hot-wire measurements are perfomed on linear 3D acoustic receptivity in a two-dimensional laminar boundary layer. A localized quasi steady surface roughness serves as the receptivity element. A plane acoustic wave with frequency f ac scatters at this vibrating source and the generated TS-wave train is measured downstream in spanwise cuts and at combination frequencies (f 1,2 = f ac ∓ f v ). After Fourier decomposition linear stabiltity theory is used for upstream extrapolation to the initial amplitudes at the roughness element. The dispersion characteristic is determined and the complex receptivity function is calculated by normalization of the initial TS-spectra with the related amplitudes and phases of the surface vibrator and the acoustics. The results are compared with Direct Numerical Simulations based on a vorticity-velocity formulation of the complete Navier-Stokes equations and a new embeded wall model. Good overall agreement is achieved.

Experimental study of resonant interactions of modulated waves in a non self-similar boundary layer

The current work is devoted to the study of weakly non-linear interactions of Tollmien-Schlichting waves in an incompressible, 2D airfoil boundary layer. Selected resonant regimes are investigated with emphasis to the regimes where more than one fundamental T-S wave are present in the flow. The results show that amplification rates of the effective sub-harmonic modes are not affected by modulations in time of the fundamental wave. It is also shown that modulations of the 2D fundamental waves tend to generate additional modes at modulation frequency. Furthermore, these modes can resonate with the fundamental ones as detuned subharmonics. This mechanism seems to be responsible for the initial seeds of subharmonics in cases of ‘natural’ transition.

Interaction of a Cylindrical Roughness Element and a Two-Dimensional TS-Wave

Roughness induced transition in the leading edge region of airfoils is one of the challenges in the design of highly efficient wind turbine blades especially with respect to various inflow turbulence levels. The present experimental investigation discusses, therefore, the interaction of a laminar boundary layer perturbed by a single, 2D disturbance mode with a cylindrical roughness element at medium height. High and low speed streaks in the roughness wake suggest the presence of a counter-rotating vortex pair. A significantly increased amplification of the disturbance mode downstream of the roughness in comparison to the zero roughness height case leads to the formation of 3D structures with a dominant spanwise length scale corresponding to the roughness diameter. However, these 3D structures are only very weakly distinctive far downstream.

Systematic investigations of 3D acoustic receptivity with respect to steady and unsteady disturbances. Experiment and DNS

In the present paper, the linear 3D acoustic-roughness receptivity of the 2D boundary layer of an airfoil is investigated systematically with respect to main influence parameters by means of wind tunnel experiments and Direct Numerical Simulations (DNS). A plane acoustic wave with frequency f ac scatters at a specially designed localized roughness element, that is capable to vibrate with frequency f v . The resulting Tollmien-Schlichting (TS) waves appear at combination frequencies (f 1,2 = f ac ∓ f v ) and develop as a wave train downstream of the surface roughness. The complex receptivity function is evaluated by normalization of the initial TS-spectra with the related amplitudes and phases of the surface vibrator and the acoustics. The main influence parameters investigated are the frequency (of the acoustic and the TS wave), the pressure gradient (given by the streamwise position of the surface roughness) and the influence of surface vibrations on the acoustic receptivity. Additionaly pure vibrational receptivity is studied. The main goal of this investigation is to study and compare systematically different types of receptivity in the same base flow developing on an airfoil at a Reynolds number typical for glider applications.