Mohammad Kamruzzaman

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.

Broadband Trailing-Edge Noise Predictions - Overview of BANC-III Results

The Third Workshop on Benchmark Problems for Airframe Noise Computations, BANC-III, was held on 14-15 June 2014 in Atlanta, Georgia, USA. The objective of this workshop was to assess the present computational capability in the area of physics-based prediction of different types of airframe noise problems and to advance the state-of-the-art via a combined effort. This documentation summarizes the results from workshop category 1 (BANC-III-1) which focuses on the prediction of broadband turbulent boundary-layer trailing-edge noise and related source quantities. Since the forerunner BANC-II workshop identified some room for improvements in the achieved prediction quality, BANC-III-1 relies on the same test cases, namely 2D NACA0012 and DU96-W-180 airfoil sections in a uniform flow. Compared to BANC-II particularly the scatter among predictions for the DU96-W- 180 test case could be significantly reduced. However, proposed adaptations of previously applied computational methods did not systematically improve the prediction quality for all requested parameters. The category 1 workshop problem remains a challenging simulation task due to its high requirements on resolving and modeling of turbulent boundary-layer source quantities.

On the Length Scales of Turbulence for Aeroacoustic Applications

Accurate evaluation of the turbulence integral correlation length scales is important for a large group of numerical flow-induced noise prediction methods. Due to the complex mathematical interrelationships of the two-point space-time velocity statistics, a direct evaluation of the integral length scale is a challenging task from the standard CFD methods. The present paper focuses on the theoretical and experimental investigations of five different integral length scales evaluation methodologies, namely i) two-point space correlation based method, ii) single point power-spectrum based approach, iii) wavenumber of the most energy containing eddy dependent method, iv) auto correlation via Taylor’s hypothesis and, v) correlation function curve fitting method. All these methods are analyzed based on the previously performed two-point turbulent boundary-layer (BL) correlation measurement data. A detail comparison study has been performed with discussions on the limitations and drawbacks of each method. Other length scales found in turbulence theory such as pseudo length scale, dissipation length scale, Prandtl mixing length, Taylor microscale and Kolmogorov scale have been also discussed elaborately. A theoretical relationship for the derivation of integral length scales based on standard RANS simulation is derived by employing Kolmogorov local isotropy hypothesis. The enhanced method allows modelling of the isotropic length scale and dissipation from the anisotropic experimental spectral data. This outcome is applied further for the RANS turbulence model validation purpose in the framework of a turbulent boundary-layer trailingedge interaction (TBL-TE) noise prediction model. Results are extensively validated with the wind tunnel measurement of high Reynolds number airfoil BL flows.

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.

Benchmarking of Trailing-Edge Noise Computations - Outcome for the BANC-II Workshop

The Second Workshop on Benchmark Problems for Airframe Noise Computations, BANC-II, was held on 7-8 June 2012 in Colorado Springs, CO, USA. The objective of this workshop was to assess the present computational capability in the area of physics-based prediction of dierent types of airframe noise problems and to advance the state-of-the-art via a combined effort. This paper summarizes the results from workshop category 1 which focuses on the prediction of broadband turbulent boundary-layer trailing-edge noise and related source quantities. 2D airfoil sections, namely a NACA0012 and DU-96-180, served as test cases. Code-to-code comparisons in this category were mainly restricted to relatively fast RANS-based methods applying statistical noise theory. Overall, the prediction capability was sufficient to capture the principal trailing-edge farfield noise scaling behavior in the mid-frequency range (about 1 kHz <= fc <= 5 kHz), i. e. the measured dependence of noise on angle-of-attack or free stream velocity. Differences in predicted trends appeared at lower and higher frequencies. Moreover, a comparatively large scatter among the DU-96-180 prediction results was observable, indicating individual room for improvement in the applied approaches.

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.

Evaluation of Measured Anisotropic Turbulent Two-point Correlation Data for the Accurate Prediction of the Turbulence Noise Sources

Noise emitted from 2D subsonic airfoil sections depends strongly on the specific properties of the turbulent boundary-layer passing the trailing edge. The intention of the present paper is three folds: (1) wind tunnel measurement of the two-point turbulent velocity correlations to analyze the trailing-edge near wake flow structure in detail, (2) assessment of the measurement data and post-processing of the turbulence properties in a way so that it can be comparable to numerical results, and (3) development of a relationship between isotropic theory and anisotropic measurement data to approximate quantities like isotropic longitudinal length scale Λf . Measurements of two-point turbulent velocity correlations were conducted in 2D turbulent boundary layer flows over two airfoils (NACA 0012 & NACA 643-418 ) at high Reynolds numbers (Re=1.5e6 and Re=2.5e6) in the Laminar Wind Tunnel (LWT) of the University of Stuttgart. Correlation tensor components were measured by two X-wire probes shortly downstream of the airfoil trailing edge with separation in vertical direction for different cases of boundary layer development (angles-of-attack, natural and fixed transition). Two different approaches to evaluate turbulence integral correlation length scales from the measured two-point correlation data are presented. The first methods provides anisotropic turbulence integral length scales estimated by fitting an exponential function to the measured two-point velocity correlation coefficient. The second method established a local relationship between the results from isotropic theory and anisotropic experimental data. This relationship permits efficient approximation of the isotropic longitudinal integral length scale (Λf), and enables the consideration of anisotropy effects in RANS predicted data. The outcome of both methods are been compared with RANS results and lead to significant improvement of the prediction of the turbulence noise sources. The benefit of these efforts will be further applied to a Turbulent Boundary-Layer Trailing-Edge (TBLTE) interaction noise prediction method (Rnoise) to analyze anisotropy effects of the airfoil trailing-edge near-wake flow.

Prediction of Flow-Induced Noise Sources of Wind Turbines and Application Examples

The consideration of aeroacoustic aspects and the minimization of noise emission is an integral part in the design process of modern onshore wind turbines. In the present paper dedicated aeroacoustic prediction methods are introduced that are applied at the University of Stuttgart for identification of noise sources and the aero-acoustic design and analysis of airfoils and rotors of wind turbines. The available portfolio of tools ranges from fast semi-empirical methods to numerical CAA schemes with substantial computational demands. The chain of tools thereby enables a separation and quantitative comparison of the different relevant noise sources. Based on dedicated wind tunnel measurements of noise sources and noise emission of airfoils the methods have been continuously verified and improved. Besides a summary of the theoretical basis, specific validation examples are given that show the strengths and the deficiencies of the available methods and motivate ongoing improvements. Finally, application examples with practical relevance and means for a passive and active noise reduction are discussed.