Measurement and Scaling of Trailing-Edge Noise

Kurzfassung

The presentation provides an overview on DLR’s research activity in trailing-edge (TE) noise source description. Current efforts aim at establish-ing the necessary data quality required for CAA validation. The specific challenges with regard to farfield TE noise measurements in open-jet wind-tunnels are pinpointed for the example of elliptic mirror measurements in the Acoustic Wind Tunnel Braunschweig (AWB) the results of which are compared to corresponding free-field microphone data. The presentation of farfield TE noise data includes the specification of numerous different uncertainties in data acquisition and analysis, imposed by extraneous noise sources, the frequency response function of the measurement system, coherence loss due to the open-jet shear-layers and par-ticular source model assumptions. The corresponding frequency range limitations of focussing measurement systems are critically reviewed. Systematic errors are estimated by means of comparisons with other published TE noise data, when acquired by alternative measurement techniques and in different test facilities. Some of the derived TE noise spectra are compared with those obtained from recent CAA simulations and empirical prediction methods. The evaluation of additional aerodynamic data required for the scaling and prediction of TE noise is also addressed. This includes hot-wire measurements within the turbulent boundary-layer and unsteady surface pressure measurements in the TE region to describe the relevant flow characteristic. Turbulent boundary layer analysis includes detailed measurements of the boundary layer thickness, the displacement and momentum loss thicknesses as well as the wall friction coefficients, which are applied for scaling of both the surface pressure and farfield noise level spectra. The applicability of existing semi-analytical and semi-empirical prediction models, such as the well-documented models by Chase [1, 2] or Goody [3], dedicated to reproduce unsteady surface pressure spectra are examined in comparison with the present data set. This includes an estimation of limits in the noise source description due to the agglomeration of different error contributions and corresponding corrections. Local unsteady surface pressure power spectral densities close to the TE as well as two-point cross spectral data are analysed. [1] Chase, D. M., “Modeling the Wavevector-Frequency Spectrum of Turbulent Boundary Layer Wall Pressure,” Journal of Sound and Vibration, Vol. 70, No. 1, 1980, pp. 29-67 [2] Chase, D. M.,”The Character of the Turbulent Wall Pressure Spectrum at Subconvective Wavenumbers and a Suggested Comprehensive Model,” Journal of Sound and Vibration, Vol. 112, No. 1, pp. 125-147 [3] Goody, M., “Empirical Spectral Model of Surface Pressure Fluctuations,” AIAA Journal, Vol. 42, No. 9, 2004, pp. 1788-1794.