Wavenumber frequency spectra of turbulence in a lifting wake for broadband noise prediction

Abstract

Measurements of wavenumber frequency spectra of velocity fluctuations have been made using two 3component hot-wire probes in the wake of a rectangular NACA 0012 half wing at a 5° angle of attack. Spectra were measured along spanwise lines in the near two-dimensional part of the wake, in the stretched region where the wake begins to wind around the vortex, directly above the vortex core, and radially from the core center. Dominated by the large-scale organized motions present in the wake, the spectra show little similarity with the von Karman isotropic turbulence spectrum. Outside the core, upwash spectra contain a single maximum at the passage frequency of these large-structures. Motions associated with this peak are highly anisotropic both in terms of velocity components and in length scales. Similar anisotropy is also seen at higher wavenumbers, suggesting either that the smaller scale turbulent motions are organized by the large eddies, or that the non-sinusoidal components of those eddies contribute significantly to the spectrum at higher wavenumbers. The implications of these results for broadband noise prediction have been assessed by considering the sound radiation from this turbulent flow over a semi-infinite flat plate. The major conclusion is that the correlation length scales of the flow are small compared to the acoustic wavelength and so are well represented by their values at zero spanwise wavenumber. However, the blade response function has a variation with spanwise wavenumber which cannot be ignored and significantly influences the spanwise directionality. 'Associate Professor, Senior Member AIAA Undergraduate Researcher, Student Member AIAA 'Professor, Member AIAA Staff Scientist, Member AIAA Copyright © 1997 by W Devenport, C Wenger, S Glegg and J Miranda. Published by the American Institute of Aeronautics and Astronautics, with permission. c c0 G ke