Measurements of unsteady pressure fluctuations on the surface of an unswept, multi-element airfoil

This paper presents an analysis of unsteady surface pressure measurements in noise source regions of an unswept wing equipped with a part-span flap and with fulland part-span slats. The test was conducted in the NASA Ames 7by 10-Foot Wind Tunnel, at Mach Numbers up to 0.22 and Reynolds Numbers up to 3.7xl0 based on wing chord. Signal processing methods included autospectra, coherence between adjacent sensors, and cross correlation between the time derivative of surface pressure sensors and one of the microphones of a phased array which was mounted in the test section wall. The spectra of signals from sensors near the flap side-edge, which were similar in shape to preliminary acoustic spectra obtained from the phased microphone array, generally decayed monotonically with frequency. Signals from sensors located in the slat region also exhibited similar features in comparison to preliminary phased microphone array spectra, including high frequency noise associated with a low slat loading condition. Modifications to the flapedge and slats which resulted in reduced radiated noise levels generally had the effect of reducing the autospectral levels of individual sensors as well as reducing the coherence between adjacent sensors in the noise source regions. * Aerospace Engineer, Low Speed Aerodynamics Branch, Mail Stop 247-2, Senior Member AIAA. t Aerospace Engineer, Branch, Mail Stop 247-2 Low Speed Aerodynamics ** Aerospace Engineer, Low Speed Aerodynamics Branch, Mail Stop 247-2, Senior Member AIAA. tt Aerospace Engineer, Sterling Federal Systems, Mail Stop 247-2, Member AIAA. Copyright ©1997 by the American Institute of Aeronautics and Astronautics, Inc. No copyright asserted in the United States under Title 17, U. S. Code. The U. S. Government has a royalty-free license to all rights claimed herein for Governmental purposes. All other rights are reserved by the copyright owner. Nomenclature f frequency, Hz p(x,t) radiated acoustic pressure sensed at farfield location x P0(y,t) unsteady pressure at surface location y tf flap maximum thickness U0 free stream velocity, Introduction This paper describes recent measurements of fluctuating pressures in several noise source regions of an unswept wing equipped with leading and trailing edge high lift devices. The measurements were obtained as part of a detailed aerodynamic and acoustic investigation undertaken to improve the understanding of noise associated with high-lift systems, and to motivate techniques to alleviate such noise. This effort was supported by NASA's Advanced Subsonic Technology Noise Reduction Program with the goal of allowing future transport aircraft to operate at reduced noise levels without penalizing aerodynamic performance. Past studies, such as those reviewed by Crighton, have identified flap side-edges as significant noise sources and this is a major focus of the present study. Noise due to leading edge slats is another known source which will be examined. An important objective of the current effort is to gain improved knowledge of the unsteady flow characteristics in the flap-edge and slat noise source regions, and of the effects of changes in test model configuration on the local unsteady flow and the radiated noise. It is anticipated that these measurements will be used to develop and validate new physical models of the noise source mechanisms for improved prediction and control of high-lift noise. Background The steady and unsteady flowfields associated with the flap side-edge and leading edge slat are known to be highly complex combinations of turbulent boundary layers, multiple interacting vortices, and accelerated jet-like flows through gaps. The difficulty of the problem is compounded by the geometrical complexities of typical flight configurations with effects of sweep, exposed non-streamwise edges of deployed 481 American Institute of Aeronautics and Astronautics flaps and slats, as well as multiple support brackets with separated turbulent wakes. Fink and Schlinker used an acoustic mirror to locate noise sources on a wing model equipped with part span flaps and slats, to a Re of 2x10 based on wing chord. Neither aerodynamic characteristics or unsteady surface pressures were measured during this test. Kendall and Ahtye conducted a similar investigation of an unswept wing with a part span flap, to Re of 2.8xl0, a value close to that of the present study and about 10% to 40% of full scale. The noise spectra reported by both of these studies exhibited smooth, broadband distributions generally absent of tones or other narrow band features. Miller, et al4 obtained measurements of radiated noise and surface pressure fluctuations near the edges and midspan of a triple slotted flap on a 6.7 m semispan wing, with a root chord Re of about 8xl06. Estimates of radiated rms noise levels were obtained by three approaches: 1) cross correlations between surface sensors, 2) cross correlations between pressure transducers and far-field microphones, and 3) a seminormalized correlation method. In the latter method, an estimate of the portion of sound rms level p'(x) due to surface pressure fluctuations with rms level Po'(y)> s given as: