EXPERIMENTAL STUDY OF A TIP LEAKAGE FLOW - PART TWO: WAVELET ANALYSIS OF WALL PRESSURE FLUCTUATIONS

Advanced post-processing techniques based on the wavelet transform are applied to pressure signals measured at the surface of an instrumented airfoil installed within the anechoic wind tunnel available at the Laboratory of Fluid Mechanics and Acoustics of the Ecole Centrale de Lyon. Two flow configurations, with and without a variable gap at the airfoil tip, are investigated. The scope of the post-processing procedure is to extract the most energetic non-periodic contributions, localized in time and in space, and to detect the fluid dynamic structures which may act as noise sources. The events tracking method is based on the computation of time-frequency energy maps from which it is possible to select events, determine their time of appearance, and perform conditional averages. The conditioning procedure has shown that the amplitude of the oscillations of the averaged pressure signature becomes larger for increasing width of the gap, probably as an effect of a roll-up phenomenon occurring at the tip edge of the airfoil. In addition, the pressure-velocity cross analysis, including data obtained from PIV measurements, yielded the location of the major fluid dynamic structure statistically related to the largest pressure fluctuations at the wall.