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Wall pressure fluctuations induced by turbulent boundary layers over surface discontinuities
No full textAn experimental investigation of wall pressure fluctuations generated by turbulent boundary layers over surface
irregularities has been conducted in a backward–forward-facing step geometry simulating a large aspect-ratio cavity. This
simplified incompressible flow model was designed to reproduce geometrical discontinuities present, for example on an
aircraft fuselage, and responsible for interior noise generation. This study considers the effects of the main dimensionless
parameters, such as the Reynolds number, the normalized incoming boundary layer thickness and its space averaged
turbulence intensity, on the Sound Pressure Levels (SPL) and auto-spectra frequency estimates at the wall. Phase averaged
temporal quantities are obtained by the application of a wavelet based auto-conditioning method and by velocity/pressure
conditional statistics and cross-correlations. This analysis clarifies some relevant properties of the wall pressure
fluctuations in the two-steps configuration. Satisfactory scaling for both the SPL and the frequency spectra by
dimensionless parameters are obtained and empirical correlations which might be useful for predicting wall pressure
properties in practical applications are derived
Propagation of wall pressure perturbations in a large aspect-ratio shallow cavity
No full textWall pressure fluctuations generated by turbulent boundary layers over a shallow cavity are studied experimentally in a low-speed wind tunnel facility. The scope of the present work is to characterize the propagation of the pressure perturbations at the wall by means of pressure cross-correlations and cross-spectra measured through a microphone pair translated along the cavity floor. It is found that the mechanism characterizing the pressure propagation close to the backward facing step and in the middle of the cavity is similar to what is commonly observed in equilibrium boundary layer being the convection velocity smaller than the external mean velocity. On the other hand, in the close vicinity of the forward-step, the hydrodynamic contribution of the pressure fluctuations is accompanied by a relevant acoustic effect characterized by a convection velocity close to the speed of sound. Furthermore, in the regions close to the two steps, the spectral decay of the coherence function, even though of exponential type, is faster than that obtained in the quasi-equilibrium region
Characterization of a Separated Turbulent Boundary Layer by Time-Frequency Analyses of Wall Pressure Fluctuations
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