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Shear sheltering is defined as the effect of the mean flow velocity profile in a boundary layer on the turbulence caused by an imposed gust. In aeroacoustic applications turbulent boundary layers interacting with blade trailing edges or roughness elements are an important source of sound, and the effect of shear sheltering on these noise sources has not been studied in detail. Since the surface pressure spectrum below the boundary layer is the primary driver of trailing edge and roughness noise, this thesis considers the effect that shear sheltering has on the surface pressure spectrum below a boundary layer. This study presents a model of the incoming turbulence as a vortex sheet at a specified height above the surface and shows, using canonical boundary layers and approximations to numerical results, how the mean flow velocity profile can be manipulated to alter the surface pressure spectrum and hence the associated trailing edge noise. The results from this model demonstrate that different mean velocity profiles drive significant changes in the unsteady characteristics of the flow. The surface pressure fluctuations results also suggest that boundary layers where the shear in the mean velocity profile is significant can be beneficial for the reduction of trailing edge noise at particular frequencies. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2020. / FAU Electronic Theses and Dissertations Collection

The far field acoustics generated by turbulent flow over rough surfaces has been experimentally investigated in an acoustically treated wall jet facility. The facility allows direct measurement of the far field sound from small patches of surface roughness, without contamination from edge or other aerodynamic noise sources. The facility is capable of generating turbulent boundary layer flows with momentum thickness Reynolds numbers between 450 and 1160. The variation of surface conditions tested cover the range from hydrodynamically smooth surfaces through most of the transitional range, with h+ variations from 3 to 85. Single microphone narrow band acoustic spectra, measured in the far field, show sound levels as much as 15 dB above the background from 0.186 m2 roughness patches. The measurements revealed the spectral shape and level variations with flow velocity, boundary layer thickness, and roughness size; providing the first data set large enough to assess the affects of many aerodynamic properties on the acoustic spectra. Increases in the size of grit type roughness produced significant increases in acoustic levels. Patches of hydrodynamically smooth roughness generated measurable acoustic levels, confirming that acoustic scattering is at least one of the physical mechanisms responsible for roughness noise. The shapes of the measured spectra show a strong dependence on the form of the surface roughness. The acoustic spectra generated by periodic two-dimensional surfaces have a much narrower louder peak than that generated by three-dimensional grit type roughness. Measurements also show the orientation of the two-dimensional surface significantly affects the acoustic levels and directivity. The variation of sound levels with flow velocity and roughness size suggests the acoustic field is significantly affected by changes in the near wall flow due to the presence of the roughness. Current models of noise generated by rough surfaces predict the general trends seen in measurements for flows over grit and two-dimensional roughness in the range of 20<h+<50. However, in cases with a low Reynolds number or large grit size, where the roughness is likely to significantly affect the hydrodynamic pressure field, the scattering models did not perform as well. / Ph. D.

Various researchers are interested in the structure of the surface pressure fluctuations for the development and use of noise prediction techniques for helicopter and turbomachinery rotors. This study, conducted in the Virginia Tech low speed boundary layer wind tunnel, covered the effects of zero and favorable streamwise pressure gradient flows on the surface pressure fluctuation spectra, coherence and convective wave speeds in turbulent boundary layers for momentum Reynolds numbers from 3000 to 18,800. The acceleration parameter, pressure gradient flow. K is near 2x10⁻⁷ for the favorable Small pinhole condenser microphones were used to obtain the surface pressure fluctuation data for all test cases. The longitudinal and lateral coherence functions and the convective wave speeds were obtained for both streamwise pressure gradient flows. The results presented are for the surface pressure fluctuation spectra nondimensionalized by different groupings of the outer and inner boundary layer variables. The grouping using the outer variables, U_e, π_w and δ₁ collapse the spectra for the low to middle range of frequencies for most test cases. The grouping using the inner variables, U_π and ν, collapse the spectra for the middle to high range of frequencies for all test cases. The value of p¹/r_w was near 3.8 and 2.8 for the smallest values of d⁺ in the zero and favorable pressure gradient flows, respectively. The spectral data was corrected using the correction developed by G.M. Corcos, but the pinhole correction developed by Bull and Thomas was not used in the data reduction process. However, some discussion is included on the effects of the pinhole correction for the results of this study. The coherence exhibits a decay that is not exponential in some cases, but the Corcos similarity parameters ωΔx/U_c and ωΔz/U_c collapse the data for all test cases. C The ratio of U_c/U_e shows an increase with increasing ωδ₁/U_e up to a certain value of ωδ₁/U_e where U_c/U_e becomes constant. This was observed in the present results for both streamwise pressure gradient flows. The experimental results presented show good agreement with previous research. / M.S.