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An analytical approach has been developed to model the rapid term contribution to the unsteady surface pressure fluctuations in wall jet turbulent boundary layer flows. The formulation is based on solving Poisson’s equation for the turbulent wall pressure by integrating the source terms (Kraichnan, 1956). The inputs for the model are obtained from 2D time-resolved Particle Image Velocimetry measurements performed in a wall jet flow. The wall normal turbulence wavenumber two-point cross-spectra is determined using an extension of the von Kármán homogeneous turbulence spectrum. The model is applied to compare and understand the baseline flow in the wall jet and to study the attenuation in surface pressure fluctuations by unidirectional canopies (Gonzales et al, 2019). Different lengthscale formulations are tested and we observe that the wall jet flow boundary layer contributes to the surface pressure fluctuations from two distinct regions. The high frequency spectrum is captured well. However, the low frequency range of the spectrum is not entirely captured. This is because we have used PIV data only up to a height of 2.3𝜹, whereas the largest turbulent lengthscales in the wall jet are on the order of 𝒚𝟏/𝟐≈𝟔𝜹. Using the flow data obtained from PIV and Pitot probe measurements, the model predicts a reduction in the surface pressure due to canopy at low frequencies. 

The flow field around a series of streamwise rods, referred to as canopies, is investigated using two-dimensional two-component time-resolved particle image velocimetry (PIV) and large eddy simulations (LES) to characterize the changes in the flow field responsible for reducing the low and high-frequency surface pressure fluctuations previously observed. It was found that an axisymmetric turbulent boundary layer (ATBL) develops over the rods, whose thickness grows at a greater rate above the rods than below. This boundary layer reaches the wall below the rods at a point where previously a saturation was found in the low-frequency noise attenuation, revealing that the ATBL is responsible for the low- frequency noise attenuation. The flow is displaced by the presence of the rods, particularly above them, which offset was primarily caused by the blockage of the ATBL. The flow below the rods exhibits the properties of a turbulent boundary layer as its profile still conforms to the logarithmic layer, but the friction velocity was found to drop. This viscous effect was found to be responsible for the high-frequency noise attenuation reported previously. 

Previous studies have demonstrated that structures such as a canopy or finlets placed within a boundary layer over an aerodynamic surface can attenuate pressure fluctuations on the surface without compromising aerodynamic performance. This paper describes research into the fundamental mechanisms of this pressure shielding. Experiments and analysis are performed on elemental canopy configurations which are arrays of streamwise rods placed parallel to the wall in order to eliminate the confounding effects of a leading edge support structure. Experiments show that such a canopy produces attenuation in three distinct frequency ranges. At low frequencies, where convective scales are much greater than the canopy height, attenuation spectra scale on the canopy height Strouhal number, but at high frequencies, a dissipation type frequency scaling appears more appropriate. There is mid-freqeuncy region which shows reduction in attenuation and is observed for all canopy structures tested. Attenuation in this region appears to scale with Strouhal number based on canopy spacing. 

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. It has been studied extensively in applications involving boundary layer transition, where the primary concern is flow instabilities that are enhanced by turbulence in the flow outside the boundary layer. 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, we will consider the effect that shear sheltering has on the surface pressure spectrum below a boundary layer. We will model the incoming turbulence as vortex sheets at specified heights above the surface and show, using classical boundary layer profiles and approximations to numerical results, how the mean flow velocity can be manipulated to alter the surface pressure spectrum and hence the radiated trailing edge noise. 

Previous studies have demonstrated that structures such as a canopy or finlets placed within a boundary layer over an aerodynamic surface can attenuate pressure fluctuations on the surface without compromising aerodynamic performance. This paper describes research into the fundamental mechanisms of this pressure shielding. Experiments and analysis are performed on elemental canopy configurations (parallel arrays of streamwise rods) that eliminate the confounding effects of a leading-edge support structure. Experiments show that such a canopy produces attenuation in two distinct frequency ranges. At low frequencies (convective scales much greater than the canopy height) attenuation spectra scale on the canopy height Strouhal number, but at high frequencies (canopy scales of the order of the height) a dissipation type frequency scaling appears more appropriate. RANS calculations are performed simulating the canopy geometry directly and as a porous layer. Pressure fluctuation spectra predicted from the RANS results by separately accounting for inner and outer layer contributions are able to accurately recreate the wall pressure spectra both with and without the canopy and thus the major features of the attenuation spectra. 

A new Anechoic Wall Jet Wind Tunnel was built at Virginia Tech. A detailed design based on the old wall jet tunnel was done to improve the quality of the resultant flow. Aerodynamic and acoustic calibrations were performed in order to understand properties and characteristics of the flow generated by this new facility which can be used for various aeroacoustic studies. Far-field acoustics were measured using half-inch B&K microphones in a streamwise array to characterize and reduce the background noise. Sound pressure levels were lower by 10 dB for frequencies up to 700Hz in comparison to the old facility. The turbulent surface pressure fluctuations of the wall-jet flow were studied using Sennheiser microphones placed along streamwise and spanwise locations to record surface pressure fluctuations. Comparison of the autocorrelation plotted for microphones along the same span indicate uniform flow features. A decay in the turbulence levels is observed along the downstream direction as expected. Aerodynamic calibrations included mean velocity measurements along different spanwise locations, wall-jet boundary layer profiles and streamwise cross-sections. Spanwise and cross-sectional velocity profiles show good uniformity of the flow. Detailed boundary layer analyses were performed with the parameters obtained from the experiments.