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Browsing by Author "Ayton, Lorna J."

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    Experimental Investigation of Bio-inspired Unidirectional Canopies
    Nurani Hari, Nandita; Szőke, Máté; Devenport, William J.; Glegg, Stewart A. L.; Priddin, Matthew; Ayton, Lorna J. (2022-02-08)
    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.
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    Trailing-edge serrations: improving theoretical noise reduction models
    Ayton, Lorna J.; Szőke, Máté; Paruchuri, Chaitanya; Devenport, William J.; Alexander, William (2021-08-02)
    This paper discusses the elements that make up a theoretical model for predicting the noise generated by a serrated trailing edge, and in particular, the required input to such a model; the wavenumber frequency spectra of the turbulence at the edge. It is proposed that this input, which is often modeled empirically, varies between a straight edge and a serrated edge and this fundamental difference in turbulence structure at the trailing edge leads to inaccurate theoretical predictions of noise reduction. Experimental measurements are therefore taken for straight and serrated trailing edges, with a particular focus on measuring the quantities which arise in typical wavenumber-frequency spectra models such as the TNO model. Whilst elements like the boundary layer thickness, shear profile, and skin friction velocity are found to be similar across the straight and serrated edges, the normal velocity wavenumber spectra is observed to vary substantially between the straight edge, and different locations along the serrated edge. In particular, the high-frequency decay rate of this spectra is reduced for a serrated edge versus a straight edge. This observed difference is then incorporated into a simple Chase-style empirical wavenumber-frequency spectrum; theoretical predictions which take account of a weaker high-frequency decay rate for a serrated edge agree much better with the experimental far-field noise predictions than theoretical predictions which assume an identical turbulent structure for both straight and serrated edges.