Experimental Investigation of Bio-inspired Unidirectional Canopies

Abstract

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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