Abstract
Large-Eddy Simulations of the development of Kelvin-Helmholtz instability and the transition to turbulence in an atmospheric boundary layer flow over a canopy are carried out using the hydrodynamic module of FireStar3D. The simulations were conducted by combining five different wind velocity (1, 3, 5, 8, and 12 m/s at the top of the canopy) and three leaf-area indices (0.5, 2, and 8) with variable leaf-area density. As long as the average wind speed was maintained at the top of the canopy, it was observed that the flow field was not significantly affected by canopy density, despite the higher drag exerted by the denser canopy. The wave-number and frequency spectra of the stream-wise velocity at the canopy surface was analysed at steady state. The analysis revealed the presence of a continuous spectrum of frequencies and wavelengths, indicating a fully developed turbulent flow. More importantly, the analysis describes the wavelength and frequency dependence of the normalized amplitude of the Fourier transforms of the stream-wise velocity at the top of the canopy. This normalized amplitude exhibits an exponential decay with a slope of − 0.5 with the normalized wave-number. At low frequencies, the normalized amplitude is globally constant, while it decays at high frequencies with the Strouhal number according a power law of slope − 3.