Intermittent Turbulence in Stratified Flow over a Canopy

During the night turbulence can often be very intermittent, occurring in sudden vigorous bursts after prolonged periods of low-intensity. Several mechanisms have been proposed to explain intermittency. The present study focuses on the role of porous surface elements, which influence the mean wind profile near the surface and therefore the shear characteristics. The shape of this profile plays a vital role in the mechanism that causes the flow to destabilize. Direct numerical simulations of a pressure driven stably stratified flow are used to study the development of initial instabilities into breaking waves and the possibility of a subsequent collapse of turbulence and return to a weakly turbulent state. To better understand the parameter space associated with the occurrence of intermittency, we conduct a linear stability analysis which provides critical values for the Richardson number in relation to the canopy characteristics. This enables one to make an a-priori educated guess of the conditions that favor intermittency, which are subsequently studied in more detail using the DNS.