Linear stability analysis of turbulent flows over dense filament canopies

The aim of this work is to study the effect of the variation in canopy parameters on the Kelvin-Helmholtz-like instabilities triggered over them. The appearance of these instabilities over filament canopies has been widely studied, but the present work seeks to explore whether the instability can be manipulated by changing canopy properties. To this effect, a parametric study using linear stability analysis is conducted. For the analyses, the canopy is modelled using two methods. The first models the canopy as a permeable substrate. The second accounts for the canopy through drag forces acting on the flow within it. Some effects of canopy dynamics, namely the average bending of the canopy elements and the dynamic clustering of the canopy are also individually studied. Using the porous medium analogy it is shown for rigid canopies that the onset of the instabilities is governed by the geometric mean of the streamwise and wall-normal permeabilities The drag model exhibits an additional feature missed by the porous model i.e. an optimum value of drag coefficient at which the amplification of the instability is maximum. It is also observed that the clustering of filaments caused by the waving of the canopy can significantly increase the amplification of the instability, and has a greater impact than the mean filament bending.