Large-eddy simulations of flows over rigid canopies in different canopy-flow regimes

We have performed fully resolved large-eddy simulations of turbulent flows over rigid canopies in an open channel of depth H at low Reynolds number (Reb=Ub H/ν = 6000, b stands for bulk) in sparse, transitional and dense canopy-flow regimes (Nepf ARFM 44, 2011).The different regimes are obtained by varying the height h of the rigid filaments that constitute the canopy while maintaining unchanged the filaments thickness and number per wall unit surface. In particular, we have considered variations from H/h=2.5 to H/h=20 using an immersed boundary method to model each rigid filament of the canopy. The variations of the canopy height determines the locations of the mean velocity internal inflection point in the canopy and of the virtual origin seen by the external flow. The latter has a strong impact in determining the outer flow regime (i.e. the skin friction Reynolds number of the outer flow) and the eventual appearance, size and intensity of the Kelvin-Helmholtz rollers induced by the inflection points.More details on the effect of the canopy height (i.e., the λ parameter proposed by Nepf et al. to characterise canopy flows) in establishing the mechanisms of the interaction between the flow inside and outside the canopy layer.