
Zhengbing Chen Alejandra Ortiz Lijun Zong Heidi Nepf, The wake structure behind a porous obstruction and its implications for deposition near a finite patch of emergent vegetation, Water Resources Research, 2012
This experimental study describes the mean and turbulent flow structure in the wake of a circular array of cylinders, which is a model for a patch of emergent vegetation. The patch diameter, D, and patch density, a (frontal area per volume), are varied. The flow structure is linked to a nondimensional flow blockage parameter, C_{D}aD, which is the ratio of the patch diameter and a drag length scale (C_{D}a)^{−1}. C_{D} is the cylinder drag coefficient. The velocity exiting the patch, U_{e}, is reduced relative to the upstream velocity, U_{∞}, and U_{e}/U_{∞}decreases as flow blockage (C_{D}aD) increases. A predictive model is developed for U_{e}/U_{∞}. The wake behind the patch contains two peaks in turbulence intensity. The first peak occurs directly behind the patch and is related to turbulence production within the patch at the scale of individual cylinders. The second peak in turbulence intensity occurs at distance L_{w} downstream from the patch and is related to the wakescale vortices of the von Karman vortex street. The presence of the flow U_{e} in the wake delays the formation of the von Karman vortex street until distance L_{1} (<L_{w}) behind the patch. Both L_{1} and L_{w}increase as U_{e} increases and thus as the flow blockage (C_{D}aD) decreases. L_{1} sets the distance behind the patch within which fineparticle deposition can occur. Beyond L_{w}, turbulence associated with the wakescale vortices inhibits deposition. 
