Local and global flow structures as well as transfer and transport of a passive scalar from a circular colony of uniformly distributed cylindrical vegetation are investigated at Re = 2100. The number of cylinders in the colony is varied from 1 to 284 yielding solid fraction of 0.00 < ϕ < 0.65. Three flow regimes are identified: Co-shedding flow regime prevails at low solid fraction where wakes of individual cylinders have minimal interaction. Bleeding-wake flow regime is identified at intermediate solid fraction in which stream-wise bleeding flow delays the formation of colony-scale vortices yielding a steady wake between two separated shear layers. Single-body flow regime is observed at high solid fraction and is accompanied by the commencement of colony-scale vortex shedding. As the solid fraction increases, drag and Sherwood number increases linearly and at a reducing rate at low and intermediate solid fractions, respectively, while the net lift is negligible. At high solid fraction, the commencement of colony-scale vortex shedding is accompanied by a jump in lift force and base suction. Both pressure and friction lift/drag increase and decrease with increase in solid fraction, respectively, towards the value experienced by a solid cylinder. Sherwood number decays exponentially towards the value experienced by a solid cylinder at high solid fraction. Colonies at intermediate solid fraction exhibit the highest scalar transfer but weakest transport (diffusion) in their near field wake. Scalar transfer in colonies with high solid fraction deteriorates with increase in solid fraction yielding less scalar concentration in their downstream wake.

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This study investigates the local and global flow structures and mass transfer characteristics for a group of interacting side-by-side cylinders in unbounded flow. Configurations with 2, 3, 4, and 5 members are considered for a range of pitch-ratios (1.05≤S/D≤4) at the Reynolds number Re = 90. The focus is laid on the time-averaged and instantaneous local flow features including wake field, jet flow, vortical structures, λ2, pressure coefficient, and mass transfer coefficient as well as integral variables including hydrodynamic forces. Four flow regimes are identified based on the vortical structures and average stream-wise velocity field. At low pitch-ratio, the whole structure behaves like a single bluff body, while each member in the configuration behaves like an independent isolated bluff body when the pitch-ratio is large. Between these two regimes, asymmetrically and symmetrically deflected wake regimes are observed. Flow regimes dictate hydrodynamic and mass transfer characteristics such that a jump in the hydrodynamic forces and mass transfer coefficient is observed as the flow regime evolves from a single body to a deflected wake. The configuration's size and the position of member cylinders in the structure have more profound effects on hydrodynamic forces and mass transfer characteristics at small pitch-ratio. For all configurations, drag increases as one progresses deeper into the structure, while lift forces are always repulsive between any two neighboring cylinders. Member cylinders have comparable mass transfer coefficients at intermediate and high pitch-ratios, while exterior cylinders exhibit a higher mass transfer coefficient at low pitch-ratio. Overall, the structure's size has a more profound effect on the values of scalars at low pitch-ratios.

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