Flow and Scalar Transfer Characteristics for a Circular Colony of Vegetation

Local and global flow structures, as well as transfer and transport of a passive scalar from a circular colony of uniformly distributed vegetation stems, are investigated at Re = 2100, Re = 4200, and Re = 8400. The number of stems in the colony is varied from 1 to 284 yielding a solid fraction of 0.0<𝜙𝜙<0.65. The following three flow regimes are identified: a co-shedding flow regime prevails at low solid fraction where wakes of individual cylinders have minimal interaction; a 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; and a single-body flow regime is observed at high solid fraction and is accompanied by the commencement of colony-scale vortex shedding. As Reynolds number increases, the separated shear layers observed at intermediate solid fraction break up to form stem- scale vortices that organize themselves in colony scale coherent structures. As the solid fraction increases, drag and Sherwood number experienced by colonies increases linearly and at a reducing rate at low and intermediate solid fractions, respectively, while the net lift remains negligible. At high solid fraction, the commencement of colony-scale vortex shedding is accompanied by a jump in lift and base suction. Pressure and friction lift/drag increase and decrease with an increase in solid fraction, respectively, toward the value experienced by a solid cylinder. Sherwood number, on the other hand, decays exponentially toward the value experienced by a solid cylinder at high solid fraction. Colonies at intermediate solid fraction exhibit the highest scalar transfer but weakest transport in their near field wake. Scalar transfer in colonies with high solid fraction deteriorates with an increase in solid fraction yielding less scalar concentration in their downstream wake. Each case consist of about 14M computational points and computations were performed on TACC LS6 clusters. A typical case converges in 128,000 processor hours.