Scalar transport in flow past finite circular patches of tall roughness

Simultaneous particle image velocimetry-planar laser-induced fluorescence (PIV-PLIF) measurements are conducted on the flow around and downstream of patches of tall cylinders with various solidity ϕ≡Nc(d/D)2, where Nc is the number of cylinders in a patch, d and D are the diameters of the cylinder and the patch, respectively. Results suggest that the bleeding and scalar concentration in the wake and above the patches are dependent upon ϕ. Recirculation bubble appears at downstream of the patch boundary, limiting the flow from convecting downstream and forcing it (and the scalar) to escape vertically from the top of the patch. Above the wake, a shear layer generated by the velocity difference between the free flow and the wake itself shields the near-wake region. The net effect of the solidity on the wake topology is the appearance of an elongated patch as ϕ decreases and a taller patch as ϕ increases. In terms of scalar transport, both advective and turbulent vertical scalar transport are enhanced as ϕ increases. High advective vertical scalar transport CV is concentrated around the canopy (above the canopy due to vertical bleeding and at the TE due to high scalar concentration), while high turbulent transport c′v′¯ largely coincides with the shear layer. Octant analysis of the conditionally averaged fluctuating components (u′, v′, and c′) shows an enhanced mixing inside the shear layer and entrainment of turbulent flows between the shear layer, the wake region below and the freestream above this layer. In terms of eddy diffusivity models, the complexity of the eddy diffusivity constant increases with ϕ.