Local eddy viscosity and diffusivity models are widely used to understand and predict turbulent flows. However, the local approximations in space and time are not always valid for actual turbulent flows. Recently, a non-local eddy diffusivity model for turbulent scalar flux was proposed to improve the local model and was validated using direct numerical simulation (DNS) of homogeneous isotropic turbulence with an inhomogeneous mean scalar (Hamba 2022 J. Fluid Mech. 950, A38). The model was modified using the scale-space energy density in preparation for application to inhomogeneous turbulence (Hamba 2023 J. Fluid Mech. 977, A11). In this paper, the model is further improved by incorporating the effects of turbulence anisotropy, inhomogeneity and wall boundaries. The needed inputs from the flow to evaluate the model are the Reynolds stress and the energy dissipation rate. With the improved model, one- and two-dimensional profiles ofthe non-local eddy diffusivity in turbulent channel flow are evaluated and compared with the exact DNS values. The DNS results reveal a contribution to the scalar flux from the mean scalar gradient in a wide upstream region. Additionally, the temporal profile of the non-local eddy diffusivity moves downstream, diffuses anisotropically and is tilted towards the bottom wall. The model reproduces this behaviour of mean flow convection and anisotropic turbulent diffusion well. These results indicate that the non-local eddy diffusivity model is useful for gaining insights into scalar transport in inhomogeneous turbulence.

A ground vortex engendered by the interaction of uniform flow over a plane surface with suction into a cylindrical conduit whose axis is normal to the cross-flow and parallel to the ground plane is investigated in wind tunnel experiments. The formation and evolution of the columnar vortex and its ingestion into the conduit’s inlet are explored using planar/stereo particle image velocimetry over a broad range of formation parameters that include the speeds of the inlet and cross-flows and the cylinder’s elevation above the ground plane with specific emphasis on the role of the surface vorticity layer in the vortex initiation and sustainment. The present investigations show that the appearance of a ground vortex within the inlet face occurs above a threshold boundary of two dimensionless formation parameters, namely the inlet’s momentum flux coefficient and its normalised elevation above the ground surface. Transitory initiations of wall-normal columnar vortices are spawned within a countercurrent shear layer that forms over the ground plane within a streamwise domain on the inlet’s leeward side by the suction flow into the duct. At low suction speeds, these wall-normal vortices are advected downstream with the cross-flow but when their celerity is reversed with increased suction, they are advected towards the cylinder’s inlet, gain circulation and stretch along their centrelines and become ingested into the inlet at a threshold defined by the formation parameters. Finally, the present investigations demonstrated that reduction of the countercurrent shear within the wall vorticity layer by deliberate, partial bypass of the inlet face flow through the periphery of the cylindrical duct can significantly delay the ingestion of the ground vortex to higher level thresholds of the formation parameters.