Effects of vortex formation and interaction on turbulent mass transfer over a two-dimensional wavy wall

The present paper conducted a large-eddy simulation of turbulent flow and mass transfer over two-dimensional wavy walls with different wave slopes (ratio of amplitude to wavelength ranging from 0.0375 to 0.1) to investigate the generation and interaction of vortices and the effect on turbulent mass transfer. The present wavy wall induces flow separation and reconstructs a turbulent shear layer having a concave feature, and thus triggers streamwise vortices via the centrifugal instability mechanism. We confirm that these vortices originate from separation and reattachment points. The convex wall enhances the spanwise vorticity traced to the upstream trough, such that spanwise vortices form via the Kelvin-Helmholtz instability mechanism deviating from the crest. There are four vortex modes between crests controlling the turbulent scalar statistics. (1) The interaction between spanwise and streamwise vorticity activates strong shear, high-level turbulent kinetic energy (TKE) and Reynolds shear stress (RSS) near the separation point while upstreaming the origins of high streamwise and vertical turbulent scalar fluxes (VTSF). (2) The shear effect in the separation zone maintains the TKE and RSS, whereas the trapped separation bubble causes dominated scalar diffusion on the leeward side. (3) Vortex pairs are generated at the reattachment point to eject low-momentum and low-scalar fluids upward into the average flow. (4) The layout of vortices on the windward side is reversed to that in the separation point, and the restriction of the convex wall on the streamwise vorticity weakens the shear and forms a region of low RSS and VTSF.