The influence of wavy walls on the transport of a passive scalar in turbulent flows

We carry out an experimental study to investigate the turbulent transport of a passive scalar in turbulent flows between a flat top wall and two different complex surfaces and describe the influence of the different surface geometries on the mixing properties. In order to achieve a homogeneous and inhomogeneous reference flow situation two different types of surface geometries are considered: a sinusoidal bottom wall profile and a profile consisting of two superimposed sinusoidal waves. The laboratory investigations were performed by the use of a combined digital particle image velocimetry (DPIV) and planar laser-induced fluorescence (PLIF) technique to examine the spatial variation of the streamwise, spanwise and wall-normal velocity components and to assess the concentration field of the scalar. Statistically sufficient image sequences (frame rate 4 Hz) are obtained from both DPIV and PLIF to calculate time-averaged statistics of the velocity field and the scalar field. We discuss the influence of the complex surfaces on large-scale structures present in the flow field and the transport of the passive scalar. A higher spanwise spreading of the scalar plume is observed compared to plane channel flows. Due to the influence of the complex surfaces a higher transport normal to the mean flow direction is found. The mixing properties of the turbulent flow are enhanced for the surface composed of two superimposed waves compared to the sinusoidal wavy surface. This is consistent with the scaling of large-scale structures found for this surface geometry.