Measurements of the concentration field in nonlinear travelling-wave convection

We present observations of travelling-wave convection in ethanol/water mixtures in a rectangular container using shadowgraphic flow visualization along the axes of the convective rolls. The entrainment of solute into the centre of the fluid layer from the boundary layers at the top and bottom of the cell is clearly observed in the shadowgraph images as sharp features at the roll boundaries. The shadowgraph images are integrated to yield a map of the two-dimensional refractive-index field. The contributions to this field from the convective temperature and the concentration fields have sufficiently different spatial character to be reliably separated using image-processing techniques. This has allowed us to produce a map of these two flow fields that is in good quantitative agreement with numerical integrations of the full Navier–Stokes equations in the Boussinesq approximation. We also present qualitative visualizations of transients caused by abruptly triggering and quenching travelling-wave convection. These experiments illustrate the formation of concentration boundary layers and reveal the existence of large-scale currents which are an inherent part of travelling-wave convection.