  The efficient mixing of fluids is key in many applications, such as chemical
reactions and nanoparticle precipitation. Detailed experimental measurements of
the mixing dynamics are however difficult to obtain, and so predictive
numerical tools are helpful in designing and optimizing many processes. If two
different fluids are considered, the viscosity and density of the mixture
depend often nonlinearly on the composition, which makes the modeling of the
mixing process particularly challenging. Hence water-water mixtures in simple
geometries such as T-mixers have been intensively investigated, but little is
known about the dynamics of more complex mixtures, especially in the turbulent
regime. We here present a numerical method allowing the accurate simulation of
two-fluid mixtures. Using a high-performance implementation of this method we
perform direct numerical simulations resolving the spatial and temporal
dynamics of water-ethanol flows for Reynolds numbers from 100 to 2000. The
flows states encountered during turbulence transition and their mixing
properties are discussed in detail and compared to water-water mixtures.
