Combined particle image velocimetry and thermometry of turbulent superstructures in thermal convection

Turbulent superstructures in horizontally extended three-dimensional Rayleigh-B\'enard convection flows are investigated controlled laboratory experiments water at Prandtl number $Pr = 7$. A cell with square cross-section, aspect ratio $\Gamma l/h 25$, side length $l$ and height $h$ is used. Three different Rayleigh numbers the range $10^5 < Ra 10^6$ considered. The accessible optically, such that thermochromic liquid crystals can be seeded as tracer particles to monitor simultaneously temperature velocity fields a large section of horizontal mid-plane for long time periods up 6 h, corresponding approximately $10^4$ convective free-fall units. joint application stereoscopic particle image velocimetry thermometry opens possibility assess local heat flux bulk thus analyse characteristic large-scale transport patterns flow. direct comparison existing numerical simulation data same parameter $Pr, Ra$ $\Gamma$ reveals superstructure global turbulent transfer scaling $Nu(Ra)$. Slight quantitative differences traced back violations isothermal boundary condition water-cooled glass plate top. scales fall into size range, but systematically larger. It confirmed experimentally an important backbone transfer. present enable, furthermore, study gradual evolution time, which challenging simulations large-aspect-ratio convection.