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Turbulent superstructures in horizontally extended three-dimensionalRayleigh-B\'enard convection flows are investigated in controlled laboratoryexperiments in water at Prandtl number $Pr = 7$. A Rayleigh-B\'enard cell withsquare cross-section, aspect ratio $\Gamma = l/h = 25$, side length $l$ andheight $h$ is used. Three different Rayleigh numbers in the range $10^5 < Ra <10^6$ are considered. The cell is accessible optically, such that thermochromicliquid crystals can be seeded as tracer particles to monitor simultaneouslytemperature and velocity fields in a large section of the horizontal mid-planefor long time periods of up to 6 h, corresponding to approximately $10^4$convective free-fall time units. The joint application of stereoscopic particleimage velocimetry and thermometry opens the possibility to assess the localconvective heat flux fields in the bulk of the convection cell and thus toanalyse the characteristic large-scale transport patterns in the flow. A directcomparison with existing direct numerical simulation data in the same parameterrange of $Pr, Ra$ and $\Gamma$ reveals the same superstructure patterns andglobal turbulent heat transfer scaling $Nu(Ra)$. Slight quantitativedifferences can be traced back to violations of the isothermal boundarycondition at the extended water-cooled glass plate at the top. Thecharacteristic scales of the patterns fall into the same size range, but aresystematically larger. It is confirmed experimentally that the superstructurepatterns are an important backbone of the heat transfer. The presentexperiments enable, furthermore, the study of the gradual evolution of thelarge-scale patterns in time, which is challenging in simulations oflarge-aspect-ratio turbulent convection.

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