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