Assessment of horizontal velocity fields in square thermal convection cells with large aspect ratio

Transparent heating plates, consisting of glass coated with a transparent conductive metal oxide, are applied in large aspect ratio turbulent Rayleigh-Bénard convection (RBC) to investigate the large-scale patterns of velocity fields with optical flow measurement techniques across the whole horizontal cross section. The square convection cell with an aspect ratio G =L/h = 10 was tested inside the scaled convective airflow laboratory experiment (SCALEX) facility which enables experiments with gases as working fluids for pressures of up to 10 bar to achieve very high Rayleigh numbers Ra. For the current study, Ra = 2 × 104 was applied. The velocity fields are measured with 2D3C particle image velocimetry (PIV). The possibility of reliable PIV measurements with reproducible homogenous temperature boundary conditions was demonstrated in the SCALEX facility. The seeding of the tracer particles, their illumination and data evaluation are addressed in detail. The final comparison of experimental data and numerical simulations shows a good agreement for the probability density functions of the horizontal velocity components. Deviations for the vertical out-of-plane velocity component and their dependence on the thickness of the laser sheet are discussed in detail and quantified by measurements with light sheets of different thickness.