Structure of the thermal boundary layer for turbulent Rayleigh-Bénard convection of air in a long rectangular enclosure

Measurements of the temperature distribution were performed in the upper (cold) boundary layer of a rectangular Rayleigh-Bénard cell with aspect ratios Gamma(x) = 5 and Gamma(y) = 1 using air with Prandtl number Pr = 0.71 as the working fluid. The range of investigated Rayleigh numbers was from Ra approximately 6 x 10(7) to Ra approximately 6 x 10(8), and the measurements were taken at two different positions with the purpose of understanding the variation of the properties of the thermal boundary layer along the cell. We present profiles of the mean temperature, rms temperature fluctuations, skewness, and kurtosis as a function of the distance from the cooling plate from which we extract scaling exponents and boundary layer thicknesses. Whereas most of these quantities are found to depend monotonically on the Rayleigh number for the peripheral measurement position, their values at the central measurement position exhibit a high degree of variability. These observations indicate that the properties of the thermal boundary layer in large-aspect-ratio convection can have strong spatial variations.