Statistical analysis of turbulent thermal free convection over a horizontal heated plate in an open top cavity

An experimental study of the thermal field established, over a horizontal heated plate (1000 × 500 × 12 mm3) in an open top cavity, of aspect ratio Γ = 2 has been conducted by means of local temperature measurements as a function of distance from the wall. The working fluid was air at atmospheric pressure and the imposed temperature conditions correspond to Rayleigh numbers (Ra) of order 108. Temperature measurements were accomplished on the centerline over the plate using a fine thermocouple (d = 0.025 mm). The thermal boundary layer thickness was estimated close to 2.0 mm. Distributions of mean and rms temperatures are presented and discussed along with the corresponding skewness and flatness distributions. The dependence of skewness and flatness factors of the overall probability density function (PDF) of an intermittent random process is illustrated. Based on these results the form of the temperature PDFs over the plate is discussed. It is shown that the exponential form of the distributions observed in the present experiments, as in several experiments in hard turbulence, is compatible with the intermittent presence of fluid structures with different origin and past history, which did not have the time to become homogenized. Power spectra density results at the same locations indicate the presence of a characteristic angular frequency ωp, whereas secondary peaks observed at higher frequencies provide further support to the concept of an intermittent process and the presence of fluid structures with different past history.

► Experimental study of free convection over a horizontal plate in open top cavity.
► Thermal field similar to closed cell R-B convection near the boundary.
► PDFs with exponential form are related to intermittent combination of normal ones.
► They indicate intermittent presence of fluid structures with different past history.
► Secondary peaks in power spectrum further support this concept.