Temperature fluctuations in a convection cell with rough upper and lower surfaces.

A turbulent convection experiment is conducted in a cell with rough upper and lower surfaces. Temperature statistics, frequency power spectrum, and thermal dissipation are measured over varying Rayleigh numbers in the central region of the cell. The temperature histogram in the rough cell is found to have the same exponential shape as that in the smooth cell, but the width of the distribution is increased by approximately 25%. The measured power spectrum shows that temperature fluctuations in the rough cell are increased uniformly across the whole frequency range. The cutoff frequency f(c) of the power spectrum and the time averaged square temperature time derivative <( partial differentialT/ partial differentialt)(2)> are used to characterize the thermal dissipation in turbulent convection. It is found that the normalized f(c) as well as <( partial differentialT/ partial differentialt)(2)> in the smooth and rough cells with different aspect ratios can all be superposed onto a single curve, indicating that the thermal dissipation in these cells is determined by the same mechanism. The experiment suggests that the enhanced heat transport observed in the rough cell is determined primarily by the local dynamics near the upper and lower boundaries.