Fluctuations of the heat-flux in turbulent convection

We report measurements of the temporal fluctuations of the heat-flux in Rayleigh-Benard turbulent convection, Although being a global quantity, i. e. spatially averaged on the flow field, we show that its probability density function (PDF) is non-gaussian and asymmetric. The ratio of the rms fluctuations of the heat flux to their mean display a power law. Ra-gamma with gamma = 0.25, on more than two decades in Rayleigh number (2 10(7) < Ra < 4 10(9)), Thus, the fluctuations are roughly proportional to the temperature difference. We discuss this law and we propose a simple mechanism to explain the non-gaussian character of the PDF.