The origin of a characteristic frequency in hard thermal turbulence

New methods are proposed for filtering the time series of heat flux which is useful for detecting the characteristic frequencies in hard turbulent convection. High-resolution solutions have been obtained for a Rayleigh (Ra) of 10⁸ and infinite Prandtl number in a box with an aspect ratio of 1.8, in which the finest grid consisting of 140×400 bicubic splines was used. Successively higher temporal derivatives and high-pass spectral filtering of the Nusselt number at this high Ra reveal the existence of bursts. They are closely related to the presence of plumes in the thermal boundary layer in that there is a relative absence of activity in the boundary layer just before the onset of a burst and, on the other hand, there is a period of intense activity shortly before the end of a burst. These bursts are spaced evenly in time, thus yielding a single characteristic frequency, which may be related to a period associated with a pulsation mechanism in hard turbulent convection.