Abstract
Our life is strongly affected by turbulent convective flows, driven by time-dependent thermal forcing, especially diurnal heating of the Earth's surface by the Sun. In a laboratory experiment, we investigate their analogues: We study complex and extraordinary properties of turbulent buoyancy driven flows generated due to periodic modulation of the temperature of the plates of a Rayleigh-Bénard cell, with amplitudes both smaller and larger than either the positive or negative mean temperature difference between the top and bottom. We probe the turbulent flow of our working fluid - cryogenic helium gas - using temperature sensors placed in the cell interior and embedded in its plates. We discuss spatial and temporal structure of the heat flow, generalize validity of Nusselt versus Rayleigh number scaling Nu ∝ Ra γ with γ ≈ 1/3 at very high Ra for modulated convection and argue that this system represents a benchmark model which helps us understand the energy budget of ocean currents or weather formation on Earth subject to diurnal Sun heating as well as similar natural flows on Earth-like planets.