### abstract

- We study experimentally and theoretically the transition phenomena caused by external forcing from Rayleigh-B\'enard convection with large-scale circulation (LSC) to the limiting regime of unstably stratified turbulent flow without LSC, where the temperature field behaves like a passive scalar. In the experiments we use the Rayleigh-B\'enard apparatus with an additional source of turbulence produced by two oscillating grids located near the sidewalls of the chamber. When the frequency of the grid oscillations is larger than $2$ Hz, the LSC in turbulent convection is destroyed, and the destruction of the LSC is accompanied by a strong change of the mean temperature distribution. However, in all regimes of the unstably stratified turbulent flow the ratio $[({\ensuremath{\ell}}_{x}{\ensuremath{\nabla}}_{x}T){}^{2}+({\ensuremath{\ell}}_{y}{\ensuremath{\nabla}}_{y}T){}^{2}+({\ensuremath{\ell}}_{z}{\ensuremath{\nabla}}_{z}T){}^{2}]/\ensuremath{\langle}{\ensuremath{\theta}}^{2}\ensuremath{\rangle}$ varies slightly (even in the range of parameters where the behavior of the temperature field is different from that of the passive scalar). Here ${\ensuremath{\ell}}_{i}$ are the integral scales of turbulence along the $x,y,z$ directions, and $T$ and $\ensuremath{\theta}$ are the mean and fluctuating parts of the fluid temperature. At all frequencies of the grid oscillations we have detected long-term nonlinear oscillations of the mean temperature. The theoretical predictions based on the budget equations for turbulent kinetic energy, turbulent temperature fluctuations, and turbulent heat flux, are in agreement with the experimental results.