Bolgiano scale in confined Rayleigh-Taylor turbulence

We investigate the statistical properties of Rayleigh Taylor turbulence in a three-dimensional convective cell of high aspect ratio, in which one transverse side is much smaller that the others. By means of high-resolution numerical simulation we study the development of the turbulent mixing layer and the scaling properties of the velocity and temperature fields. We show that the system undergoes a transition from a three- to two-dimensional turbulent regime when the width of the turbulent mixing layer becomes larger than the scale of confinement. In the late stage of the evolution the convective flow is characterized by the coexistence of Kolmogorov-Obukhov and Bolgiano-Obukhov scaling at small and large scales, respectively. These regimes are separated by the Bolgiano scale, which is determined by the scale of confinement of the flow. Our results show that the emergence of the Bolgiano-Obukhov scaling in Rayleigh Taylor turbulence is connected to the onset of an upscale energy transfer induced by the geometrical constraint of the flow.