Plume Dynamics in Two-Dimensional Thermal Convection

We investigated single-point velocity statistics of convective turbulence driven by a thermal gradient in a freely suspended soap film in the plume dominant regime in the absence of the large scale circulation. The velocity probability density function (pdf) measured at the center of the convective region is strongly non-Gaussian and is asymmetric along the vertical direction. For positive fluctuations (velocity against gravity), the pdf tail is close to an exponential form with P(v$_{y})\sim $exp(-v$_{y}$/v$_{yrms})$ whereas for negative fluctuations (velocity parallel to gravity), the pdf is super-Gaussian with P(v$_{y})\sim $exp(-$\vert $v$_{y}$/v$_{yrms}\vert ^{3/2})$. The pdf of horizontal velocity component v$_{x}$ is more symmetric and shows a similar super-Gaussian form with P(v$_{x})\sim $exp(-$\vert $v$_{x}$/v$_{xrms}\vert ^{3/2})$. These observations can be understood in terms of plume dynamics and are consistent with the instanton formulation that relates input and output statistics of turbulence.