Three-dimensional velocity measurements of Rayleigh-Bénard convection in a cylinder

This paper reports on the first large-scale three-dimensional investigation of thermal convection inside a cylinder by time-resolved tomographic particle image velocimetry. Rayleigh-Bénard convection is one of the most intensely studied problems in fluid mechanics due to its relevance to several fields of application, ranging from the geophysics and astrophysics to industrial engineering. Despite the huge amount of works on this subject, only few works have focused on three-dimensional experimental measurement of convection inside a cylindrical sample. In the present study, Rayleigh-Bénard convection is investigated in a cylinder with one-half aspect ratio at Rayleigh and Prandtl numbers equal to about 2E-8 and 6.5, respectively. The behavior of the turbulent flow is analyzed both in the time-average and instantaneous evolution. Focus is given to the structure and dynamics of the large–scale circulation. The azimuthal motion of the large-scale circulation is instantaneously traced and its relationship with the time-averaged flow field is investigated. Proper orthogonal decomposition is also used to identify the characteristic modes of the thermal convection. The most energetic modes are strictly related to the existence of the large-scale circulation, but interestingly the third and fourth modes exhibit a double-roll configuration, which is found to be the dominant pattern in the instantaneous flow field at some time instants.