Three-dimensional study of the Rayleigh-Bénard convection by particle image velocimetry measurements

Abstract

It is often assumed that most of the Rayleigh-Benard flow pattern (at low Rayleigh number) is basically 2-D: the well known flow structure in horizontal 2-D rolls is surrounded by a 3-D boundary flow region, close to the walls of the (rectangular) box. But the elucidation of the flow dimensionality and other particular features of the flow pattern strictly requires the determination of the three velocity components in the entire flow field. We present here some experimental work devoted to this task.The particle image velocimetry, with a 5 mW He-Ne laser as the illuminating source, has been used to measure the projections of the local velocity vectors on several parallel planes along the whole flow. A systematic processing of these data allows to know spatial derivatives of the velocity, the vorticity (out of plane component), 2-D streamlines and the flow rate through several lines in each analyzed plane. Then, by using the continuity equation and assuming that the velocity is equal to zero just in the walls, the third component of the velocity field can be easily calculated in the whole flow region. From that, all the components of the vorticity and the 3-D streamlines have been calculated.Results show the 3-D structure of the flow in a small box and allows to conclude that the particle image velocimetry technique is particularly well suited for this type of 3-D flow field analysis.It is often assumed that most of the Rayleigh-Benard flow pattern (at low Rayleigh number) is basically 2-D: the well known flow structure in horizontal 2-D rolls is surrounded by a 3-D boundary flow region, close to the walls of the (rectangular) box. But the elucidation of the flow dimensionality and other particular features of the flow pattern strictly requires the determination of the three velocity components in the entire flow field. We present here some experimental work devoted to this task.The particle image velocimetry, with a 5 mW He-Ne laser as the illuminating source, has been used to measure the projections of the local velocity vectors on several parallel planes along the whole flow. A systematic processing of these data allows to know spatial derivatives of the velocity, the vorticity (out of plane component), 2-D streamlines and the flow rate through several lines in each analyzed plane. Then, by using the continuity equation and assuming that the velocity is equal to zero just in the wa...