RAYLEIGH BENARD CONVECTION IN A SMALL BOX - SPATIAL FEATURES AND THERMAL-DEPENDENCE OF THE VELOCITY-FIELD

An experimental study of the spatial features of Rayleigh-Benard convection in a small box is presented. Experiments are carried out in a rectangular cell (aspect ratios GAMMA(x) = 2.03, GAMMA(y) = 1.19) filled with silicone oil (Prandtl number, Pr = 130) for different Rayleigh numbers, Ra (up to Ra = 75 Ra(c), Ra(c) 1707). The basic structure of the flow field for this range of Ra consists of two rolls with their axes parallel to the shorter horizontal side. Both senses of rotation for the rolls are observed, corresponding to the two branches of the bifurcation. Particle image velocimetry, with a 5 mW He-Ne laser as the illuminating source, is used to measure the velocity field in the midplane of the cell. From it the vorticity field (out of plane component) and two-dimensional streamlines are calculated. The flow has been measured to be three-dimensional, even for very low Ra, owing to the sidewall influence. The spatial features of the flow are shown to be dependent on both Ra and the sense of rotation of the rolls. Finally, a Fourier analysis of the velocity field is presented. The spatial and thermal dependences of the different Fourier terms are reported. The velocity field, in a first-order approximation, is quantitatively described.