Unavoidable pretransitional macroscopic fluid motion and its effect on a possible critical fluctuation in a finite Benard system

Light scattering from Brownian particles suspended in a viscous fluid was used to measure the macroscopic velocity of the fluid confined within a finite lateral system whose top and bottom were kept at different temperatures. The results indicated that a finite heat conduction of the side wall causes a macroscopic fluid motion even much below the critical Rayleigh number for a finite system, and that the subcritical flow could suppress critical fluctuations which are expected for an infinite system. The measured value of the macroscopic fluid motion agreed qualitatively as well as quantitatively for $-{10}^{-2\mathrm{}}\lesssim \epsilon =\frac{(\Delta T-\Delta T_c)}{\Delta T_c}\lesssim -0.5\mathrm{}$ with the theoretical prediction of Daniels and with that of Hall and Walton. Required experimental conditions necessary to observe critical fluctuations are discussed.