Experimental study of Rayleigh–Benard convection at intermediate Rayleigh numbers using interferometric tomography

Abstract

An experimental study of Rayleigh–Benard convection in an intermediate aspect ratio box that is square in plan is reported. An intermediate range of Rayleigh numbers has been considered in the study. The fluid employed is air. A Mach–Zehnder interferometer is used to collect the line-of-sight projections of the temperature field in the form of interferometric fringes. Images have been recorded after a sufficient time has elapsed for the initial transients to have been eliminated. Interferograms have been collected from four to six view angles. These are used to obtain the three dimensional temperature field inside the cavity by using tomography. An algebraic reconstruction technique has been used for the inversion of the projection data. The convergence of the iterative inversion procedure was unambiguous and asymptotic. The reconstructed temperature field with a subset of the total data was found to be consistent with the remaining unused projections. Results for two Rayleigh numbers, namely 13 900 and 40 200 have been reported. These were found to correspond to two distinct flow regimes. At these Rayleigh numbers, a well-defined steady state was not observed. At the lower Rayleigh number, the fringes away from the wall showed mild unsteadiness. At the higher Rayleigh number, the fringes were found to switch between two patterns. Results for the dominant mode alone have been presented for this problem. At a Rayleigh number of 13 900, three dimensional flow structures, whose influence is equivalent to longitudinal rolls have been observed. At a Rayleigh number of 40 200, cubic cells have been noted in the cavity. The associated flow pattern is inferred to be a plume rising from the heated plate. The local Nusselt number variation is seen to be consistent with the observed flow patterns.