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We present results of experimental study of turbulent flow with large scale structures in Rayleigh-Bénard convective cell. Experiments were conducted in air flow in a rectangular chamber with a heated bottom wall and a cooled top wall. Velocity field was measured using a digital particle image velocimetry (PIV) in 15 cross-sections parallel to the vertical walls of the chamber. Experiments revealed three distinct flow components in a large scale circulation (LSC): (i) a main roll with a length scale of a size of the chamber that been observed in previous studies and (ii) two elongated vortex rings adjacent to the bottom and top of the main roll. A characteristic length scale of a vortex in two eddy rings is by a factor of 5 smaller than a length scale of a main roll. A mean horizontal velocity of the main roll and a mean vorticity of two eddy rings are aligned in the center of a cell so that the helicity of the mean flow is quite high. We estimated helicity of turbulence using the measured cross-correlation functions of the velocity field. Spectra of turbulence reveal two distinct ranges in the inertial interval with slopes close to -5/3 and -7/3. Remarkably, similar inertial subranges in turbulent velocity spectra were observed in various geophysical and astrophysical turbulent flows. We found that the magnitude of the length scale where a slope of the velocity spectra changes and the magnitude of the length scale defined as a ratio of turbulent energy to the helicity are close. Since the mean and turbulent components of the flow velocity field in a Rayleigh-Bénard convective cell are helical, the Rayleigh-Bénard convection allows investigating properties of helical turbulence which is quite difficult in other flow configurations.
Author(s):
Alexander Eidelman
Ben-Gurion University of the Negev
Israel