The Fluid Velocity and its Reorientation in Simulated Turbulent Rayleigh-Benard Convection
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The velocity and reorientation of the large-scale circulation (LSC) in Rayleigh-Benard convection (RBC) at the Rayleigh numbers R = 1x107 < Ra <1 x108 and Prandtl numbers P=0.021 (mercury), 0.4, and 0.7 (air) are studied. The dynamics of Rayleigh-Benard convection is strongly dependent on these two dimensionless parameters which characterize the strength of the heat input driving the turbulence and the fluid being used in our simulations. Using the spectral element code Nek5000, we numerically simulate Rayleigh-Benard systems in a cylinder whose diameter is equal to its depth. These fluid-filled cells are surrounded by conducting walls with the top and bottom plates fixed at cold and hot temperatures or insulating, respectively. As we increase the heat input into the cell, a turbulent and chaotic motion is produced, and a characteristic of this motion is a noisy yet regular large-scale circulation. In all of our simulations, we witnessed a large-scale circulation which is described as the global motion of the fluid as it becomes less dense at the bottom of the cell and rises from one side of the cell as the cooler fluid from the top of the cell falls into its place on the opposite side. We measure the vertical velocity VZ and the temperature of the fluid near the lateral surface of the midplane. Using Fourier transforms, we are able to identify cessations of our large-scale circulation and also analyze the amplitude and phase of other Fourier modes that are present.