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Title:Development of experimental techniques and studies of spatial structure in turbulent thermal convection
Author(s):Offutt, Peter William
Doctoral Committee Chair(s):Adrian, Ronald J.
Department / Program:Physical Oceanography
Engineering, Mechanical
Discipline:Physical Oceanography
Engineering, Mechanical
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Physical Oceanography
Engineering, Mechanical
Abstract:Particle-image velocimetry (PIV), shadowgraph, and thermochroic liquid crystal techniques are applied to the study of spatial structure of turbulent thermal convection in wide horizontal water layers. A large-aspect-ratio test section for the study of turbulent Rayleigh convection in water has been constructed, instrumented and characterized. Boundary temperature measurements reveal significant finite Biot number effects for water bounded by aluminum.
Particle-image velocimetry measurements in a vertical plane in Rayleigh convection gave approximately 8500 velocity vectors in each of 62 instantaneous realizations at a Rayleigh number of 1.0 $\times$ 10$\sp{8}$. The vector fields reveal structures ranging in size from a few millimeters in the boundary regions to large-scale motions spanning the 10 cm layer depth. Ensemble one-point statistics of the velocity field, including the mean velocity field, root-mean-square profiles, and probability-density functions of the vertical velocity as a function of height are presented. The PIV data have also provided the first measurements of the two-point spatial velocity correlation in turbulent Rayleigh convection.
The PIV measurements are supplemented by thermochroic liquid crystal visualizations of temperature fields on the lower boundary, and by shadowgraph projections of the temperature fields in the bulk. The shadowgraphs reveal well-defined, viscous thermal plumes which generally cross the layer intact; midlayer transit measurements from videotaped shadowgraphs in Rayleigh convection and unsteady nonpenetrative convection give ascent rates of 1.4 to 1.5 times Deardorff's inviscid convective velocity scale. The thermochroic boundary temperature visualizations showed plumes emanating from nodes in a network of warm lines enclosing patches of cooler fluid.
Deardorff's inviscid velocity and temperature scales for the core are refined by enforcing equality of bulk turbulent production and spectral energy transfer. The collapse of velocity fluctuation data under the new "bulk" scales for different modes of turbulent thermal convection shows the old systematic discrepancies arose from dimensionally correct but numerically arbitrary scale definitions. Also, under the new scaling, a monotonic Rayleigh number dependence emerges among the core temperature fluctuation profiles. This trend indicates that true high-Reynolds-number invariance is not achieved at Rayleigh numbers accessible to direct numerical simulation and laboratory-scale experiments.
Issue Date:1995
Rights Information:Copyright 1995 Offutt, Peter William
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9543687
OCLC Identifier:(UMI)AAI9543687

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