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| 2000-2001 Fluids Seminar Page |
Graduate Aeronautical Laboratories
Caltech
Abstract-
Experiments on mixing and the isosurface geometry of turbulent jets injected
into a crossflow are described. Jet-fluid concentration fields were
measured with laser-induced fluorescence and digital imaging techniques for
velocity ratio Vr = 10.1 jets in the jet Reynolds number range 1.0 x 10^3 <=
Rej <= 20 x 10^3. Enhanced scalar mixing with increasing Reynolds number is
seen in the evolution of PDFs of jet-fluid concentration fields. Turbulent
mixing is found to be flow dependent, based upon differences between
transverse jets and jets discharging into a quiescent reservoir. The PDF of
scalar increments is also studied in the far field of the transverse jet,
and found to trend from Gaussians to exponential distributions with
decreasing separation distances. The scalar field is seen to be anisotropic
from two-dimensional power spectra, directional PDF of scalar increments.
The isosurface geometry of transverse jets is also studied with fractal geometry. Generalized coverage statistics are introduced for anisotropic, non-self-similar sets. This generalized coverage counting involves covering with parallelepipeds of varying size and aspect ratio. A scale-dependent measure of anisotropy is introduced, and it is shown to transform the coverage count, N(lambda_1,lambda_2), to isotropy through a scale-dependent normalization of the coordinates. Isosurfaces of the transverse jet are found to be anisotropic at both large and small scales. For the transverse jet, jet-fluid-concentration isosurfaces are vertically stretched at small length scales, and horizontally stretched at large length scales. In the special case of isotropic box counting, the scale-dependent coverage dimension is found to vary from unity, at the smallest length scales, to 2, at the largest length scales, indicating that the isosurfaces produced by turbulent mixing are more complicated than can be described by power law fractals.
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