Measurement and analysis of scalar fluctuations in turbulent flows.

摘要

Scalar concentration fluctuations are analyzed and measured in turbulent flows. The scalars examined are the mass fraction in a gaseous mixture and mild temperature fluctuation in an atmospheric boundary layer. Data from jet flows of propane and methane into air are analyzed; air flow in the boundary layer over a flat ground is both measured and analyzed. In both flows the data consist of components of the fluid velocity and the scalar concentration measured at the same point over time, at various spatial positions. The jet flow Reynolds number is 10,000 and the atmospheric flow Reynolds number is of the order of 106.; The conditional scalar dissipation rate is found to exhibit similar behavior in these two dissimilar flows, under certain restrictions. A parabola for the conditional scalar dissipation is found to fit the data, and the fitting coefficients for the atmospheric flow are found to obey simplified equations. The first fit coefficient exhibits a roughly power-law dependence with the height from the ground.; For the atmospheric flow the temperature differences over a scale separation r are studied. The moments of the temperature differences are found to have power law dependencies on the distance r, with the exponent values depending on the height from the ground. Two-exponent power-law behaviors with distance r are found to fit the structure functions for the temperature differences.; For the atmospheric flow, for even and odd moment orders, even and odd parity statistical quantities are defined on the basis of the probability density function (pdf) of temperature differences over a distance r, in a plane parallel to the ground. The usefulness of these quantities is explored using the spherical harmonics approach to extract power-law scaling behavior with distance r. The scaling properties are also examined using the Extended Self Similarity approach. New fitting formulas are proposed for the pdf of the temperature differences.; In summary, this thesis is a contribution to the understanding of the statistical properties of passive scalars mixed by turbulent flows. The work is largely experimental but has analytical aspects to it as well.