Turbulent Diffusion in Uniformly Sheared Flow

Abstract

The objective of this thesis research is to further the understanding of turbulent diffusion by experimentally studying the turbulent diffusion of a plume of dye released in uniformly sheared flow generated in a water tunnel. The flow studied was nearly homogeneous but strongly anisotropic and had a turbulent Reynolds number of 150. Maps of the turbulent velocity and dye concentration were measured simultaneously using stereoscopic particle image velocimetry and planar laser-induced fluorescence. A thorough analysis of the planar laser-induced fluorescence technique was performed; several previously unconsidered sources of error were identified and corrections were proposed. The measured evolutions of the mixed velocity-concentration statistics of the plume were compared with previous studies. The turbulent scalar flux vector was related to the mean concentration gradient through a first-order gradient transport model and, for the first time in an experimental flow, all components of the turbulent diffusivity tensor were measured directly. The turbulent diffusivity tensor was found to be highly anisotropic and its streamwise component appeared to be counter-gradient. The relative diffusion of the plume was also investigated and the evolution of the mean square particle separation was found to be consistent with Richardson-Obukhov scaling, with a value of Richardson's constant equal to 0.35. The fine structure of the concentration field and the mixed velocity-concentration statistics were also documented. Because of the high level of intermittency of the present plume, the scalar probability density function was strongly non-Gaussian and the conditional expectations of the velocity components and the scalar dissipation, conditioned upon the scalar value, were distinctly non-linear. Lastly, the role of coherent structures on scalar diffusion was investigated and a conditional eddy analysis demonstrated that hairpin vortices were associated closely with large scalar flux events.