Coherent Structures, their Interactions, and their Effects on Passive Scalar Transport and Aero-Optic Distortion in a Turbulent Boundary Layer

Abstract

This thesis focused on the characterization of coherent structures and their interactions in a turbulent boundary layer using data from particle image velocimetry (PIV) measurements performed at Caltech and from a direct numerical simulation (DNS) of Wu et al. (2017). Connections were identified between instantaneous and statistical descriptions of coherent velocity structures, through the analysis of representative models for their structures derived from the resolvent analysis of McKeon and Sharma (2010). The representative models were used in a novel conditional averaging technique to identify the average behavior of small scales about variations in the large-scale streamwise velocity field. Based upon the results of this analysis, a hypothesis for a scale interaction mechanism was proposed involving three-dimensional critical layers. The modeling and analysis methods were then applied to the aero-optic problem in which optical beams are observed to be distorted after passing through variable-density turbulent flows. Measurements using simultaneous PIV and an aero-optic sensor called a Malley probe (Malley, Sutton, and Kincheloe, 1992) were conducted in an incompressible, mildly-heated turbulent boundary layer with Prandtl number of 0.7. A conditional averaging analysis of the data identified that the nonlinear interaction of two scales was most correlated to the aero-optic distortion. The modeling of this interaction using resolvent modes led to new insights regarding the instantaneous relationship between the velocity and scalar fields over a range of Prandtl numbers.