Conditional POD and Space-Time POD for a Turbulent Boundary Layer with Passive Scalar Transport and Aero-Optic Distortion

The velocity field associated with the deflection of a laser beam passing through a heated incompressible boundary layer was studied using variants of the classical proper orthogonal decomposition (POD) technique. Due to the heating of the boundary layer, a passive density gradient was present in the flow creating an index of refraction gradient. This index of refraction gradient resulted in the beam being deflected from its original position. The influence of coherent structures in the flow was studied using conditional POD and space-time POD. Using velocity and laser beam deflection angle measurements from a Malley probe, variants of POD were performed to decompose the flow field. Both variants were also performed on the Gaussian filtered small-scale velocity field. Conditional POD was performed for large upstream and downstream deflections and compared to the POD modes for all snapshots. The results from conditional POD for the wall-normal component of velocity were found to be shifted in the streamwise direction based on the condition and the size of the mode evaluated. Space-time POD resulted in the same modes as the conditional POD and showed the modes convecting in the streamwise direction without a significant change in the size of the structures. This led to the hypothesis that the large scale Malley probe deflection events are highly correlated to the positions of specific structures relative to the Malley probe.