Planar and volumetric turbulent mode dynamics of a supersonic separated/reattaching flow

Three-component, planar, stereoscopic particle image velocimetry (S-PIV) data and three-component, volumetric, tomographic PIV (T-PIV) data acquired in the near wake a Mach 2.49, axisymmetric, separated/reattaching flow field are modally deconstructed and analyzed using proper orthogonal decomposition (POD). The S-PIV data were analyzed throughout the entire measurable near wake, and the T-PIV data and analysis were limited to a subsection of the conical shear layer of this flow field. Analysis of the S-PIV data using POD identified high-energy turbulent mechanisms present in the flow field associated with the exchange of fluid between the shear layer and the bounded recirculation bubble corresponding to entrainment/detrainment events. The phase spectra associated with these POD modes were used in conjunction with a turbulent quadrant analysis in order to conditionally average turbulent events associated with a corresponding flow field mechanism. This analysis was able to successfully filter turbulent events to reveal the flow field dynamics occurring both in the immediate vicinity of an acting turbulent mechanism, as well as far away from where the mechanism is acting. The POD analysis of the T-PIV data revealed the presence of coherent, three-dimensional, high energy-containing turbulent structures present within the shear layer. Hairpin and counter-hairpin vortices were found to be associated with the highest energy-containing POD mode. The second POD mode identified the existence of a quasi-streamwise oriented structure that exists in the subsonic portions of the shear layer throughout the adverse pressure gradient region. The three-dimensionality of this structure was resolved and displayed using linear stochastic estimation. Many of the lower energy-containing modes exhibited similar dynamics to the turbulent structures of the higher energy modes, occurring at smaller spatial scales and/or higher spatial frequencies. This indicates that the coherent structures identified by POD are responsible for much more of the total turbulent kinetic energy present in the shear layer than just the energy contributions of the first two POD modes.