|Abstract:||The current work provides a detailed characterization of the turbulence structure in the near-wake of a blunt-based cylinder aligned at zero angle-of-attack in a Mach 2.49 freestream. Particular emphasis is placed in this work on the identification of turbulence mechanisms in the flow regions both upstream of, and in the immediate vicinity of reattachment of the separated shear layer, as these mechanisms are the main drivers of critical flow properties, such as the low pressure in the separated region. This work was experiment-based, utilizing non-intrusive optical measurement techniques, including stereoscopic particle image velocimetry (SPIV), which measures all three components of velocity along a large planar region, and tomographic PIV (TPIV), which measures all three components of velocity throughout a volumetric region. Large ensembles of measurement samples were acquired and processed for each set of experiments, which allowed for statistical convergence in the implementation of various novel turbulence analysis techniques, such as linear stochastic estimation (LSE), proper orthogonal decomposition (POD), and several others.
The volumetric TPIV data allowed for the identification and characterization of 3-D coherent turbulent structures within this flow, including upright hairpin vortices, inverted hairpin vortices, and streamwise- elongated quasi-axial vortices. Upright hairpin structures are demonstrated to commonly exist throughout this flow, both upstream and downstream of the reattachment point, while the presence of inverted hairpins was limited to the subsonic flow regions. An LSE-based analysis provided robust evidence regarding the statistical prevalence, size, and spatial growth of these structures. It is demonstrated that the amplified shearing mechanism along the inner-arch of these structures generates high-energy velocity fluctuations aligned with consistent directions, which ultimately dominate the turbulent energy spectrum throughout this flow. It is also demonstrated, using conditional flow statistics derived from the POD results, that the dynamics of these coherent structures can be directly linked to the growth rate and subsequent reattachment length of the separated shear layer, which is well understood in the literature to be a key driver of the large pressure drag in these massively separated flows through mass entrainment from the recirculation bubble. It was found that an increased statistical prevalence of upright hairpin vortices acts to reduce the shear layer growth rate and increase the reattachment length, which is then postulated to result in a significant reduction in the pressure drag. The inverse was also found for the inverted hairpins, which have a directly opposite influence on the developing flow. Additionally, an analysis of the turbulence statistics along the interface separating the shear layer from the recirculation bubble reveals a consistent spatial organization to the upright and inverted hairpin structures along this interface, providing further evidence of their strong and direct influence on mass entrainment.
This work also provides a detailed and high-spatial-resolution characterization of the 3-D Reynolds- averaged turbulence field, including all components of the kinematic Reynolds stress tensor, the fluctuating velocity triple product tensor, and the mean flow-induced turbulence production tensor, among others. This statistical information provides further insights into the spatial development of coherent turbulence mechanisms, and also serves as a detailed benchmark experimental data set for the comparison and validation of computational simulations of this flow.