Time-Resolved Self-Aligned Focusing Schlieren Applied to a Transonic Turbine Cascade

Self-Aligned Focusing Schlieren (SAFS) is a recently developed variant of focusing schlieren that overcomes the demanding alignment requirements of conventional focusing schlieren while preserving a narrow depth of field in a compact and flexible configuration. In this work, a double-lens SAFS setup is applied to a turbomachinery test case, imaging the exit flow of a transonic low-pressure turbine cascade. Time-resolved schlieren measurements were acquired at 180 kHz around the trailing edge of one blade, and image pre-processing was applied to highlight vortical, acoustic, and weak pressure-wave features. The resulting data were analyzed using three complementary approaches. First, space–time maps of the wake region and the corresponding two-dimensional power spectral density (PSD) distributions were used to extract the vortex-shedding frequency, the associated Strouhal number, and the convection velocity, all found to be consistent with previous measurements and literature. Second, pointwise PSDs and Fourier maps were employed to visualize the spatial distribution of spectral energy and to identify frequency coupling between vortex shedding and acoustic-wave generation. Third, a proper orthogonal decomposition analysis was conducted to isolate the dominant coherent structures and to further confirm their coupling.