Comparison of Self-Aligned Focusing Schlieren and Conventional Schlieren for Compressible Turbine Cascade Experiments

The increasing need for quantitative flow information in turbomachinery testing motivates the development of optical diagnostics that offer higher sensitivity while enabling compact experimental setups. In this context, the present work introduces self-aligned focusing schlieren (SAFS) and compares its performance against conventional schlieren imaging. The two techniques are applied to a linear turbine cascade operating at Ma = 0.93 and 1.10 to assess their capability to resolve the relevant aerodynamic structures and unsteady phenomena. The results demonstrate that SAFS produces clearer instantaneous visualizations, enabling improved detection of wakes, pressure waves, and shock-wave structures. In the transonic regime, SAFS allows a clearer identification of vortex shedding coexisting with trailing-edge acoustic radiation and upstream-propagating pressure waves. In the supersonic condition, the shock waves captured with SAFS appear thinner, consistent with reduced line-of-sight integration effects. A quantitative analysis based on two-dimensional spatial autocorrelation maps confirms that SAFS reconstructs the vortical wake with higher fidelity, recovering the characteristic vortex-shedding spacing. Spectral analyses along selected flow paths further demonstrate the superior capability of SAFS to resolve the dominant unsteady flow dynamics. The resulting spectral content is consistent with Strouhal-number ranges reported in the literature for vortex shedding and shock-boundary-layer interaction.