In high-loading compressors, the three-dimensional flow separation in the blade corner region significantly affects aerodynamic performance, efficiency, and stability. Conventional Reynolds-Averaged Navier-Stokes (RANS) methods and steady experimental approaches fall short in fully capturing the unsteady, multi-scale, and intricate vortex structures that drive corner separation and its associated losses. To address this, the chapter employs advanced Detached-Eddy Simulation (DES) and Delayed Detached-Eddy Simulation (DDES) techniques to achieve high-fidelity, time-accurate predictions of corner flow fields in a controlled diffusion cascade representative of modern compressor geometries. Complemented by comprehensive experimental data, these simulations serve as a robust platform for understanding the fundamental mechanisms underlying corner separation.The chapter first validates the DES-series methods against oil-flow visualizations, pressure-sensitive paint (PSP) measurements, and traditional pressure measurements. Results confirm that DES and DDES significantly outperform conventional RANS in predicting not only the location and extent of corner separation but also in reproducing key unsteady flow features such as passage vortices, horseshoe vortices, trailing-edge shedding vortices, and complex vortex interactions downstream.Subsequent analyses focus on identifying the dominant unsteady flow structures and their evolution. By employing advanced modal decomposition techniques—such as Dynamic Mode Decomposition (DMD) and Proper Orthogonal Decomposition (POD)—the study disentangles the spatiotemporal complexity of the turbulent flow field. These methods reveal the presence of coherent vortex structures, including passage vortices and hairpin vortices at the cascade exit, as well as the interplay between corner vortices and suction-surface separation vortices (SSV) under varying incidence conditions. Under near-stall conditions, these vortex interactions intensify, driving instability and expanding low-energy regions that degrade aerodynamic performance.In essence, this chapter provides a comprehensive, high-fidelity view of the corner separation phenomena in compressors, demonstrating how DES-series simulations, when integrated with experimental validation and advanced flow-field decomposition techniques, yield new insights into the intricate unsteady mechanisms governing compressor stability and efficiency. The findings offer theoretical foundations and practical guidelines for future compressor designs and flow control strategies aimed at mitigating corner separation and enhancing overall compressor performance.