Mechanisms of velocity redistribution and secondary flow evolution induced by rotational compressibility effects in rotating square duct
High-speed rotating cooling passages in aero-engine turbine blades experience significant centrifugal compression, leading to strong coupling between thermodynamic states and flow structures, which challenges the validity of incompressible assumptions. In this study, large eddy simulations are performed to investigate the flow evolution induced by rotational compressibility effects in an adiabatic rotating square duct. The results demonstrate that rotational compressibility does not act as an independent additional force term but instead modifies the balance among centrifugal force, Coriolis force, and pressure gradient through density redistribution. The density variation induces a streamwise centrifugal-buoyancy effect, which drives high-momentum fluid toward the trailing side and generates an intensified adverse pressure gradient near the leading side, resulting in progressive boundary-layer thickening and large-scale flow separation. With increasing rotational compressibility, the cross-sectional secondary-flow topology evolves from the classical four-vortex structure to a six-vortex configuration and eventually develops into an eight-vortex topology. This transition may be associated with the expansion of the leading side separated reverse-flow region, which reverses the local Coriolis-force direction and disrupts the original secondary-flow balance. Furthermore, a separation prediction model based on centrifugal work number (CW) and rotation number (Ro) is established to identify the dominant parameters governing rotational-compressibility-induced separation in finite-length rotating ducts. These findings suggest strong coupling among centrifugal compression, flow separation, and secondary-flow topology transition, providing insight for advanced turbine blade cooling design.