Optimization design and flow physics of end wall contouring for aeroengine bypass duct with a large radius change-to-length ratio

To address the urgent demand for compact turbofan engines driven by distributed and cooperative technologies, and to solve the challenges of high flow losses and severe separation in S-shaped bypass ducts with large radius change-to-length ratios, this study aims to reveal the physical mechanisms by which coupled inner and outer end wall curvatures regulate corner vortex evolution and turbulent dissipation. Based on the fourth-degree polynomial parameterization method, a streamlined and efficient parametric model was established by introducing the extremum points of the inner and outer end wall profiles as core design variables. Through Reynolds-Averaged Navier–Stokes numerical simulation and entropy generation analysis, the influence of extremum point positions on the streamwise adverse pressure gradient, radial pressure distribution, and corner secondary flow structures was systematically investigated. The research revealed a low-loss correlation zone between the extremum points of the inner and outer end wall profiles. The synergistic regulatory mechanism is identified as follows: the position of the outer wall extremum point governs the adverse pressure gradient distribution in the outlet section, whereas the inner wall extremum point primarily controls the adverse pressure gradient distribution in the inlet section, altering the flow direction of low-momentum fluid and consequently affecting the onset of flow separation and the generation and development of corner reverse flow. The optimized extremum point combination (inner end wall: x_c1 = 0.061 and outer end wall: x_c2 = 0.048) effectively suppresses the corner vortex and reverse flow cell, reduces turbulent dissipation, achieving a 35.8% reduction in fluctuating flow dissipation, and significantly enhances the duct's flow capacity. This study elucidates the mechanism by which end wall curvature influences losses through modulation of the pressure field and secondary flow structures, providing a theoretical foundation for the design of high-performance bypass ducts.