Numerical investigation and mode analysis of the S-duct

This study investigates the flow field and turbulence characteristics within two types of S-shaped ducts through both experimental and numerical methods. The experiments include pressure measurements and constant temperature anemometry, while the numerical simulations employ the Reynolds-averaged Navier–Stokes (RANS) approach. To ensure the accuracy of the simulations, a mesh independence study was conducted, and the results were compared with experimental data. Additionally, three common turbulence models—k−ϵ, k−ω, and the shear stress transport (SST)—were tested. After comparing the two S-shaped ducts, the configuration with superior performance was selected for further unsteady simulations. Modal analysis was subsequently performed using proper orthogonal decomposition, dynamic mode decomposition, and spectral proper orthogonal decomposition. The results show that the S-shaped diffuser performs better when the diffusion rate follows an S-shaped curve rather than a linear increase. In low-velocity experiments, a 1 kHz sampling frequency and 1-second period suffice for turbulence analysis, as higher values only yield minimal differences. Regarding the simulation, although RANS simulations can match the experiment, the discrepancies with experimental data can also not be ignored. Among the three turbulence models, the SST model best matches the experiment. However, modal analysis of the transient flow field reveals that RANS makes it hard to capture turbulence details in low-speed conditions, and only the k−ϵ model has modes with clear dynamic patterns. Last, the functionalities and features of each modal analysis method are demonstrated.