ETD

• Axial turbine stage
• Azimuthal Modal Analysis
• Entropy generation
• Phase-locked averaging
• Reynolds-Averaged Navier-Stokes (RANS)
• Rotor-stator interaction

This thesis introduces and applies a robust and flexible methodology for space–time modal analysis of periodic flow fields in turbomachinery, referred to as Azimuthal Modal Analysis (AMA). The procedure first processes experimental data through phase-locked averaging over the blade-passing period, in order to obtain phase-consistent periodic flow fields. A two-dimensional Fourier transform is then applied in the azimuthal and temporal directions, from which the modal spectrum is obtained. Binary spectral masks are used to isolate families of modes (rotor-locked, stator-locked, axisymmetric, and rotor–stator interaction components) and to selectively reconstruct their contributions in the physical domain through an inverse double Fourier transform. The energy associated with each mode is subsequently used to build meridional maps and radial distributions describing the evolution of unsteady components within the stage.

The same framework is extended to flow fields computed through Reynolds-Averaged Navier–Stokes (RANS) simulations, enabling a unified decomposition approach for both experimental and numerical data and exploiting the full knowledge of the numerical solution throughout the machine volume. The methodology is not limited to conventional aerodynamic variables but is also applied to thermodynamic quantities associated with losses, in particular entropy and volumetric entropy generation rate. This allows the distribution of modal energy associated with these quantities to be analyzed along the meridional channel and in the radial direction, and related to the three-dimensional organization of the flow.

The methodology is applied to the SPLEEN test case of the CT3 experimental facility at the von Kármán Institute, concerning a high-speed low-pressure axial turbine stage. Overall, the work provides a reusable framework for modal and thermodynamic diagnostics of periodic flow fields in axial turbomachines, supporting the connection between spectral structures, flow dynamics, and loss mechanisms.