SATURN researchers have produced two new papers documenting key insights into marine propeller physics.
Journal of Fluid Mechanics
Antonio Posa, Riccardo Broglia, Mario Felli, Marta Cianferra, and Vincenzo Armenio.
The acoustic analogy is adopted to characterise the signature of a seven-bladed submarine propeller, relying on a high-fidelity large-eddy simulation, performed on a computational grid consisting of 840 million points. Results demonstrate that the nonlinear terms of the Ffowcs-Williams and Hawkings equation quickly become dominant moving away from the propeller along the direction of its wake development. While the linear terms experience a decay moving downstream, the nonlinear terms grow in the near wake, as a result of the development of wake instability. In particular, this growth affects frequencies lower than the blade frequency. Therefore, the acoustic signature of the propeller is mainly tonal in the near field only, due to the thickness and loading components of noise from the surface of the propeller and the periodic perturbation caused by its tip vortices. They develop instability at a faster rate, compared with the hub vortex, triggering the process of energy cascade towards higher frequencies and contributing in this way to broadband noise.
Journal of Ocean Engineering
Antonio Posa, Mario Felli, Riccardo Broglia
The acoustic analogy is utilized to analyze the sound from a propeller–rudder system, exploiting data from high-fidelity Large-Eddy Simulations, performed on a cylindrical grid consisting of 3.8 billion points. Simulations were conducted with the downstream hydrofoil at incidence angles of , , and . The analysis demonstrates the significant effect of the downstream hydrofoil in increasing the sound pressure levels, especially due to the loading noise from its surface, but also as a result of the enhanced complexity of the wake topology. For increasing angles of incidence, the wake experiences a growing elongation in the direction of the span of the hydrofoil and higher values of cross-stream velocity, reinforcing the shear between the wakes shed by the propeller and the hydrofoil, respectively. A very complex directivity of the acoustic signature develops, whose dependence on the orientation of the hydrofoil is variable across both the azimuthal and the streamwise directions. Separation on the suction side of the hydrofoil for large incidence angles has the effect of reducing the loading component of noise from its surface, which is the leading one, with the exception of the radial coordinates within a few diameters from the axis of the propeller and its wake.