Dr.-Ing. Julian Kimmerl, Technische Universität Hamburg/SCHOTTEL GmbH, Georg-Weinblum-Preis
Underwater radiated noise of propulsor-hull combinations is an important challenge for protection of the environment and military applications. Typically only single acoustic sources and propellers in open water condition have been investigated in detail. For geometrically complex and cavitating propulsor flow in behind ship condition with volumetric sound sources, the finite volume method is the only viable simulation approach and requires high mesh resolution at the locations of known acoustic sources, such as the trailing vortices of the propeller. In the marine industry, the incompressible, isothermal Eulerian mixture model is the standard and requires additional means to estimate far-field noise, such as coupled acoustic analogies. The limitations in capturing the volumetric sound sources of this combined approach are inclined flow, slipstream interactions with obstacles, and upstream wake fueling slipstream distortions, as the propeller slipstream flow is not rotationally symmetric to the axis. A combination of methods specific to the posed problem is developed, with a wall resolved implicit Large Eddy simulation to capture most turbulent length scales in combination with the Volume-of-Fluid Eulerian mixture model and the OpenFOAM Schnerr-Sauer phase transition model. To realize the propeller rotation, a multi reference frame or a dynamic mesh functionality with sliding mesh interface is used. To resolve the propeller slipstream phenomena a manual and automatic AMR method is implemented based on Q-criterion and vapor regions. The permeable surface Ffowcs-Williams-Hawkings method deals with the acoustic part of the simulation and the only viable placement of the passive surface is determined as being directly connected to the geometry of the sliding mesh interface for complex cases with numerical resource restrictions. Different test cases with increasing complexity are investigated beginning with hydrofoils, open water propellers, propeller-rudder combinations and ending with propulsor-hull combinations that consist of a twin-screw mega yacht, a single screw container vessel and a double ended ferry with a SCHOTTEL SRE propulsor centrally located at each end. The test cases are evaluated with the developed methods regarding turbulence, with a focus on trailing vortices, sheet and trailing vortex cavitation and structure interaction in the propeller slipstream and validated with model test results, where available. Numerical and physical phenomena occurring with these specific test cases that affect acoustic sources are identified and described, such as the time-dependent flow within a cavitating tip vortex of a propeller. The simulation approach is validated regarding the correct prediction underwater sound with open water propeller test cases and compared with full-scale measurements, leading to good agreement for propellers and acceptable agreement for the full-scale cases. Pressure pulses at propeller blade frequencies in wetted and cavitating condition on the hull are well predicted by the incompressible pressure with errors of approximately +/-0.1kPa and far-field noise determined with the acoustic analogy reaches satisfactory agreement with deviations of around +/-10dB depending on the frequency. However, the approach is technically plagued by numerical issues, which depending on the spurious noise source mainly affect higher frequency noise in cavitating condition in full scale. Besides, more advanced and intuitive visualization methods to evaluate the near- and far-field pressure field are developed that lead to better interpretability of underwater acoustics.