City, University of London with sponsorship from Finnish engineering firm Wärtsilä has conducted a research project into identifying the specific design parameters that create the risk of propellers singing and has developed a new methodology for design stage risk detection of the singing propeller phenomenon
Some propellers in service produce a periodic tonal noise that can be heard in the steering flat, shaft tunnel, engine-room or even in the accommodation. The singing noise has been reported to be of both harmonic and non-harmonic nature and can take a variety of forms ranging from a deep sounding grunting noise to a high-pitched warbling noise, typical of an incorrectly set turning operation on a lathe. The phenomenon rather than being anything particularly harmful is a matter of annoyance to the human ear and so has a negative effect on comfort levels on-board operating vessels.
For many years since the end of the 2nd World War it was considered that the problem of propeller singing could be solved by the application of sharp edged, single or double, chamfers to the tips and trailing edges of propellers. Indeed, for most propellers when they have a tendency to sing, such standard remedies, dating back to the 1940s and 50s, would have shown themselves to be effective in curing the problem. However, this has not been the case with some of the more modern advanced propeller designs.
There have been many exploratory studies on the origins of singing and how to prevent it. The generally accepted reason why a propeller sings is that the natural frequency of a vibrating blade coincides with the vortices being shed from the outer trailing edge and, rarely, the tip region of the blades. This results in a resonance condition known as ‘lock-on’. This mechanism is also known as ‘Vortex Induced Vibration’ (VIV), where the exciting force is generated continuously by the shedding of parallel vortices.
The propeller singing phenomenon is a complex and sensitive problem in that, for example, from series of nominally identical propellers that have been produced from a common design, perhaps only one member of the series will sing. Such an observation suggests that either small propeller blade dimensional changes or minor differences in the wake fields may give rise to the problem. The numerous and extensive studies on the topic have shown the sensitivity of the vortex shedding, known as von Kármán streets, from the trailing edge geometry of the blade sections and the influence of the development of the boundary layer over the hydrofoil form.
The new research began in 2011 and set out to investigate the parameters governing the phenomena of singing. The research was undertaken as part of a PhD project at City, University of London and added to Wärtsilä’s own internal investigations. This research showed that the singing problem can be controlled by altering the propeller main design parameters; careful attention to the vibration modes of the blade and modifying the trailing edge of the blade to eliminate the shedding vortices and, therefore, also the hydrodynamic excitation forces (Figure 1).
The results of this research established three major findings:
1. Singing is generated from the lock-on between vortex shedding at the trailing edges and the modal response from the propeller blades.
2. Propellers potentially at risk of singing can be distinguished from non singing propellers (such as with a certain bandwidth) by a careful evaluation of vibration modes.
3. The occurrence of the singing behaviour requires periodic excitation forces, and so in this respect, trailing edge and ASE details are very important and will determine whether vortices are shed or not.
The new approach examined Computational Fluid Dynamic (CFD) analysis of flow and vortex shedding at the trailing edge, as well as the influence ofcavitation; mechanical exploration, such as 3D measurements of geometries; vibration measurement in air and water; modal analysis using Finite Element Analysis (FEA) and the analysis of impact of other affecting factors including a parametric analysis and sensitivity studies previous design records as well as a detailed literature review.
The results, concluded in December 2015, have now led to a method by which vessels at risk for singing are identified in the design process. City, University of London’s Professor of Marine Engineering, Professor John Carlton FREng, is quoted to say: “This has been an extremely successful project in dealing with an important propeller design issue. As an industry, we thought we had discovered a pragmatic solution to the singing propeller problem many years ago. However, some recent advanced propeller designs did not respond to the conventional treatments. As such, this research has now led to a method which enables designers to assess the singing potential of a propeller at the design stage.”
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