Thesis
Ducted propeller underwater radiated noise mitigation through leading-edge tubercle modifications
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2023
- Thesis identifier
- T16519
- Person Identifier (Local)
- 201967947
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Anthropogenic underwater radiated noise (URN) has a negative impact on marine life, disrupting key biological functions such as communicating, navigating and catching prey. One of the largest contributors to URN is the various types of marine vessels that occupy the world’s oceans today and of this, the most significant proportion of the noise is emitted from the propulsor. Due to increasing public awareness on this topic, international bodies such as the International Maritime Organization (IMO) published non-mandatory guidelines in 2014 to accelerate the reduction in shipping URN. Over the last few decades, there has been growing interest and research around leading-edge (LE) tubercles which are located on humpback whale pectoral fins. They are believed to enhance the manoeuvrability of the marine mammal through prolonged flow attachment and have shown to improve the hydrodynamic and noise performance of marine applications through the introduction of counter-rotating streamwise vortex pairs which alter the local flow-field. But, this concept has yet to be applied to ducted propellers and based on the available literature, they could have the capability to address the needs of the shipping industry by reducing the URN signature of marine vessels. Therefore, this research study focuses on the noise mitigation capability of LE tubercles on ducted propellers. Two key areas of noise mitigation were identified through a review of the state-of-the-art literature; LE tubercles applied to the duct to alter the vortex development in the ducted propeller slipstream and therefore, mitigate turbulence and vorticity-induced noise and LE tubercles applied to the blades to influence the sheet cavitation development over the blades and thus, mitigate cavitation-induced noise. This study aims to establish a proof of concept for both identified noise mitigation techniques and understand the core fundamental fluid dynamic mechanisms behind the performance changes using a numerical methodology known as Computational Fluid Dynamics (CFD). In summary, it was found that through an initial design and optimisation study, LE tubercle modifications on the duct could improve the duct thrust performance, although this was dependant on the amplitude and wavelength of the tubercle geometry, the change in amplitude was more significant than the change in wavelength. Through further detailed analysis of the optimised LE tubercle modified duct it was found the duct thrust could be improved by over 7% due to the compartmentalisation of the flow separation on the outside of the duct with minimal impact on the overall hydrodynamic performance of the propulsor. Also, it could reduce the far-field URN by over 3dB overall sound pressure level (OASPL) through disruption of the coherent wake structure in the propeller slipstream. It was found that LE tubercle modified ducted propeller blades could improve propulsive efficiency by up to 6.5% when comparing at the same thrust loading condition. Additionally it was found that the LE tubercle modified ducted propeller blades could produce a noise reduction in the far-field at most test conditions considered to a maximum of 6dB OASPL and reduce the blade load fluctuation. Through analysis of the flow-field, it was found that this was predominantly due to the introduction of the counter-rotating vortex pairs and subsequent alteration of the local pressure field over the blade suction side, which ultimately reduced the sheet cavitation severity by a maximum of up to 50% over the blade surface by funnelling the cavitation behind the tubercle trough region. To compliment the previous studies in the model scale, an investigation into the scaling effect of LE tubercle modified duct and propeller performance was conducted by performing an analysis of the optimised geometries at a larger scale. It was found that the duct thrust performance could be improved by a maximum of 3.6% with minimal impact on the overall hydrodynamic performance of the propulsor. The increase in scale and subsequent increase in Reynold’s number resulted in the inception of LE flow separation to occur at a much later advance ratio on the outer duct section than in the model scale study and therefore no flow separation compartmentalisation was observed by the LE tubercles within the propulsor operational envelope. A reduction in far-field noise of 4dB OASPL was predicted and this was predominantly due to the disruption on the coherent wake structure in the propeller slipstream. For the LE tubercle modified ducted propeller blades in the full scale study, it was found that a cavitation reduction of over 60% was observed due to the cavitation funnel effect and comparing at the same thrust loading condition, the propulsive efficiency could be enhanced by 7.5%. In the far-field, a maximum reduction of 13dB OASPL was observed while the blade load fluctuation was reduced which was pre- dominantly due to the reduction in cavitation severity over the blades due to the LE tubercle cavitation funnel effect. Therefore, although a difference in magnitude was predicted in LE tubercle performance change between model and full scale, the general conclusions regarding hydrodynamic and hydroacoustic performance enhancement agreed well with the previous model scale studies.
- Advisor / supervisor
- Atlar, Mehmet
- Shi, Weichao
- Resource Type
- DOI
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