Spiteri, A (2024) AN INVESTIGATION OF THE HYDRODYNAMICS OF AIR LUBRICATION SYSTEMS AND THE DEVELOPMENT OF A SCALING TECHNIQUE FOR DIFFERENT DESIGNS AND OPERATIONAL CONDITIONS USING CFD. Doctoral thesis, Liverpool John Moores University.

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Abstract

An air lubrication system (ALS) is a green technology that has been increasing in popularity to reduce ships’ emissions and improve their Carbon Intensity Indicator (CII) ratings. ALS works by injecting air underneath the hull of the ship, this modifies the turbulent boundary layer (T.B.L.) in a positive way and reduces drag. A full understanding of how this technology works and how it can be optimised is still not fully understood, due to a lack of research and many uncertain influencing variables. Also, currently, there is no clear and verified method of understanding how a scaling law can be applied to extrapolate results from model testing to full-size testing. A better understanding of the system would make ALS more efficient, optimisable at different operating conditions and a scaling law would be useful to extrapolate towing tank testing to full-size results, giving ship owners further trust in this technology.
A robust Computational Fluid Dynamics (CFD) methodology was first developed, this was done with extensive testing of boundary conditions, research on turbulence models and the addition of compressibility to the model. The CFD model was validated by comparing the behaviour and patterns from experimental results to other behaviours. Three types of testing regimes were undertaken: i) the same plate size with different depths to understand the effect of hydrostatic pressure, ii) three different plate lengths with different Reynolds numbers (Re) to understand the effect of Re, iii) three different plate sizes and depths that replicate a model test and a full-size model with a length of 65 m. It was discovered that there is almost a linear relationship between drag reduction loss and an increase in hydrostatic pressure, with a statistical analysis r value > -0.954. A non-dimensional exercise was undertaken and whilst the final equation was not highly accurate, plotting the value of mass flow rate (MFR) and drag reduction resulted in a predictive model with over 86 % accuracy. Increasing the Re number up to Re x10^9 resulted in a loss in the drag reduction effect for a mixing ratio, which is the ratio of air to water in the boundary layer, of 0.4, whilst results for a higher mixing ratio were more stable. Finally, three plate sizes were simulated representing a model size, full size and an in-between to fully understand what happens when scaling up an ALS. This resulted in an agreement of over 84 % from model scale to full scale. The difference in the drag reduction effect is hypothesised to come from an increase in Re, an increase in turbulence and computational errors. It was concluded that when scaling up the technology the air injection area and the MFR should be scaled with the same scaling ratio.

Item Type:	Thesis (Doctoral)
Uncontrolled Keywords:	hydrodynamics; CFD; Air Lubrication Systems
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Divisions:	Engineering
SWORD Depositor:	A Symplectic
Date Deposited:	05 Apr 2024 09:53
Last Modified:	05 Apr 2024 09:54
DOI or ID number:	10.24377/LJMU.t.00022813
Supervisors:	Armin, M, Blanco-Davis, E and Wang, J
URI:	https://researchonline.ljmu.ac.uk/id/eprint/22813

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