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Research
From the physics of a lubricant film a few micrometres thick to the alignment of a 320 m shafting line: the group’s research themes, the questions behind them and the methods we use.
Why friction in marine propulsion matters
Ship machinery is a chain of lubricated contacts, and each one takes a share of the engine’s output.
- 5–7.5% of BHP — mechanical losses inside the diesel engine (piston rings, piston pin, connecting-rod big end and main crankshaft bearings).
- 0.5–1% of BHP — shafting losses in the journal bearings of the line shafting and stern tube.
- 1–2% of BHP — gearbox losses.
- >90% of running cost — for a 176,000 dwt Capesize bulk carrier, fuel of the order of €9.5 M per year.
Even a small improvement in a bearing’s friction coefficient, film thickness or alignment therefore pays back immediately in fuel, emissions, maintenance and availability.
Hydrodynamic lubrication and CFD thermohydrodynamic analysis
How a converging film of oil carries hundreds of tonnes of thrust, and how we compute it accurately enough for design decisions.
In a sector-pad thrust bearing the lubricant is dragged into a converging geometry and builds up pressure, which keeps stator and rotor apart. Performance is described by the load carrying capacity (the pressure integral over the rotor area), the friction force (shear stress integrated at the rotor–oil interface) and the friction coefficient, the ratio of the two. Pads may be fixed with a given convergence ratio, or tilting / pivoted.
Fixed-pad bearings
Three-dimensional CFD thermohydrodynamic models of sector-pad thrust bearings with rectangular dimples resolve the flow in the film, conjugate heat transfer into the pad and rotor, and the energy equation in the lubricant. Dimple meshes of 13×13×14 elements (2,366 elements per dimple) and 14 elements across the film in the cross-flow direction are typical. Simulations reproduce measured pressure and temperature distributions on a test bearing (for example 2,000 rpm at 1,860 N), which is how the models are validated.
Tilting-pad and turbocharger thrust bearings. For all geometries investigated, texturing starts at the inflow and extends circumferentially for about 70% of the total length, covers 80% of the width and uses a texture depth of 30 µm. Turbocharger thrust bearings add their own difficulties: rotational speeds from 10 to 200 kRPM, casing temperatures typically above 100 °C, strong shear in a very thin film (shear thinning), cavitation at high lubricant temperature and meshes fine enough to resolve the film. The design goals remain the same—carry the thrust, minimise friction power losses, cut fuel consumption.
Surface texturing, hydrophobic surfaces and coatings
Deliberately shaping the stator surface — at the micro- and nanoscale — to improve load capacity and reduce friction.
Artificial surface texturing introduces small periodic irregularities — rectangular, trapezoidal or spherical dimples, or grooves — whose resolution can now reach the sub-micron scale. Methods used include micro-stereolithography, laser surface texturing and chemical etching. Hydrophobic and superhydrophobic (bio-inspired, extremely difficult to wet) surfaces behave in a different way again: the solid–fluid interface develops small shear forces and a slip velocity, so the wettability depends on both the geometrical microstructure and the chemical composition of the surface.
Work in the group has coupled parametric bearing models with CFD and evolutionary optimization to find optimal textures — for example maximizing load carrying capacity over a range of convergence ratios by choosing textured length and groove depth — and to quantify what hydrophobic stator surfaces can add.
- For partially hydrophobic sliders, load capacity is maximal at slightly diverging geometries without texturing, and the friction coefficient can be cut by roughly 60% compared with the optimal smooth bearing.
- Introducing texturing on top of slip can reduce some of those benefits — the two mechanisms compete.
- Stochastic surface roughness on stator and rotor, and magnetic-fluid-based bearings with rough surfaces, are studied analytically and numerically, including higher-order moments of the operational parameters.
- Coating routes are explored together with materials scientists, e.g. encapsulated liquid lubricants incorporated in metal-matrix thermal-spray coatings.
Manufacturing tolerances and wear
A texture that is optimal on paper is only useful if it can be manufactured and it survives. The group therefore quantifies how deviations in convergence ratio, dimple depth and dimple shape, stator concavity or convexity and stator waviness (wavenumbers of one and three) change performance, and turns those results into manufacturing tolerances for textured micro-thrust bearings.
Wear is treated as a design objective in the same optimisation loop: maximize the mean load carrying capacity while minimizing the mean wear rate over the expected life of the bearing, taking into account both translational motion along the thrust direction and tilting motion about a principal axis of symmetry of the rotor, and the resulting stiffness and damping.
Engine tribology
Piston rings, piston pin and crankshaft bearings: the contacts that decide engine friction and wear.
Lubrication models for piston rings of two-stroke marine diesel engines and for their main and crankpin bearings have been developed in-house in C++, for both steady-state and transient operation, including the effect of misalignment, wear and spatial variation of lubricant viscosity, with Elrod–Adams mass-conserving cavitation modelling. The piston-ring model has been used to quantify the effect of hydrophobicity and artificial surface texturing on power efficiency.
Typical outputs of these codes are the film thickness field, the pressure contour with its cavitation region, the oil volume fraction and the operating parameters that follow from them. A companion line of work addresses the dynamics of shaft–machine systems including planetary gearboxes, and combustion-related engine studies with partners in the same School.
In-house software.
- Shaft-alignment calculations (elastic line, bearing reactions, offsets).
- Journal-bearing solutions, steady-state and transient, with misalignment, wear, spatially varying viscosity and Elrod–Adams cavitation.
- Single- and multi-objective optimization based on evolutionary algorithms with local search, and meta-model assisted variants.
Shaft alignment and shafting–hull interaction
Can the alignment of a ship’s shafting be improved over the whole range of anticipated loading conditions?
Conventional shaft alignment assumes rigid bearing foundations, a rigid hull and a static, non-operating bearing. Each of those assumptions is optimistic: the foundation of a bearing deflects, the hull changes shape as the ship is loaded, and the bearing itself develops a film whose thickness redistributes the load. The group therefore developed elastic shaft-alignment methods and software that account for all three, in line with the rule guidance released by classification societies (BV Rule Note NR 592 DT R01 on elastic shaft alignment, April 2015; ABS Guide for Enhanced Shaft Alignment, October 2015).
Alignment has to establish the number of support bearings, their principal dimensions and longitudinal position, and mainly their vertical offsets — and it must deliver an equidistribution of bearing loads, no unloaded or negatively loaded bearing, minimized crankshaft stresses from shafting loads, acceptable misalignment (especially at the aft stern-tube bearing) and acceptable lubricant film characteristics.
Case study — a VLCC of 320,000 t deadweight (LBP 320.00 m, B 60.00 m, D 30.00 m, scantling draft 22.50 m). Elastic alignment changes the picture seen by conventional methods: elasticity of the bearing foundation is taken into account, bearing film state is taken into account, and part of the elastic deformation is compensated by the oil film, which removes the negative loading of the first engine bearing predicted by conventional alignment. For the aft stern-tube bearing (L = 1.83 m, D = 0.82 m, clearance 1.1 mm) the analysis gives a bearing load of about 1,043 kN with a maximum pressure of 1.3 MPa and a mean pressure of 0.695 MPa. Additional features include the effect of hull deflections on shaft and bearing performance, the influence of superstructure installation, foundation stiffness, and the change of alignment characteristics during a voyage as consumables alter hull deflections.
Vibration of shafts, hull and machinery
A ship is a complex dynamic system with an endless series of natural frequencies, excited from several sources at once.
Main engine, propeller, generators and auxiliary equipment load the structure simultaneously, and the structure itself responds through many closely spaced modes. Our approach combines onboard measurements and a computational route: modal analysis of the ship structure, resonance diagrams to place the operating ranges relative to the natural frequencies, and then mitigation measures derived from the model.
Related work covers torsional vibration analysis of marine propulsion shafts, shaft and engine condition monitoring, early failure detection and fault diagnosis, as well as the dynamic performance of shaft bearings — bearing loading during normal vessel operation and the identification of critical operating regimes.
- Torsional, lateral and axial vibration analysis of propulsion shafting.
- Modal analysis of the ship structure and resonance (Campbell-type) diagrams.
- Vibration mitigation: design changes versus operational measures.
- Onboard measurements with accelerometers, proximity probes and torque/strain instrumentation.
- Bearing loading and dynamic coefficients during operation.
Monitoring, diagnostics and machine learning
Turning measurements into an operating decision.
Recent work uses octave-band analysis of sound and vibration measurements with machine-learning models to predict journal-bearing performance and to identify the loading condition of a bearing experimentally. Other studies go beyond the classical Sommerfeld number with AI techniques to evaluate the performance of misaligned journal bearings.
For the monitoring of shafting and machinery the group works with data from proximity probes, accelerometers, strain gauges, LVDTs and shaft torque measurement, and with the concept of “marginally acceptable” shaft-line states: operating windows in which every bearing still carries an acceptable load, with an on-time alert when the system approaches the boundary.
Beyond the bearing itself, mixed-reality tools are being developed for the efficient design, monitoring and operation of marine systems, and the analysis of dynamic systems uses modal identification methods based on proper orthogonal decomposition (POD) of coupled structural–acoustic fields.
Smart lubricants and acoustics
Two lines of work that extend the group’s core themes.
Electrorheological and magnetic fluids
Electrorheological lubricants are based on an electrically insulating oil containing conductive particles that form chains when an electric field is applied, changing the rheological behaviour of the lubricant macroscopically and allowing the viscosity distribution in the film to be controlled. Design variables and performance indices have been optimised for such films, and magnetic-fluid-based hydrodynamic bearings with stochastic roughness are analysed with a generalised Reynolds-type equation.
Sound, vibration and acoustics
The group’s acoustic work began with standard-compliant procedures to calculate sound transmission loss (design of transmission rooms, numerical measurements) and the redistribution of low-frequency room modes by finite-element-based optimisation, and continues with proper-orthogonal-decomposition techniques for identifying and decoupling coupled structural–acoustic systems and optimising the acoustic quality of enclosed spaces.
Methods and tools
The same four ingredients appear in every project: physics-based modelling, numerical simulation, experiment and optimisation.