Rolling bearings are among the most widely used machine elements. As a result, reducing bearing frictional losses can have a significant impact on the overall energy efficiency of the vast majority of engineering systems. Given that rolling bearings are typically lubricated with grease, one of the most effective ways to improve bearing efficiency is through a suitable grease formulation. However, at present there is a lack of understanding of grease frictional behaviour in typical non-conformal, highly stressed bearing contacts. This limits the development of low-friction bearing greases, and thus further improvements in bearing efficiency. The current research work attempts to address this topic through an experimental investigation. Friction and film thickness tests were conducted on a wide selection of greases in ball-on-disc tribometers, under conditions broadly representative of rolling bearing contacts. Initial work was carried out with a series of commercially available greases, selected to cover a wide spectrum of thickener types, base oil types and viscosities, and practical bearing applications. These tests served to identify the most influential aspects of grease formulation on friction, and provide a reference line in terms of the current state of the art in low-friction bearing greases. Subsequently, the work was focussed on the study of a series of custom lithium-thickened greases. These were designed and manufactured with the purpose of isolating and investigating the effects of particular aspects of interest in grease formulation, including base oil type, base oil viscosity and the presence of friction modifiers. In addition, the performance of the corresponding bled and base oils was also studied to help shed some light on the prevailing mechanisms. The results show that two distinct operating regions, at high and low speeds, can be identified in the tribological performance of greases. At high speed greases behave approximately as their base oils. In this region, oil type was found to be the major parameter of influence on friction, with synthetic oils (particularly PAO) providing the lowest friction coefficients. In the same high speed region, the influence of base oil viscosity is that expected from a classical Stribeck relationship for oils, i.e. higher base oil viscosities in the grease formulation tend to produce higher friction as a result of thicker films. In contrast, at low speed grease behaviour is much more complex and is strongly influenced by both base oil and thickener properties. In this region the combination of lithium and PAO was shown to provide both thick films and low friction. No benefit in friction reduction was observed when a friction modifier, namely oleic acid, was added to the formulation of the greases. The influence of mechanical degradation of greases on their tribological performance was also evaluated. It was established that mechanical degradation has the effect of diminishing the friction and film thickness benefits provided by some of the greases at low speeds. A remarkable exception was found in the tribological behaviour of greases formulated with lithium and PAO oil, which was overall practically unaffected by mechanical degradation. Finally, based on the observed results, the combination of a low viscosity PAO base oil and lithium thickener was suggested as the most suitable formulation for a low-friction rolling bearing grease.

The research of this thesis focuses on the study of sprays produced by twin fluid air-blast atomisers and the main objective is to study the liquid jet break-up mechanism and relate it to the downstream spray characteristics. Two different air-blast atomiser geometries are used; coaxial, where the liquid co-flows with the gas stream, and the liquid jet in a gaseous cross-flow. The thesis describes advanced and novel measurements to reveal the temporal and spatial development of the liquid flow and its interaction with the surrounding gas stream. Initially, the break-up process is studied by measuring the characteristics of the continuous liquid jet. Techniques such as electrical conductivity, high-speed shadowgraphy and optical connectivity were used to characterise the atomisation process. The latter is a novel laser-based technique used to illuminate internally the continuous liquid column by introducing a laser beam within the liquid nozzle, while a fluorescent dye in the liquid ensures that the whole volume of the liquid is visualised. The laser light propagates downstream while reflecting on the gas-liquid interface to be interrupted at the break-up position, where the light is scattered and diffuses widely. In the case of a jet in a cross-flow gas stream the fluorescent intensity images were recorded from two different angles to reveal the various features involved in the liquid jet structure. The study of the spatial and temporal characteristics of the instabilities and the developed surface waves on the liquid column can provide information on jet morphology and a better understanding of the physics that elicit the break-up phenomenon. For that purpose, Proper Orthogonal Decomposition (POD) is applied to reveal the various flow scales and elucidate the mechanism of transfer of momentum from the gas to the liquid flow. The most energetic modes are used to describe the jet interface dynamics that may well define the formation of the downstream droplet sizes. Interferometric Laser Imaging for Droplet Sizing (ILIDS) was also used for planar measurements of droplet sizes and velocities. ILIDS images the scattered light from droplets in an out-of-focus mode at different streamwise distances from the nozzle exit to obtain interference fringe patterns associated with each droplet. The spacing of each fringe pattern is proportional to the corresponding droplet diameter. Instantaneous droplet clustering is measured along with the primary atomisation process and the liquid jet break-up characteristics are correlated with the downstream droplet sizes. Several time delays are used between optical connectivity and ILIDS measurements to capture the various classes of droplet sizes that travel with different velocities from the break-up region to the downstream spray location. The small droplets travel faster and move with a velocity similar to the gas flow, in contrast to the larger droplets, which are conveyed to the size measuring region with a lower velocity and, therefore, higher time delays. A conditional correlation method was developed to reduce statistical uncertainties. Negative correlations were found between the break-up length of the liquid jet and downstream number of droplets, indicating that they are inversely proportional. The relation between the break-up length and droplet Arithmetic and Sauter Mean Diameters seems to be more complex since a sinusoidal relation was extracted. The estimated correlation coefficients varied with time delay and a repeatable trend was observed which exposed the coherent behaviour of the break-up process and its frequency, revealing that it is not a random phenomenon, but rather a multifaceted mechanism governed by physical laws.

The present doctoral work investigates wake flow behind a square-back Ahmed body at laminar and turbulent Reynolds numbers. At low Reynolds numbers the Ahmed body wake undergoes a series of bifurcations towards symmetry breaking state. Using Large Eddy Simulations, which resolve the boundary layer down to the wall, the sequence of symmetry breaking bifurcations is captured numerically for the first time. Large Eddy Simulations are then used to investigate the reflectional symmetry breaking behaviour in the turbulent regime, where it manifests as slow and random bi-modal switching between the asymmetric states. The bi-modal switch from one state to the other is captured for the first time numerically, allowing to study the effects of wake displacement on the instantaneous wake structures. A linear feedback control strategy, designed to attenuate base pressure force fluctuations is tested on the Ahmed body flow. At low Reynolds numbers the strategy is successful; a moderate base pressure recovery and a concomitant drag reduction are achieved and the wake is re-symmetrised. A similar control approach is also benchmarked at higher Reynolds numbers, where the flow exhibits turbulent separation. The preliminary results suggest that the controller is unlikely to reduce drag and other actuation/sensing arrangements need to be investigated. This work motivates future attempts to develop a linear feedback controller based on the strategy described herein, but with better selected actuation and sensing. Further work is also required to better understand the physical mechanism behind bi-modal switching in the wake.

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