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The experiments detailed within this thesis have measured the distortion of the superfluid energy gap in high magnetic field and the dissipation for an object in uniform linear motion through superfluid 3He-B. The latter experiments led to an astonishing discovery of no discontinuity in the dissipation for an object in uniform linear motion at the Landau critical velocity. The experiments were performed in a “Lancaster style” nested experimental cell at ultra-low temperatures within the ballistic limit. In the first set of experiments we studied two almost identical quartz tuning fork resonators with different vibration directions with respect to the vertical magnetic field. One vibrated along the field direction and the other vibrated in the horizontal plane. Our measurements have shown that the critical velocity for the vertical fork decreases significantly with increasing field, dropping to almost 60% of its original value as the highest field is approached. However, there is very little change of the critical velocity for the horizontal fork. Our data shows good agreement with theoretical predictions and previous experiments using vibrating wires. During measurements at high magnetic fields, 300 mT to 330 mT, we observed discontinuities in the velocity response for very small changes in driving force. This behaviour might be due to vortex generation around the vibrating object and a subsequent shielding effect, previously observed in the response of a large vibrating wire (with diameter of 100 µm, similar to a typical fork dimension). Intriguingly the detailed behaviour also appears to depend on the orientation of the tuning fork with respect to the magnetic field direction. The second set of experiments used a novel measurement tool referred to as the “flopper”. The idea behind the development of the flopper was to have a low frequency device with low Q-factor that could be moved in an arbitrary fashion. The flopper is a large 25 × 9 mm goalpost-shaped NbTi vibrating wire. With AC current the wire can be driven at its resonance frequency or by using a DC linear stroke it can be moved over a controlled distance within the cell. By adding a high frequency “probe” signal on top of the DC signal we calibrated the position of the flopper with respect to the cell. We performed DC strokes of the flopper over short distances within the cell at various velocities. This led to us discovering that the dissipation of uniform linear motion at velocities exceeding the Landau velocity did not show any discontinuity. Since the critical Landau velocity is so fundamental in the understanding of superfluidity, this was a considerable surprise. The comparisons between AC and DC motion led to the development of a model to describe the dissipation processes in 3He-B.

The review of current technologies for measurement of gas velocity in stack flow applications is undertaken and it is shown that the ultrasonic time-of-flight method is the most suitable and offers a number of advantages over alternatives. Weakness of current piezoelectric based transducers are identified as the inability to operate at temperatures above 400 °C due to limitation of piezoelectric materials used, and a case for development of an alternative high temperature material is put forward. A novel and highly enhanced, lead free piezoelectric material, suitable for continuous operation at temperatures above 400 °C has been engineered for ultrasonic gas velocity sensor applications. Structural modification of pure bismuth titanate (Bi4Ti3O12) or BIT compound, through multi-doping at the Ti-site, has been found to enhance piezoelectric properties accompanied with a mild reduction in Curie temperature, Tc. Initially, compounds doped with tungsten and chromium were found to increase the piezoelectric coefficient (d33) from around 5 pC N¯¹ in pure bismuth titanate, to above 20 pC N¯¹ in doped compounds. This increase is attributed to lower conductivity and improved poling conditions. Further increases in d33 (up to 35 pC N¯¹) were then realised through controlled grain growth and reduction in conductivity for niobium, tantalum and antimony doped compounds. The Curie temperature of the material with best properties is found to be 667 °C, which is a slight reduction from 675 °C for pure bismuth titanate ceramic. The enhancements in modified bismuth titanate achieved in present work allow the material to be considered as suitable for high temperature ultrasonic transducer applications. Integration of bismuth titanate material into a working high temperature transducer is then considered and the investigation of suitable, high temperature bonding method is undertaken. It is shown that reactivity of bismuth titanate with the titanium based fillers makes brazing unsuitable as a bonding method between piezo-ceramics and stainless steel. A novel assembly method, using liquid gallium as an electrically conductive bond, and a mechanical restraint for the piezo actuator is then presented as an alternative with the potential to reduce the negative effects of differences in thermal expansion coefficients between constituents of the transducer assembly.

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The aims of this project were the implementation of Computational Fluid Dynamics (CFD) to the study of supersonic ejectors, and the investigation of the flow processes that occur. The conventional ejector has been in existence for more than a century yet the design has remained largely unchanged and is difficult to optimise. This has been attributed to a lack of understanding of the complex flow processes and phenomena that occur. CFD provides the ability to study these processes, and to rapidly assess geometrical influence upon operational performance. The CFD model was assessed through systematic appraisal of the numerical parameters that influence solution stability and simulation accuracy. Two proprietary CFD codes were utilised; a structured segregated code and an adaptive mesh coupled code. Assessed parameters included; mesh dependency, discretisation schemes, turbulence models, and boundary layer models that are shown highly influential. Simulation was validated through comparison of predicted and experimental entrainment values. Simulations of an ejector that is part of a steam-jet refrigeration cycle were used to assess the influence of geometry and operating conditions. The structured code was found suitable for geometrical studies however the coupled code was required for detailed flow analysis. Geometrical studies showed current ejector design guidelines to be well set. Operational studies highlighted the dominant influence of motive fluid flow rate upon entrainment levels. Shock systems and flow processes could be clearly identified. Simulations of ejectors utilised in vacuum and thrust augmenting applications were also conducted in assessment of the general applicability of CFD. CFD has the potential to be an effective and powerful tool III simulating and understanding ejectors. Qualitative and quantitative results can be obtained dependent upon the optimisation and validation of the mathematical model. This however can only be performed properly if the user fully understands the t10w physics and applied numerics.

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