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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Improve the understanding of uncertainties in numerical analysis of moored floating wave energy converters

Vickers, Andrew William January 2012 (has links)
The wave energy industry, still in its infancy compared to similar activities offshore, must look to the oil and gas industry for guide lines on design criteria for survival, safety and operational optimisation for installations at sea. Numerical analysis tools for prediction of the response of floating moored structures have become an important part of the design task for the offshore industry offering a low cost and low risk option compared to scale tank testing. However, rather than having only a task of station keeping and survival, the moorings for a wave energy converters (WECs) would also be required to provide the ability of not adversely affecting the power capture task. The main aim of this work is to gain an understanding and reduce the uncertainties in the numerical modelling of WECs. Experimental work designed and performed under the HydraLab III project of which the author was a member were used to evaluate the response characteristics of a 1:20 scale “generic WEC” device with a 3 point mooring system. The investigation was enhanced through further tests implemented by the author at Heriot-Watt wave tank using a single WEC device. The outcomes from these experiments were used to aid in the implementation of the aim identified above. Two numerical model categories were set up to understand the uncertainties apparent to the mooring simulations. The first category included only the calculation of the mooring line response using experimental data to inform the motion of the floating body. The second category included the motion response of the floating body coupling the complex behaviour to the moored system. The mooring tension results for the first category shows an error between the numerical prediction and the experimental results up to 16 times that of the experimental value. This was mainly during slack conditions where the mooring line tension was lower than the pretension in the line at still water. During the higher tension events the average error was 26%. For the second category it was found that the numerical predictions of the WEC motion response in six degree of freedom (6DOF) were generally over predicted. The tension predictions for the coupled simulations identified an error of between 1.4 and 4.5%. The work presented here contributed to the understanding of uncertainties in numerical simu- lations for WEC mooring designs. The disparity between the simulation and experimental results re-enforced the requirement for a better understanding of highly dynamic responding moored cou- pled systems. From this work it is clear that the numerical models used to approximate the response of moored WECs could provide a good first design step. Whilst this work contributed to the understanding of uncertainties and consequently reduced some of these, further work is rec- ommended in chapter 6 to investigate the definition of some of the mechanical and hydrodynamic properties of the mooring line. It is also suggested that external functions should be included 2 that would allow to model the coupled effect of Power-Take-Off (PTO) system. It is intended to conduct future work deriving a fully dynamic mooring simulation including the effects of PTO.

The coalessence of droplets in settlers

Ali, F. A. January 1969 (has links)
No description available.

The application of optimising control to an hydraulic drive

Steel, G. K. January 1968 (has links)
No description available.

Nonlinear dynamics and rolling-sliding wear in a free floating cam follower with seven degrees-of-freedom

Stewart, Donald January 2006 (has links)
This thesis considers the dynamics of a free floating cam follower in a hydraulic motor.  The follower consists of a rod which forms a metal-to-metal seal against a rotor and is subject to rolling-sliding wear. A mathematical model was constructed to study the nonlinear dynamics and wear of the rod.  In particular, two wear phenomena frequently observed in the field were of interest, namely a barrelling type of wear in which the rods would suffer significantly higher wear towards the ends than in the middle, and a polygonisation type of wear in which the rod would be worn to a generally hexagonal, pentagonal or square profile. Initially a 3 degree-of-freedom planar model was constructed.  An analysis of the friction coefficients of the rotor and stator was carried out, and revealed the importance of maintaining a lower coefficient of friction on the stator than on the rotor in order to ensure that the rod continued to roll on the rotor. This was followed by a 7-degree-of-freedom modelling 3 dimensions.  In this model the rod is modelled as two shorter rods connected at a pivot point with rotational stiffness located at the centre of mass of the real rod.  CFD analysis was used to model the fluid forces acting on the rod. A wear rig was constructed to carry out wear experiments with differing rolling:sliding ratios.  The outputs from these experiments were used to calibrate an Archard-type wear model in the mathematical model.  Wear patterns could then be iteratively fed back in to the mathematical model and the evolution of the wear pattern studied.  The results of this approach are contrasted with models where wear is only calculated at the end of a simulation, and demonstrate that it is necessary to iteratively feed back wear into the geometry of the model to correctly model wear over a long period.  Mechanisms for both polygonisation and barrelling wear are shown to exist from the dynamics of the 7 degree-of-freedom model.

The use of hoses and hose inserts to reduce pressure ripple in hydraulic circuits

Way, T. M. January 2004 (has links)
No description available.

A double-acting hydraulic ram pump for deep-well water pumping

Law, Thomas Robert January 2015 (has links)
Many existing deep-well water pumping technologies that are affordable to developing world smallholders suffer from reliability issues, low efficiency and/or expensive running costs. The Double-Acting Hydraulic Ram Pump (DAHR) has the potential to address these problems. An evolution of the classic hydraulic ram pump that converts kinetic energy from free-flowing water sources into a reduced flow at a much higher head, the DAHR contains virtually no moving parts or dynamic seals. The result is a deep-lift technology that can be both highly efficient and extremely reliable. This thesis investigates the potential of the DAHR, taking an initial proof-of-concept rig to a design that is ready for outdoor field trials. The beta prototype design process has been aided considerably by the development of a suite of numerical models. Like the conventional hydraulic ram, the DAHR has clearly defined acceleration and delivery phases either side of an impact event. The fluid motion during acceleration is modelled using a 1D lumped element approach whereas the delivery phase modelling is based on the shock equations for both compression and rarefaction waves. Unlike the conventional hydraulic ram, the DAHR makes full use of the kinetic energy downstream of the impact. The numerical results facilitate the selection of drive pipe diameter, inlet/delivery valves and the choice of pipe material via the resulting sound speed. A 15 m tall test facility housed within a three storey public stairwell was set up to help simulate pumping from deep underground. The DAHR sitting in a tank at the bottom would lift water to the top of the stairwell before it was returned under gravity to complete the circuit. The driving power input to create the low pressure, high volume oscillatory flow within the DAHR U-tube is provided by two custom-made pneumatic fluidynes. The data acquired over several weeks of testing with three different plastic drive pipe materials required an automated post-processing routine capable of analysing DAHR performance impact-by-impact. Computed efficiencies of up to 75 percent were achieved while pumping 350 L/h at 32 m head. Experimental observations also showed good agreement with numerical modelling. A single-acting design, capable of fitting down a smaller borehole, was considered as an alternative way forward. A further prototype, with the second drive pipe replaced by a gas spring, was designed, built and tested in the same facility. The prototype proved substantially more difficult to control and estimated efficiency was approximately half that of the DAHR validating the original double-acting approach.

High temperature embedded electrical machines for aerospace turbine applications

Rodrigues, Leon January 2013 (has links)
This thesis describes research contributions in the field of electrical machines for operation at elevated temperatures. High temperature operation of electrical machines is considered critical for the realisation of the 'more-electric aircraft' concept, which involves electrical machines embedded directly on to the shafts of the aircraft gas turbine. The particular machine of interest for this thesis is a switched reluctance machine for operation on the high pressure shaft. The hostile environment, mainly due to the high temperatures (~350°C ambient) introduces several challenges in the modelling, design and manufacture of electrical machines. In order to aid selection of materials and collect necessary data for the machine design, detailed analysis of the published magnetic and electrical data for key materials at high temperatures has been carried out. Further measurements on the high strength 50% Cobalt Iron materials were also conducted, which supplement the understanding of the materials behaviour at high temperatures, specifically in terms of the effects of the long term thermal ageing on the individual loss mechanisms in the material. The design optimisation of an SR machine for 350°C operation is also described in detail. The design procedure illustrates how the high temperature material properties influence machine performance and achievable power densities. In order to more reliably predict the performance of machines at elevated temperatures several modelling techniques have been developed. A method to calculate instantaneous core loss was introduced, which was formulated such that it could be used in circuit simulations to ensure power balance. Extensive validation of this model has also been carried out.

Experimental analysis of pressure variation, flow conditions and component losses in a single stage centrifugal pump fitted with fixed guide vanes

Grassie, J. C. January 1945 (has links)
No description available.

The design of hydraulic servos using optimization techniques

Armstrong, P. J. January 1971 (has links)
No description available.

Nonlinear hydrodynamic modelling of an oscillating wave surge converter

Crooks, David Joel January 2017 (has links)
This study develops further the conceptual model of Oscillating Wave Surge Converter (OWSC) hydrodynamics and improves the representation of hydrodynamic torques in the device's nonlinear time domain numerical model. Wave excitation torque tests were used to evaluate how the wave excitation torque experienced by a static OWSC varies with wave period and amplitude for a range of OWSC pitch angles. Forced oscillation tests were used to evaluate how the radiation torque experienced by an OWSC, due to its motion through still water, varies with oscillation period, oscillation amplitude and angular velocity. A third set of experiments, in which the restoring moment of buoyancy was augmented with mechanical springs, referred to as buoyancy simulation tests, demonstrated how modifications to the static torque of the system influences OWSC motion, The experiments validated and identified the limits of linear hydrodynamic coefficients obtained using Boundary Element Method (BEM) codes. Numerical methods were evaluated on their ability to improve the representation of the measured torques not estimated by the BEM codes. The wave excitation and buoyancy simulation tests were performed in 4.5m and 16m wide wave tanks at Queen's University Belfast (QUB). This presented the unique opportunity to observe how wave tank characteristics can influence experimental measurements. The buoyancy simulation tests also calibrated and validated an enhanced nonlinear time domain numerical model. The enhanced nonlinear time domain numerical model contained the numerical methods that were found to improve the estimates of the wave excitation and radiation torques measured during the wave excitation torque and buoyancy simulation tests. The enhanced nonlinear time domain numerical model contained a pitch angle dependent wave excitation torque, a nonlinear state-space approach to modelling radiation torque, an empirically developed drag torque term and an analytically derived nonlinear hydrostatic torque.

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