No description available.

The design of slender structures such as longspan bridges, masts, offshore risers and cables is strongly influenced by their response behaviour when subjected to unsteady loads due to wind, waves and current. Simulation of the behaviour of a viscous flow past a structural cross section is of great importance to engineers concerned with the design of such structures. Offshore engineers are concerned with estimating the magnitude of structural forces induced by the most severe storm-induced wave events. Numerous studies have been conducted in an effort to estimate the structural forces induced by both regular and irregular waves. However, estimation of the maximum extreme wave-induced structural forces, particularly for relatively small diameter horizontal components, has received less attention. Since the most widely used method for estimating the force experienced by a bluff body subjected to wave loading is the empirical drag-inertia equation developed by Morison, O’ Brien, Johnson, and Schaaf (1950), it is important to determine whether this equation is adequate to describe the forces imposed by extremely large ocean waves. A method is presented for the simulation of incompressible viscous flow past acylinder using a stream function vorticity-transport formulation discretised on a cutcell quadtree mesh. A cut-cell technique is employed to provide accurate boundary representation and to facilitate the simulation of flow past a moving boundary. The finite volume discretisation consists of second-order accurate central difference approximations within uncut flow cells and a polynomial reconstruction technique within the cut-cells that are intersected by the solid boundary. Several preliminary validation tests concerned with flow past a circular cylinder are presented to confirm the accuracy of the numerical model. Firstly, the cut-cell discretisation is applied to the solution of the Euler equations and is shown to be almost second order accurate. Comparisons of wake geometry and force coefficients for steady and oscillatory flows at low Reynolds number are then made with existing results, and show satisfactory agreement. Preliminary tests are presented to assess the accuracy of a cut-cell based method for simulating flow past a circular body that moves across a background mesh. A series of experiments is also presented concerned with the measurement of theforce experienced by a circular cylinder undergoing a pre-defined two-dimensionalmotion within a still fluid. The cylinder trajectory is representative of the motionof a fluid particle beneath an idealised large ocean wave as defined by the NewWave formulation (Tromans et al. 1991). It is observed that, whilst the magnitude of high frequency vortex induced force fluctuations varies with the ratio of wave amplitude to cylinder diameter (A=D) and the wave spectrum shape, the overall shape of both x- and y-direction force time histories is very similar for all wave groups for which the underlying spectrum has the same shape. For all of the two-dimensional cylinder motions considered, the spectrum of both measured forces closely approximates the spectrum of uq (where u is a component of the velocity vector and q the absolute velocity) and, as a result, the vector form of the well known equation developed by Morison et al. (1950) is shown to provide a satisfactory estimate of the cartesian force components. The high frequency component of the force that is not captured by the Morison et al. equation is clearly identified as a lift-type force in the radial direction. For design purposes, a reasonable estimate of the magnitude of the peak force is obtained by neglecting inertial forces and employing a drag coefficient CD = 1.0.

No description available.

In this thesis the development of a general finite element model for free surface viscoelastic flow is presented. The model uses the weighted residual finite element technique in conjunction with the VOF (volume of fluid) method in Arbitrary Lagrangian–Eulerian (ALE) and Eulerian frameworks for tracking free surface boundaries. Application of the developed model to two different case studies and obtained results are discussed and the capabilities of the model are demonstrated. The first case study is based on the simulation of an injection moulding process. The second case study is based on the modelling of a de-vulcanisation process used for recycling of rubber products.

No description available.

The contact between dissimilar materials occurs in many machine elements where one of the contacting parts is manufactured from low modulus materials such as lip seals, o-rings, and metal on polymer prosthetic hip joints. Contacts of this sort of are often operated in the iso-viscous elastohydrodynamic lubrication (I-EHL) regime. Typically, they have been studied using a numerical approach due to lack of sensor of instrumentation for measuring oil film thickness. By developing the technology of sensors such as electromagnetic radiation and magnetic resistance techniques, the phenomenon of lubrication in I-EHL regime has shown results which are better understood. However, the experimental study that has been conducted to date is only appropriate for laboratory-based measurements. This thesis deals with the ultrasonic reflection methods to measure an oil film thickness and contact between dissimilar materials where these methods could be applied in-situ. This warrants special attention because there are two drawbacks of measuring of oil film thickness and contact by using bulk longitudinal wave between dissimilar materials (such as rubber and steel) which have mismatched acoustic impedance. One is the attenuation. The ultrasonic signal will be reduced when passed through the rubber. The other is accessibility. The wave must pass normally through the interface and so the transducer must be mounted on the rubber itself. There are two methods that can be used to measure oil film thickness using ultrasonic reflection: amplitude and phase shift. The amplitude method has been proved successfully for measuring oil film thickness between two similar materials and between two materials with little difference in acoustic impedance, but it fails for contact between two acoustically dissimilar materials. In this case, the phase shift method has the potential to measure oil film thickness. The results show that this method is valid for measuring thin films (h < 40 μm) for contact between Perspex and steel. The application of ultrasonic reflection techniques to measure the lubricant film thickness in iso-viscous elastohydrodynamic lubrication regime has been investigated. The reflection of ultrasonic pulses from the interface between the nitrile sphere and Perspex disk was recorded for a range of lubricated, dry, static, and dynamic contact conditions. In this way, profiles of oil film thickness were created for various loads and sliding speeds. The phenomenon of a wedge-shaped constriction in lubricant film was observed, especially at low speeds. It was also possible to observe cavitation effects on the signal in the exit region. The measured central film thickness results are compared with published models of the lubrication mechanism and experimental data obtained from optical methods. The result shows that the oil film thickness was measured in the region of 1 to 6 µm. The data was shown to be consistent with previous published experimental work using optical methods but somewhat lower than theoretical solutions. Ultrasonic surface waves that are commercially used for non-destructive evaluation (NDE) and damage detection have been also developed to measure contact between soft and hard materials. The measurements were made by using variable and fixed wedge transducers. The reflection coefficient of Rayleigh waves at point and line contacts was measured to study the characteristic of compliant contacts in dry and lubricated conditions. The results show that the increased load causes a decreased reflection coefficient. Therefore, the reflection coefficient of Rayleigh wave at interface between soft and hard materials can be developed as a sensor for o-ring and lip seals and this sensor could be positioned remotely from the contact.

The linear stability of the Giesekus and linear Phan-Thien Tanner (PTT) fluid models is investigated for a number of planar Poiseuille flows in single, double and triple layered configurations. The Giesekus and PTT models involve parameters that can be used to fit shear and extensional data, thus making them suitable for describing both polymer solutions and melts. The base flow is determined using a Chebyshev-tau method. The linear stability equations are also discretized using Chebyshev approximations to furnish a generalized eigenvalue problem which is then solved using the QZ-algorithm. The eigenspectra are shown to comprise of continuous parts and discrete parts. The theoretical and numerical results are validated for the Oldroyd-B model, which is a simplified case of the Giesekus and PTT models, by comparing with results in the literature. The continuous and discrete parts of the eigenspectra are determined using a purely numerical scheme to solve the discretized eigenvalue problem. The continuous spectra are then more accurately determined using a semi-analytical scheme which uses an analytical solution of the Orr-Sommerfeld equation alongside a numerical solution for the base flow. A comprehensive survey of the effect of each shear thinning and extensional fluid parameter is undertaken and an instability is found for particular parameter values for the Giesekus fluid. A preliminary investigation of this instability is undertaken whereby the unstable discrete eigenvalue is investigated using an Orthonormal Runge-Kutta scheme within a shooting method which uses the results from the Chebyshev-QZ scheme as a starting point. The linear PTT fluid is found to be stable to infinitesimal disturbances within the range of shear-thinning and extensional parameters considered. The computational e ciency and accuracy of the numerical methods are also investigated.