Distributions of primary velocity and boundary shear stress were measured in a wide closed duct for a range of rectangular and asymmetric compound sections. At the interface between the shallow and deep subsections, the compound geometries included three different wall slopes, vertical, 1:1 and 1:2 (vertical:horizontal). The results will provide a data base for mathematical modelling, especially relevant to the field of single and two-stage open channel flows. The analysis is largely presented as the relationship between selected flow variables and two geometric parameters, relative depth H-h/H for the compound geometry and aspect ratio b/h for the rectangular. The flow variables include the measured distributions, and distributions of depth mean velocity, momentum and kinetic energy flux, apparent shear stress, local friction factor and lateral eddy viscosity. Also included are momentum and kinetic energy coefficients, cross-section friction factors and the width of lateral shear layers. In the compound section, the very detailed measurements indicate the presence of secondary flow cells for which there is little previous evidence. Secondary flow is discussed in relation to apparent shear stress and shear layer widths, and on the flood plain is shown to significantly influence the flow field beyond the range indicated by the primary velocity and bed shear stress distributions. For the rectangular geometry, new empirical equations are presented relating per cent shear force on the walls to aspect ratio.

The flow dynamics of a rectangular cavity with a thick incoming boundary layer at low Mach numbers is investigated by Large Eddy Simulation (LES) and parallel CFD, as a simplified model of automobile bodywork recesses. The cavity inflow is generated by means of synthetic stochastic time-dependent methods in a precursor simulation, in order to identify and analyze quantitatively the streaks in the spatially developing boundary layer approaching the cavity. In the cavity flow model, no self-sustained oscillation is found, due to the high value of the boundary layer thickness. The influence of the approaching boundary layer turbulent scales on the cavity instabilities is examined. The intermittent cavity flow behaviour is related to the injection and ejection of vortex structures across the cavity opening and downstream edge. The space and time resolution of the LES enables to identify the flow dynamics of vortical instabilities and of the three-dimensional structures in the cavity shear layer. Cavity noise sources are identified by correlation and spectral analysis. In the upstream region of the cavity, the streaks break down into smaller and less coherent structures, as shown by the reduction of the integral length scale. In the rearmost region of the cavity, a spanwise negative velocity correlation is interpreted as a dipole-type noise source, which is likely to reduce the radiated noise level with respect to a two-dimensional cavity flow. The velocity spectra show broadband amplification of modes related to the dominant scales in the cavity, as opposed to the selective mode amplification of cavities with a thin boundary layer inflow. A novelmultivariate non-dimensional analysis of the CFD parameters is presented, that explicits the modelling process for a cavity flow test case. This is used for estimating the simulation cost and the spatial and temporal resolution trade-off in the cavity flow simulation.

Compressible fluid flows governed by the Euler Equations of gasdynamics exhibit sharp features such as shocks and expansions which are difficult to capture computationally. In order to focus efficiently on these features an adaptive facility is required. A standard numerical scheme is the so-called Lagrange-plus-remap method, otherwise known as the ALE (Arbitrary Lagrangian Eulerian) scheme, on a refined mesh using AMR (Adaptive Mesh Refinement). This thesis focuses on the Lagrangian step in the ALE scheme and on an adaptive strategy based on error estimates. The strategy makes use of truncation errors of pairs of schemes of the same order of accuracy to monitor the error and to refine the mesh. Error estimates are obtained for the Isothermal and Euler Equations in both one and two dimensions. The strategy is shown to be effective in refining areas of compressibility.

This thesis describes experiments conducted on 3He B-phase in the μK temperature regime designed to thermally detect the decay of quantum turbulence. A vibrating wire grid and vibrating wire resonator housed inside a black body radiator (BBR) are driven to high velocities so as to produce both turbulence and thermal quasiparticles originating from pair-breaking of the Cooper pairs. Careful calibration of the experimental parameters allows a direct quantitative measurement of the power leaving the radiator. Measurements reveal that at high grid and wire velocities, an additional, slowly dissipating, mechanism is present which cannot be accounted for by the simple thermal recovery of the radiator from quasiparticle heating. This mechanism is attributed to a slowly-decaying quantum turbulent tangle in the radiator. Previous experiments concerned with the decay of quantum turbulence relied on measurements of an inferred vortex line density. The current experiment directly measures the power released as a quantum turbulent tangle decays and so offers a particularly direct test of the current decay models. The measurements obtained from the current experiment are compared directly with the standard model for the decay of classical turbulence involving a cascade of energy across an inertial range from large to small length scales wherein the energy is dissipated. The results are found to fit the model remarkably well and a Kolmogorov constant is extracted from the fits which is very similar to values attributed to classical fluids . This is despite the fact that the absence of a normal fluid component in the experimental regime ensures that the classical viscous dissipation mechanism is impossible.

The work presented here is concerned with the prediction of naturally occurring transition under the influence of pressure gradients and free stream turbulence, and with the effects of two and three dimensional tripping devices. Measurements have been made in zero, favourable and adverse pressure gradients. A number of transition prediction methods have been published that attempt to account for the effects of stream turbulence and pressure gradients, but they do not, in general, give similar results under particular conditions. They have been compared with each other and with some of the available experimental results, including some collected here, in an attempt to check on their limitations. In flows with low stream turbulence, it is apparently necessary to take account of the history of the laminar boundary layer's development up to transition. The method of Granville appears to be the most successful. In flows with high stream turbulence there is insufficient experimental information to indicate the best way 01 predicting transition, particularly in favourable pressure gradients. It is possible that a method using purely local criteria, rather than one assessing the boundary layer's history, is acceptable. The present measurements of the characteristics of two dimensional tripping devices (wires) have been concerned largely with measuring drag coefficients and the size of the separation bubble behind the wire. The drag coefficient of the wire varies continuously with changes in only one of the two governing non-dimensional variables. Its value is generally lower than has been assumed previously. The separation bubble can be very large and varies rapidly in size with changes in the two governing non-dimensional variables. Pressure gradients have little effect upon drag coefficients but some effect upon separation bubble sizes. Consideration has also been given to the problem of critical sizes for trip wires. The previous suggestion of a constant value of Ul:/v of 826 for I transition at the wire I, in zero pressure gradient appears to be valid only at high Reynolds numbers, the value decreasing as the Reynolds number falls. Further information has also been obtained on the effect of pressure gradients upon critical wire sizes. Measurements of the characteristics of three dimensional tripping devices (spheres) have given the critical sizes for 'transition at the element', and the movement of transition behind the element. The start of transition moves instantaneously to the element once its critical size is reached, but the end of transition approaches the element at a finite rate and progressively more gradually, until it stops a finite distance from the element. Critical element sizes for movement of the start and end of transition to the element are different. Pressure gradients affect both critical element sizes and the rate of approach of transition to the element.

Turbulence plays a large role in our everyday experiences, however, due to its enormous complexity, it is not fully understood. The hydrodynamic description of turbulence is greatly simplified by having zero viscosity, a property unique to superfluids. At sufficiently low temperatures, liquid helium is in a pure superfluid state providing us with an ideal medium to investigate turbulence. This thesis details experiments studying quantum turbulence using vibrating resonators in the B-phase of superfluid 3He below O.2Tc. At such low temperatures, there is a small number of ambient thermal quasi particle excitations which are highly ballistic, with intrinsic mean free paths approaching kilometre length scales. These few remaining quasi particles provide an ideal, non-invasive method to probe vortices and quantum turbulence in superfluid 3He-B via a process known as Andreev reflection. A macroscopic object moving through a superfluid will nucleate vorticity and create excitations once the Landau critical velocity is exceeded. In superfluid 3He, the Landau critical velocity is 27 mm/s at saturated vapour pressure. Both vorticity nucleation and the creation of excitations results in the increased damping of a vibrating object. We present damping measurements on various vibrating resonators in Superfluid 3He-B below O.2Tc and discuss the interplay between pair-breaking and vortex production, in contrast to superfluid 4He. We present evidence suggesting that in 3He-B, vortex production and pair-breaking are linked and that the onset of excess damping due to vortex production of a vibrating structure may be suppressed by submerging the resonator in vorticity produced by one of its neighbours. We investigate quantum turbulence generated by a vibrating grid resonator. The vortex line density and spatial dependence of quantum turbulence is probed by neighbouring vibrating wire resonators operating at low velocities. At low velocities, the grid emits ballistic vortex rings. As the grid velocity increases, the ring density increases, leading to the formation of a vortex tangle from the cascade of vortex ring collisions and reconnections. Using vortex line density fluctuations, we observe a 1-5/3 dependence in the power spectral density; a signature of the Kolmogorov spectrum in classical turbulence. At higher frequencies, the spectra show a 1-8 dependence, pointing to an extra dissipation mechanism. Using a beam of thermal ballistic quasiparticles and bolometric techniques, we investigated the transmission of excitations through a turbulent flow field in order to measure the absolute vortex line density. We have observed the Andreev reflection of the quasiparticle beam as vortex rings and the subsequent vortex tangle crosses the radiator's line-of-sight. From these measurements, we can estimate the propagation speed of the vortex tangle and infer the vortex line density.

The Equations of State (EoSs) ranging from cubic equations to theoretically-based molecular models are of great practical application in the modeling of physical properties of real fluids In particular, the molecular EoSs based on statistical mechanics and supported by molecular simulations are of special interest due to their ability to represent the fluid properties using only a few substance-dependent parameters with physical meaning, in principle, statistical mechanics provide theories for calculating the thermodynamics and structure of a fluid given its Intermolecular potential function. The focus in this study is on Perturbation theories for chain molecules whose reference system constitutes non-bonded segments.

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No description available.

The deceleration of flowing fluids, or diffusion as it is widely known, is a process of primary importance, whereby some of the kinetic energy is converted into static pressure rise. Hence, in the case of subsonic flows, the area of a diffusing passage increases in the direction of the flow. Most commonly used diffuser configurations are: conical, annular, rectangular and radial. The rectangular shape is used widely in vaned diffuser systems for centrifugal compressors. In general, fluid motion in diffusing passages takes place against an adverse pressure gradient, and unless great care is taken in the design of these passages, the growth of boundary layer and its ultimate separation may seriously upset the performance of the diffuser system. From the point of view of the fundamental physics of fluid motion, the study of diffusing flows is indeed very interesting. Therefore, it has attracted the attention of many investigators, and a great deal of literature has been published on the performance of commonly used diffuser configurations. However, the available performance prediction techniques are either far from being reliable, or they are limited to specific geometry and fluid conditions. In the case of rectangular diffusers, there are other aspects which do not appear to have been fully investigated, for example: (i) Interdependence of the geometrical parameters, operating conditions and the performance of rectangular diffusers on the basis of a suitable theoretical model. (ii) Correlation between the principal dimensions of the compressor and the geometry of the diffuser channels. The aim of this research was to study these aspects. Full description of the aims is given in Chapter 1. Chapter 2 gives a comprehensive survey of the published literature and a statement of the justification of the aims. Chapters 3,4 and 5 describe the theoretical development for a parametric study, flow analysis and performance analysis respectively. The results of the theoretical studies and the comparisons between the predicted and experimental performance data are discussed in Chapter 6. The conclusions and suggestions for further work are given in Chapter 7. The performance prediction method and the correlation between the relevant dimensions of the compressor and the diffuser geometry are believed to be novel. It is submitted that they should make a significant contribution to the design methodology for centrifugal compressors in general, and for two-dimensional diffusers in particular.