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Methods which allow for construction of flow feature aligned meshes in two- and three-dimensions have been developed in this thesis to investigate their potential for improvements in the numerical solution relative to globally refining the mesh. Of particular interest in the work is the generation of high-quality quadrilateral and hexahedral elements aligned with the dominant flow features. The two-dimensional techniques are applied on unstructured quad-dominant meshes, whilst the three-dimensional problems involve embedding high-quality hex-dominant mesh blocks into a hybrid volume mesh to improve their ability to capture anisotropic flow features such as shock waves, trailing shear layers/wakes and wing tip vortices. A method involving the medial axis has been studied to provide a geometric representation of two-dimensional flow features to allow feature-aligned meshes to be generated. Due to the flexibility of the approach, a range of complex features can be represented as simple geometric entities. These curves are embedded into the domain as virtual geometries to force alignment of unstructured quad-dominant surface mesh elements. The mesh locally mimics the attributes of a structured grid and provides high quality numerical solutions due to the alignment of the cell interfaces with the flow features. To improve the capability of hybrid meshes to resolve anisotropic flow physics, a method involving the extrusion of quad-dominant surface meshes has been developed. Surface meshes are extruded in the direction of extracted flow features, yielding feature-aligned semi-structured hex-dominant mesh blocks which can be embedded into the hybrid volume mesh. The presence of feature-aligned hexahedra has been shown to greatly enhance the resolution of anisotropic flow features compared with both isotropic and anisotropic tetrahedral elements, due to a significant reduction in numerical diffusion. Furthermore, improvements in the numerical solution have been also been obtained in a more efficient manner than isotropically refining the hybrid mesh. The results indicate that the type, orientation and size of the elements are significant contributing factors in the resolution of the dominant flow features.

Accelerating and decelerating turbulent channel flows are investigated to study the response of the turbulence dynamics. The objective of these investigations is to further enhance the understanding on the behaviour of turbulence and wall friction under transient conditions. Large-Eddy Simulations (LES) are carried out for step-like accelerating flows with significantly higher ratios of Reynolds number than previously covered. An experimental investigation is carried out for ramp-like accelerating flows using Particle-Image Velocimetry (PIV) and Constant-Temperature Anemometry (CTA) techniques to reproduce and validate the findings in numerical simulations. Step- and ramp-like decelerating flows are studied using Direct Numerical Simulations (DNS), the results of which are compared with observations in accelerating flows. Step-like high Re-ratio and ramp-like accelerating flows are shown to exhibit essentially the same three-stage laminar-turbulent transitional response as that described in He & Seddighi (J. Fluid Mech. 715:60-102, 2013), resembling bypass transition of boundary layer flows. The first stage is characterised by elongation and enhancement of streaks. The growing instabilities of the streak structures lead to breakdown and formation of isolated turbulent patches in the second stage, which grow in time and eventually merge with each other. The third stage is marked by the entire wall surface being covered by the newly generated turbulence. It is shown in the present study that the features of transition become more striking when the Re-ratio increases ― the elongated streaks in the pre-transitional period become increasingly longer and stronger, and the turbulent spots generated at the initial stage at the onset of transition become increasingly sparse. In a slower ramp-like flow excursion, on the other hand, the onset of transition is delayed making the flow development slower. In a step-like acceleration, a new boundary layer is formed instantly over the wall which develops into the flow with time. In a ramp-like case, however, the boundary layer development is shown to be described as an integral consequence of a continuous change of the flow. During the pre-transition stage, the time-development of the boundary layer in the step- and ramp-like accelerating flows bears strong resemblance to a time-developing laminar boundary layer described by the solution to Stokes’ first problem and can be represented by its analytical solution with a small correction. The streamwise fluctuation velocity profile in a high Re-ratio accelerating flow is shown to exhibit two peaks immediately following the onset of transition. A conditional sampling technique, based on a λ_2-criterion, is used to show that the two peaks are separate contributions of the active and inactive regions of turbulence generation. The peak closer to the wall is attributed to the ‘newly’ generated turbulence in the active region; while the peak farther from the wall is attributed to the enhanced streaks in the inactive region. Decelerating flows are shown to be also characterised by a time-developing boundary layer, similar to that in accelerating flows, bearing strong resemblance to the time-developing laminar boundary layer. The mean flow and wall friction in the early stages of the transient can be represented by the laminar analytical solution of the Stokes’ first problem. The streamwise fluctuations are shown to respond immediately following the commencement of the transient, while the response of the ‘real’ turbulence is shown to respond after a delay. Although the decay of turbulence and flow structures appear to be a gradual development herein, the decelerating flows may also undergo a transition process. However, the mechanism and stages of any such process are not clear in the present investigation. In addition, a brief investigation on the performance of the low-Reynolds number Launder-Sharma k-ε model in predicting unsteady turbulent flows is undertaken using different CFD codes. It is shown that the model performance itself is robust and insensitive to the numerical/coding framework, while slight changes in the formulation of the model have significant effect on the performance of the model.

The turbulent motions of buoyant plumes and momentum jets are ubiquitous in both the natural and built environments where a stable density stratification typically persists. The fluid mechanics of turbulent plumes and jets in stratified environments encompasses a fascinating and exigent field of research motivated by an extensive range of practical applications that include low-energy building ventilation, urban air quality management and the dispersion of pollutants in natural water bodies and the atmosphere. Merely as a consequence of vertical variations in the density of the ambient fluid, variations which often occur rapidly with depth in a shallow transitional layer (e.g. oceanic thermoclines and atmospheric inversions) separating two fluid masses, the fundamental behaviour of plumes and jets (in otherwise uniform environments) can be profoundly modified. This thesis investigates theoretically two intrinsic facets of turbulent plumes and jets in stratified environments. First, we examine the classic transport phenomenon of turbulent entrainment across a density interface driven by the localised impingement of a vertically-forced high-Reynolds-number jet. By developing theoretical models, we capture and elucidate the dominant physics at the heart of this complex physical process. Notably, the predictions of these models are in close agreement with existing experimental measurements. By unravelling the effects of confinement imposed by the physical boundaries of a box on the dynamics of interfacial entrainment, we highlight underlying physical reasons for the controversy surrounding the law governing the rate of entrainment across an interface. Second, we examine the time-dependent density stratification that develops in a confined environment following the activation of two turbulent plumes of unequal strengths. We show that the buoyancy of a bounded fluid layer can exceed, or overshoot, its steady value and that the plumes can induce a bulk overturning of the buoyant region. Finally, we discuss the wider context of this research and its application in engineering, the atmospheric sciences and oceanography.

Multi-fluid flows are omnipresent in our lives, from the fabrication of integrated circuit components in most electronics to the miniature laboratories inside medical tools, and even as a drop of rain splashes onto the wing of an aeroplane. In this thesis we use theoretical and numerical tools to investigate topics from the fascinating world of interfacial flows. The first part of this dissertation is dedicated to the study of multi-fluid systems in small scale channel geometries in the presence of electric fields. We develop the theoretical machinery to address the stabilisation (to the point of complete suppression) of the classical Rayleigh-Taylor instability under the action of an electric field acting in the plane of the liquid-liquid interface. In a related context, in many situations electric fields normal to the fluid-fluid interface may be employed in order to accurately drive instabilities towards beneficial goals. In particular, we discuss novel mechanisms to generate pumping and mixing in millimetre-sized geometries without requiring moving parts or an oncoming flow. In the second part of this thesis we turn our attention to the area of aerodynamics, thus investigating multi-fluid flows in a very different regime, dictated by high speed environments. We initially address one of the canonical problems in fluid mechanics, drop impact onto solid or liquid coated surfaces. This situation arises naturally on an aircraft in either rain or de-icing conditions. A new model for water catch on a surface is proposed, incorporating the violent splashing dynamics occurring in realistic conditions. The impingement of a large number of droplets ultimately leads to the formation of a liquid layer on the surface. We extend the powerful asymptotic framework of triple-deck theory to analyse changes in the flow separation process in the presence of an additional liquid. Flows past surface roughnesses and corners/flaps are discussed as practical examples.

The characterisation of cohesive powders for flowability is often required for reliable design and consistent operation of powder processes. This is commonly achieved by the unconfined compression test or shear test, but these techniques require a relatively large amount of powder and are limited to large pre-consolidation loads. There are a number of industrial cases where these tests are not applicable because small amounts of powders have to be handled and processed, such as filling and dosing of small quantities of powder in capsules and dispersion in dry powder inhalers. In other cases, the availability of testing powders could be a limiting issue. It has been shown by Hassanpour and Ghadiri (2007) that under certain circumstances, indentation on a cohesive powder bed by a blunt indenter can give a measure of the resistance to powder flow, which is related to flowability. However, the specification of the operation window in terms of sample size, penetration depth, indenter properties and strain rate has yet to be fully analysed. In the present work, the ball indentation process is analysed by numerical simulations using the Distinct Element Method (DEM). The flow resistance of the assembly, commonly termed hardness, is evaluated for a range of sample quantities and operation variables. It is shown that a minimum bed height of 20 particle diameters is required in order to achieve reliable measurements of hardness. A sensitivity analysis of indenter size reveals that small indenters with diameters less than 16 times the particle diameter exhibit fluctuations in powder flow stress measurements, which do not represent shear deformation. The penetration depth should be sufficiently large to cause notable bed shear deformation. It is found that this minimum penetration depth is approximately equal to 10% of the indenter radius. The hardness measurements are found to be independent of indenter stiffness within the wide range investigated. The friction between the indenter and the particles slightly increases the hardness, although its influence on the internal stresses is negligible. Cubic and cylindrical indenters measure significantly larger hardness value compared to the spherical indenter. Increasing the inter-particle friction and cohesion results in higher hardness values and internal stresses, due to the increase in resistance to shear deformation. Simulations at a range of indenter velocities confirm that the ball indentation technique can be used to analyse powder flowability over a wide range of shear rates.