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With ever growing levels of urbanisation across the globe, a good understanding of canopy flows is paramount to reduce pollution in major cities and prevent unwanted aerodynamic loading on structures. The multi-scale nature of not only urban construction but that of natural environments requires a more complex modelling system be employed. Fractal geometries have only recently been investigated in turbulent flows, their multi-scale properties make them the logical choice for modelling and simulating flows involving such complex geometries. Additionally, in recent years the usage of Lattice Boltzmann Methods (LBM) vs Computational Fluid Dynamics (CFD) has increased, since LBM offers better computational efficiency and speed over CFD. However, the shortcomings of LBM still need to be benchmarked since macroscopic quantities of the flow are extracted using a probabilistic model of the flow at microscopic scales. A plan to investigate turbulent flows over a fractal and non-fractal obstacles has been presented by implementing a LBM numerical analysis over a range of Reynolds numbers (100-49410). The suitability of LBM's multiple dynamics models including: Bhatnagar Gross Krook (BGK), Multiple Relaxation Time (MRT) and Regularised Lattice Boltzmann (RLB) have been studied for high reynolds number cases. Results from LBM cases were compared to available experimental data and published literature, although, results of fractal cases were not mesh independent compelling agreement between all three tested obstacles show a significant validation of LBM as tool to investigate high Reynolds number flows.

Civil Engineering and fluid mechanics are two fields that are not usually put into the same category. In reality however, many of the problems faced in Civil Engineering are caused by fluids. For example: the mixing of pollutants; the scouring of riverbanks; and the undesired oscillations of bridges and buildings caused by wind. Whilst it is possible to simulate the interactions of fluids and structures, it is computationally expensive and as a consequence experiments are needed to validate simplified models. Undertaking these experiments is not trivial, and unavoidably, the data collected can contain outlier / anomalous data points. There have been many attempts to create algorithms to automatically remove and or replace these outliers, the most effective or which are based on modal decomposition techniques. In fluid mechanics there are two prominent modal decomposition techniques. These are Proper Orthogonal Decomposition (POD), which can be use to determine modes that are spatially independent, and Dynamic Mode Decomposition (DMD) which can be used to determine modes that are temporally independent. The majority of the previous POD and DMD applications have been have been applied to mechanical and aerospace engineering problems. However, the focus of this thesis is the application of modal decomposition techniques solely in Civil Engineering. First, the modal decomposition technique POD, is used to create a novel computationally efficient method for filtering spurious points from experi- mental data. Second a method of combining POD and DMD to attain regions of spatial and temporal independence is proposed. Third, these methods are applied to a river groyne problem to explain the spatio-temporal mechanisms leading to the sudden expansion of a mixing layer. Finally, these methods are applied to a group of multi-scale square cylinders, resembling the layout of a city, to describe the spatio-temporal behaviour of the wake. This thesis creates a suite of tools which can be applied by Civil Engineers to understand complex mechanisms, for instance, in environmental fluid mechanics.

Previous studies characterising horseshoe vortices upstream of circular cylinders in open channels have focused on changes in the Reynolds number. This study investigates the effect of the Froude number and other flow and geometrical parameters on the nature of the horseshoe vortex system that develops in front of a circular cylinder. The results show horseshoe vortex dynamics dominated by cylinder diameter, Froude number and flow depth. Instantaneous vorticity fields were classified into either two or three groups of vortex formations based on their turbulence stress magnitudes with a high dependence shown upon the Froude number and cylinder diameter. The peak stress properties of the horseshoe vortex system was found to be controlled within a certain threshold of Froude numbers (Fr ≤ 0.3 and Fr ≥ 1.7). It was also found that a single point under the horseshoe vortex can represent the vortices better than the whole field that encompasses the whole system. The characteristics of the horseshoe vortex formed upstream of a wall-mounted circular cylinder in an open channel were studied using two dimensional Particle Image Velocimetry. Experiments were conducted for different flow conditions and focused on Froude numbers, cylinder diameters, flow depths and bulk flow velocities. Two groups of experiments were conducted. The rest consisted of twenty five experiments with a wide range of Froude numbers, including sub-critical, critical and super-critical flow conditions. This group was analysed as a whole and then the lower sub-critical experiments followed by the upper sub-critical to super-critical experiments. Another set of nine sub-critical experiments were conducted focusing on the experimental design to segregate flow and geometrical parameters. Quadrant analysis was used to analyse the contribution of near-bed turbulent stresses in the region upstream of the cylinder. Applying multivariate statistical techniques, the relationship between geometrical parameters, flow conditions and the changes in the location and magnitude of the turbulent stresses under the horseshoe vortex were examined. This study, introduces the Froude number as a new governing parameter of the horseshoe vortex system, which can be important for fluvial systems with change of hydraulics regimes as mountainous rivers.

Amid growing concern over climate change and the resultant need to reduce CO2 emissions, as well as an ever-increasing global population with a rising demand for electricity, renewable energy sources have an important role to play in future electricity generation. One such source is tidal stream energy, which offers the potential for predictable electricity generation by harnessing the energy of the tide using arrays of underwater turbines. This research explores the impact of hydrodynamic interactions between the blades of a tidal stream turbine and its support structure on turbine performance, as well as the hydrodynamic interaction of multiple turbines. Experiments were carried out using scale model turbines installed in experimental water channels, and a motorised drive system and encoder was used to record turbine performance. Due to turbulence in the region upstream of the support structure, separation distance between the turbine blades and support structure was found to significantly influence performance. An optimal separation distance between the blades and support structure was found to allow maximum performance. The turbulent wake of an upstream turbine was also found to have a notable impact on downstream turbine performance, leading to a reduction in power output. This effect is governed by upstream and offset separation distance and support structure diameter. Particle Image Velocimetry was used to record downstream turbine velocity wakes, which were found be a complex amalgamation of turbine and support structure elements. The testing of a turbine installed on four complex support structures produced large differences in power outputs, further highlighting the influence of support structure design on turbine performance, and confirming its importance to the design of tidal stream turbine arrays.