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This research aims to contribute the efforts of reducing the predictive uncertainties of Computational Fluid Dynamics (CFD) so that it can be a more reliable tool for modelling complex flows in turbomachinery. To achieve this goal, a three-dimensional (3D) flow phenomenon that embodies many key features of turbomachinery flows, that is, the corner separation flow often found in axial compressor stators, is selected as the research object. Based on the understanding of the underlying physics of this complex flow, the RANS turbulence closure models and a hybrid RANS-LES method are developed and extended. The improved RANS and hybrid RANS-LES models are shown to be able to predict the corner separation flows with significantly improved accuracy, and are expected to be applicable to other types of flows driven by the similar physical mechanisms as those of the corner separation flows. The commonly used RANS turbulence models (namely Spalart-Allmaras (SA) and Menter's Shear Stress Transport (SST) models) have been found to overpredict the size of corner separation. The physical reason is partly attributed to the underestimation of turbulence mixing between the main flow and the endwall boundary layer. This makes the endwall boundary layer unable to withstand the bulk adverse pressure gradient, and in turn leads to its premature separation near the endwall. The flow characteristics within the compressor stator cascade are then studied to facilitate understanding the physical mechanisms that drive the formation of 3D flow structures, and the physical reasons that lead to the RANS modelling uncertainties. The source terms in the SA and SST models that control mixing are identified and modified, in order to increasing the mixing process and strengthening the endwall flow. The modified turbulence models are validated to give better predictions of the extent of corner separation under various operating conditions. Further, the results of an investigation into the effects of explicit non-linear turbulence modelling on anisotropic turbulence flows are presented. In compressors, the flow is subject to 3D shear and the effects due to turbulence anisotropy shall not be ignored. It is found that increasing the Reynolds stress anisotropy within the corner region contributes to counter-rotating streamwise vortices being generated in the corner region. This leads to the higher-momentum fluid in the main flow passage being entrained into the corner region and thus enhances mixing between the corner flow and the main flow. The flow within the corner region is energized and thus is more resistant to separation. Although the proposed modifications can predict corner separations with significantly better accuracy, the results are still subject to the limitations of RANS in capturing separated flows. To further reduce the modelling uncertainties, the hybrid RANS-LES method is resorted to allow for at least some larger scales of turbulence being resolved in the regions where RANS does not work well. In this thesis, an effort is presented towards a high-fidelity hybrid RANS-LES simulation for 3D complex separated flows. The hybrid RANS-LES method is developed to resolve turbulence where necessary, while avoiding the higher cost of performing large eddy simulation (LES) everywhere. That is, reasonably modelling the near-wall turbulence behaviour in the hybrid RANS-LES context by proposing a physical-based RANS-LES blending function. The suggested future work is presented in the final chapter, which outlines the necessity of investigation into the turbulence anisotropic effects on the tip-leakage flow modelling, and the potential applicability of the proposed model variants in the prediction of the blockage due to corner separation in the multi-stage environment.

During the preliminary stages of design, designers must make decisions concerning size, speed and machine type. Misinformed choices in design parameters and architecture can lock in suboptimal performance. Design charts leveraging similitude are powerful tools that can help designers make better decisions at this point in the design process. The design charts used up until now are either partially or entirely based on lost correlations and do not include mixed flow architectures. On top of this many of the current charts contain little to no information about the various loss mechanisms, turbine characteristics and how both change through out the design parameter space. This thesis builds on the pre existing work by mapping out design parameter spaces using computation methods, more specifically using a Reynolds averaged Navier Stokes Equation Solver. This also allowed for the partial decoupling of loss mechanisms. This thesis includes a unified design methodology built around a unified set of design flow parameters. This was used to generate design charts for all architecture types, including radially fibred mixed flow architectures with fixed cone angle and radius ratio. Both total to static and total to total efficiency was presented as a function of loading coefficient and duty flow coefficient instead of the typically used non dimensional speed and diameter. This was done in order to help aid in the aerodynamic interpretation of the data. Results showed the shapes of the design spaces in the present framework can largely be explained by surface dissipation. That is, entropy generated at the surface is proportional to the surface velocity cubed with a fixed dissipation coefficient. A universal trend across all architectures was found. Turbines with high duty flow coefficient have characteristically high surface velocities and turbines with lower values of duty flow coefficient are characterised by high surface area. However, the balance between area and surface velocity of the different architectures differed significantly. Axial turbines have characteristically high surface velocities. Whereas radial turbines have characteristically high surface area and lower surface velocities. The lower surface velocities of the radial turbine was in part due to the centrifugal loading. It was shown that shifting loading from the relative acceleration term to the centrifugal term decreases relative passage velocities. It was shown that axial turbines have the broadest range across the design space, both the constrained mixed flow and radial turbines sit inside the axial space. In addition to this, axial architectures did not show a performance boundary at lower duty flow coefficients. This was attributed to the aspect ratio being fixed and other tolerances based geometric features scaling with the passage. The mixed and radial architectures were shown to suffer high losses at lower duty flow coefficients, this was attributed to the high end wall surface area, characteristic of designs in this region of the Balje chart. iv All turbines suffer a drop off in performance at high duty flow with all mechanisms con tributing to this. The characteristically high velocities of these designs result in high profile and end wall surface dissipation loss. In addition the leakage loss increases in part due to the high velocity with which the leakage flow is mixing and the high over tip driving pressure. Radial architectures show a sharper drop in performance with increasing duty flow. This was attributed to flow separation at the casing due to reducing radius of curvature. By virtue of having a lower cone angle, the mixed flow architecture have a significantly larger radius of curvature and did not exbibit any signs of flow separation. As expected the (radially fibred) mixed flow architectures have lower performance loss with increasing loading coefficients than the (radially fibred) radial architectures due to the inlet metal angle not being constrained. However axial architectures outperform both. When comparing the mixed and axial flow architectures the mixed flow architectures showed a sharper increase in surface area with increasing loading.

Transonic compressors are regularly used as part of the compression system in industrial gas turbines because their high operating speeds make them capable of producing high stage pressure ratios and they have a high efficiency. The flow inside a these compressors is inherently unsteady due to rotor-stator interactions and the flow in transonic compressors is also subject to shock waves leading to further unsteady interactions such as the interaction between the inlet guide vanes and the rotor leading edge shock waves. Despite the presence of unsteady flow, CFD simulations regularly assume the flow to be steady relative to each blade row in order to reduce the cost and time to perform simulations, however, this introduces more assumptions into the simulations compared to unsteady CFD simulations. The first aim of this thesis is to investigate the causes of the differences in predicted efficiency observed between steady and unsteady simulations in order to show the operating conditions at which the steady flow assumptions break down. It is shown that the efficiency in the rotor blade row is over predicted in the steady simulations due to a weaker shock wave. The efficiency in the IGV is also over predicted while the efficiency in the stator blade row is under predicted. The unsteady simulations are also used to show that the stage efficiency characteristic is not affected by the unsteady effects from the downstream blade rows, when the stage is embedded in a multistage machine. In the next part of the thesis the effect of the IGV-Rotor axial gap on the compressor efficiency, and the dynamic blade loading, is investigated in multistage unsteady simulations. This shows that a reduced IGV-Rotor axial spacing gives an improvement in the efficiency but leads to an increase in the dynamic loading which may prove detrimental to the blade life. It is also show that altering the IGV-Rotor spacing has an effect on the loadings of the blade rows in the downstream stages. The final part of the thesis investigates vane rescheduling in a multistage transonic compressor using steady CFD simulations. Vane rescheduling is important in multistage compressors in order to prevent the compressor stalling and to maximise the efficiency when operating at off-design speeds. Vane schedules are optimised for a range of design and off-design speeds and the effect of rescheduling on the radial flow distribution is analysed. It is shown that the flow can be affected far downstream of the rescheduled blade row and that this can affect the location of the separation in downstream blade rows.

Engine system consists of complex components, where aerodynamics can be coupled. In these coupled problems, flow separation may exist and the multiscale turbulence needs to be finely resolved to obtain an accurate solution. A high-fidelity simulation in such scenarios, however, is still infeasible for industrial applications due to the limitation of current computational resources. To make it possible, a mixed-fidelity CFD method based on the Immersed Boundary Method (IBM) is proposed. In this hierarchical method, geometries can be replaced by forces, i.e. the standard IBM or eIBM, whereas turbulence can be resolved by the Large Eddy Simulation. The thesis proposed this method and applied it to an important issue for engine design: fan-intake interaction. The method was validated on a Darmstadt Transonic Rotor with a distortion generator to replicate the unsteady distortion at incidence, the NASA Rotor 67 with steady pressure distortion, and a triangular prism for turbulence statistics. The results indicated that this method can accurately simulate the performance map, separation transfer and total pressure distributions, compared to the experimental data and Direct Mesh Resolved (DMR) case. The method was then applied to reveal the mechanism of fan influence on intake distortion. It was shown that there are two aspects of such influence: the suction effect of a fan can accelerate the flow in the upstream and directly change its streamline curvature; on the other hand, the recirculating flows can also intensify the turbulence, indirectly increase the mixing process and finally alleviate the distortion. The main flow effect was further investigated in different parameters of fan type, location and distortion size. Results showed that a tip-loaded fan is more effective in suppressing intake separation; a nearer fan to the upstream has more significant reduction of distortion; a greater distortion can be suppressed more. These results demonstrate that a fan can be an essential component for intake distortion control. Further investigations of the results from RANS and LES interpreted the dominance of the influence via main flow or turbulence. It was found that stronger main flow acceleration by a fan can mitigate the inaccuracy of turbulence models. This indicates that for a short intake design, conventional turbulence models may be capable of predicting flow separation.

The low pressure (LP) exhaust system presents a promising avenue for improving the performance of large steam turbines. For this reason, LP exhaust systems have attracted the attention of the research community for decades. Nevertheless, we still lack understanding of the flow physics and loss mechanisms in the exhaust system, especially at part-load conditions. It is also unclear how the exhaust system should be designed when its required operating range widens. This thesis provides solutions to these aerodynamic issues through experimental and numerical investigations, and provides tools that could contribute to better designs of LP exhaust systems. Firstly, the Computational Fluid Dynamics (CFD) solver ANSYS CFX was validated against experiments performed on a scaled test rig under representative part-load flow conditions. This validation exposed the weakness of Reynolds-averaged Navier-Stokes (RANS) CFD when there is a highly swirling flow and large separation regions in the exhaust diffuser. To facilitate the numerical studies, a series of tools were also developed. A design suite, ExhaustGen, was used to automate the pre- and post-processing of CFD calculations. The exhaust diffuser was parametrised using "Minimum Energy Curves", which reduce the dimension of parameter space. Further, a suitable stage-hood interface treatment (Multiple Mixing Planes) was chosen to predict the circumferentially non-uniform flow in the exhaust hood at low computational cost. Numerical investigation of the baseline geometry provided insights into the key flow features and loss mechanisms in the exhaust system, over a wide range of operating conditions. In particular, the bearing cone separation was identified as a key source of loss at part-load conditions. The effect of stage-hood interaction on the performance and design of the exhaust system was studied by varying the rotor blade design, which can positively influence system performance. Finally, a global sensitivity study was performed to identify the most influential design parameters of the exhaust hood. These findings allow, for the first time, LP exhaust hood performance maps to be constructed, so that the benefits of choosing a suitable hood geometry and blade design can be revealed. The thesis also offers contribution towards formulating LP exhaust system design guidance for a wide operating range.

Deficient performances of turbocharger compressors inside turbo-charged engines limit the behaviour of the drive train. This problem has shifted the design space for compressors towards their performance at part-speed and low-flow conditions. The most dominant feature of these flow conditions is inlet recirculation. It causes a large portion of flow to be expelled through the rotor inlet, creating a blockage ring on the casing. While on the one hand, inlet recirculation is the main loss-source at low-speed and low-flow within centrifugal compressors, on the other hand, it also keeps the compressors functioning because it reduces incidence. This thesis aimed towards increasing the understanding of inlet recirculation, with the scope on improving the part-speed, low-flow performance of automotive turbocharger compressors. The phenomenon was investigated regarding its key features, the conditions at which it occurs and its impact on performance. Furthermore, a reduced order model was derived and the influence of the tip gap size as a design parameter was analysed. The research was carried out on an automotive turbocharger compressor which was investigated experimentally and numerically. Inlet recirculation is a phenomenon which takes place in the tip region of the rotor, extending far downstream and far upstream of the leading edge. The flow within the recirculation bubble features a strong positive swirl component, affecting the work input into the machine. The phenomenon is non-periodic in a time-averaged sense. An investigation of the rotor flow-field regarding inlet recirculation, carried out for the first time, revealed that the starting point of inlet recirculation is located far inside the rotor passage. An analysis based on mass, momentum and energy allowed the derivation of a low-order model to account for inlet recirculation in preliminary design. In the compressor map, inlet recirculation was present over 40% of the map width at low speeds. It maintained its presence with increasing rotor speed beyond the point where the inlet flow became transonic. The losses in the inlet recirculation zone were shown to be up to 35% of the total compressor loss at low speed. A loss analysis showed that inlet recirculation was the main loss source at low-flow conditions. The tip clearance study showed that the size and intensity of inlet recirculation was independent of the tip gap size. Efficiency gains due to reduced tip leakage were marginalised by the presence of inlet recirculation but the rotor maintained enhanced pressure rise capabilities for reduced tip gap sizes.