Multi-Fidelity Study of Complex Multi-Scale Flows Concerning Stagnation Probes
The work in this thesis aims to highlight, quantify the loss sources related to the use of stagnation probes in aero-engines through the use of RANS and high fidelity wall-resolved Large Eddy Simulations (LES), and help develop methods and techniques to aid in improving their design to reduce the impact of these losses. The measurement of stagnation temperature is an important performance metric for the calculation of component efficiency in aero-engines, particularly due to reductions in the margin of error over concurrent improvements in jet engine performance and measurement techniques. Hence, developing an understanding of the flow physics and evaluating any loss sources help aid in reducing calibration errors. Initial work exploring the flow physics of such a stagnation probe through LES is shown to exhibit a rich set of fundamental flow features (e.g. jet in cross flow behaviour along the probe exit with the development of counter-rotating vortices). Furthermore, conduction effects through the solid region are considered, with results showing close agreement to experimental work. A fully distributed parallelised volume mesh morphing code is developed that is used to study a wide design space created through parametrisation of the baseline probe geometry. Through evaluation of this design space and the initial LES studies, improvements to the baseline design are suggested that either reduce downstream pressure loss between 10 to 35% or maximise the temperature recovery for a range of inflow conditions. Although an isolated probe can by itself present a range of flow features, the complexity was increased to simulate a real-world condition by placing the probe upon the leading edge of a turbine blade on an aero-engine and conducting a series of high-fidelity wall-resolved LES calculations. The end-walls are omitted and both periodic and spanwise periodicity (Modelling a span consisting of 32% of the axial chord length) are applied to model an infinite cascade. Its sensitivity to free-stream turbulence effects is also studied at length, with the free stream turbulence shown to have a significant impact on the separation bubble that forms upon the trailing edge of the blade, along with the presence of the probe itself. With the addition of free stream turbulence, the loss observed within the trailing edge wake is reduced with more than 50% of the losses at the cascade exit being shown to be generated by the leading edge probe. The application of Immersed Boundary Method (IBM) and LES is shown to enable the use of an extremely simple mesh to observe the primary flow features generated due to the blade and probe interaction effects, as well as quantify downstream pressure loss. IBM is utilised to approximately model the probe, while fully resolving the blade itself through a series of LES simulations. This method has shown to be able to capture downstream loss profiles as well as integral quantities compared to both experiment and fully wall-resolved LES without the need to spend a significant amount of time generating the ideal mesh. Additionally, it is also able to capture the turbulence anisotropy surrounding the probe and blade regions.
Computational Fluid Dynamics, CFD, Turbo-machinery, Large Eddy Simulation, Fluid Mechanics, Measurement, Probe, Temperature, Aerodynamics, Turbulence, Turbulence Modelling, Unsteady Aerodynamics, Instrumentation
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