The O-type transition caused by a pair of small-amplitude oblique waves in a vertical buoyancy layer of a fluid with Prandtl number $0.71$ at a Reynolds number of $200$ is investigated using linear stability analysis and three-dimensional direct numerical simulation. The small-amplitude oblique waves experience linear growth and undergo nonlinear interactions to generate streamwise vortices/streaks, two-dimensional streamwise waves and harmonic oblique waves. The streamwise vortices/streaks and two-dimensional streamwise waves have twice the spanwise or streamwise wavenumber of the original perturbation, respectively. Unlike the O-type transition in isothermal flat-plate incompressible and compressible boundary layers where streaks dominate the transition, in the vertical buoyancy layer, either streaks or two-dimensional streamwise waves can dominate the flow field during the early stages of oblique transition. The growth rates of streaks and two-dimensional waves are dependent on the wavenumber of the initial oblique waves. Streaks dominate the flow for high streamwise wavenumbers, while two-dimensional streamwise waves dominate the flow for low streamwise wavenumbers. Analysis of the turbulent kinetic energy production and the Reynolds stresses reveals that the early stages of the transition differ depending on the wavenumber of the oblique waves. An increase in the initial amplitude of the oblique waves causes a faster transition from laminar flow; however, the growth rates of the streaks and two-dimensional streamwise waves are independent of the initial amplitude. Even though different modes are dominant during the early stages of the O-type transition, the onset of chaotic flow is caused by the breakdown of streak modes.

Gas turbine engine components are subject to both low-cycle fatigue (LCF) and highcycle fatigue (HCF) loads. To improve engine reliability, durability and maintenance, it is necessary to understand the interaction of LCF and HCF in these components, which can adversely affect the overall life of the engine while they are occurring simultaneously during a flight cycle. A fully coupled aeromechanical fluid-structure interaction (FSI) analysis in conjunction with a fracture mechanics analysis was numerically performed to predict the effect of representative fluctuating loads on the fatigue life ofbliskfan blades. This was achieved by comparing an isolated rotor (IR) to a rotor in the presence of upstream inlet guide vanes (IGVs). A fracture mechanics analysis was used to combine the HCF loading spectrum with an LCF loading spectrum from a simplified engine flight cycle in order to determine the extent of the fatigue life reduction due to the interaction of the HCF and LCF loads occurring simultaneously. The results demonstrate the reduced fatigue life of the blades predicted by a combined loading of HCF and LCF cycles from a crack growth analysis, as compared to the effect of the individual cycles. In addition, the HCF aerodynamic forcing from the IGVs excited a higher natural frequency of vibration o f the rotor blade, which was shown to have a detrimental effect on the fatigue life. The findings suggest that FSI, blade-row interaction and HCF I LCF interaction are important considerations when predicting blade life at the design stage of the engine. The lack of available experimental data to validate this problem emphasizes the utility of a numerical approach to first examine the physics of the problem and second to help establish the need for these complex experiments.

Numerical modelling of internal cooling passages in gas turbine blades is a challenging task due to their physical characteristics, such as rounded duct corners, the presence of rib turbulators and their staggered locations between surfaces. This results in complex fluid dynamic phenomenon such as counter-rotating vortices and other secondary flow structures that can drive the heat transfer. Heat transfer mechanisms in such passages are inherently coupled with momentum transport and diffusion. Current industry practices for numerical modelling of such passages use unstructured mesh generation tools, steady Reynolds-averaged Navier-Stokes (RANS) equations and two-equation turbulence models such as k-ε and k-ω SST. This paper investigates two generic, engine-representative rib geometries using current numerical practices to determine their limitations. Three mesh generation tools and two turbulence models are compared across two rib geometries. The results are qualitatively and quantitatively compared to detailed experimental Nusselt numbers on the passage walls. It was found that as long as the rib geometry results in a secondary flow that directly impinges onto the wall, the meshing tools and turbulence models agree reasonably well with experiments. When the passage includes wall-wrapped ribs resulting in more complex secondary flows, this decreases the validity of the numerical tools, suggesting that more sophisticated modelling techniques are required as rib geometries continue to evolve.

Gas turbine engine components are subject to both low-cycle fatigue (LCF) and high-cycle fatigue (HCF) loads. To improve engine reliability, durability, and maintainability, it is necessary to understand the interaction of LCF and HCF in these components, which can adversely affect the overall life of the engine. The LCF loads result from the aircraft flight profile and are typically high stress, nominally rotational and aerodynamic loads. HCF loads are a consequence of high frequency vibrations, such as the fluctuating loads on blades as they rotate through the wakes from the upstream stator vanes. This paper demonstrates the importance of a fully coupled FSI analysis in conjunction with a fatigue analysis to predict the effect of representative fluctuating loads on the fatigue life of blisk fan blades. The fully-coupled FSI analysis is compared to the partially coupled FSI analysis and it is found that the former better predicts the the structural response of the titanium alloy blade to the wake impingement from the upstream stator. This results in a non-linear stress history compared to the linear response of the partially coupled system which also under-predicts the peak stress by 24%. The fatigue analysis shows the blade will fail near the root with a maximum damage of 1.079(10−17) using Miner’s rule to calculate cumulative damage. The implications of this research can influence future experimental studies that aim to generate meaningful fatigue data, which will assist in the management of safe operation of gas turbines.