This paper presents an efficient ‘Phase-Lagged’ method developed for turbomachinery applications. The method is based on the generalized-shape-correction model. Moving average techniques as well as double-passage domain formulation were adopted in order to reduce memory requirements and improve the model robustness. The model was used to evaluate the aerodynamic performance of the high-pressure transonic turbine stage CT3, experimentally studied at the von Kármán Institute for Fluid Dynamics in the framework of the EU funded TATEF2 project. The results are discussed and compared with both the available experimental data and the results obtained by means of both steady and unsteady scaled full-annulus approaches. Computational requirements of the generalized-shape-correction model are evaluated and discussed showing that nowadays unsteady results can be obtained at an affordable computational cost.

Due to the low level of profile losses reached in low-pressure turbines (LPT) for turbofan applications, a renewed interest is devoted to other sources of loss, e.g., secondary losses. At the same time, the adoption of high-lift profiles has reinforced the importance of these losses. A great attention, therefore, is dedicated to reliable prediction methods and to the understanding of the mechanisms that drive the secondary flows. In this context, a numerical and experimental campaign on a state-of-the-art LPT cascade was carried out focusing on the impact of different inlet boundary layer (BL) profiles. First of all, detailed Reynolds Averaged Navier-Stokes (RANS) analyzes were carried out in order to establish dependable guidelines for the computational setup. Such analyzes also underlined the importance of the shape of the inlet BL very close to the endwall, suggesting tight requirements for the characterization of the experimental environment. The impact of the inlet BL on the secondary flow was experimentally investigated by varying the inlet profile very close to the endwall as well as on the external part of the BL. The effects on the cascade performance were evaluated by measuring the span-wise distributions of flow angle and total pressure losses. For all the inlet conditions, comparisons between Computational Fluid Dynamics (CFD) and experimental results are discussed. Besides providing guidelines for a proper numerical and experimental setup, the present paper underlines the importance of a detailed characterization of the inlet BL for an accurate assessment of the secondary flows.

This paper discusses the importance of the unsteady interaction in transonic turboma chinery stages. Although the flow in a turbomachine is inherently unsteady, most cur rent calculations for routine design work exploit the steady state assumption. In fact, unsteady flow effects are often taken into account for mechanical integrity checks, such as blade flutter or forced response, or heat transfer issues associated with circumferen tial nonuniformities, whereas steady state calculations are usually selected for the aer odynamic design. In this work, some cases are discussed in which significant departures are found between steady and time-averaged results, and the basic fluid mechanisms responsible for them are examined. Finally, a current perspective of unsteady computational fluid dynamics (CFD) calculations for the aerodynamic design is given.

Airline companies are continuously demanding lower-fuel-consuming engines and this leads to investigating innovative configurations and to further improving single module performance. In this framework the low pressure turbine (LPT) is known to be a key component since it has a major effect on specific fuel consumption (SFC). Modern aerodynamic design of LPTs for civil aircraft engines has reached high levels of quality, but new engine data, after first engine tests, often cannot achieve the expected performance. Further work on the modules is usually required, with additional costs and time spent to reach the quality level needed to enter into service. The reported study is aimed at understanding some of the causes for this deficit and how to solve some of the highlighted problems. In a real engine, the LPT module works under conditions which differ from those described in the analyzed numerical model: the definition of the geometry cannot be so accurate, a priori unknown values for boundary conditions data are often assumed, complex physical phenomena are seldom taken into account, and operating cycle may differ from the design intent due to a nonoptimal coupling with other engine components. Moreover, variations are present among different engines of the same family, manufacturing defects increase the uncertainty and, finally, deterioration of the components occurs during service. Research projects and several studies carried out by the authors lead to the conclusion that being able to design a module whose performance is less sensitive to variations (robust LPT) brings advantages not only when the engine performs under strong off-design conditions but also, due to the abovementioned unknowns, near the design point as well. Concept and preliminary design phases are herein considered, highlighting the results arising from sensibility studies and their impact on the final designed robust configuration. Module performance is afterward estimated using a statistical approach.

The Smith diagram, originally published in 1965, has been largely exploited as a preliminary design (PD) tool for axial turbines. Currently, it is applied to aeronautical Low Pressure Turbines (LPTs) in order to define basic characteristics during the feasibility study and to compare different configurations. The Smith diagram represents a correlation of stage performance (η) as function of flow coefficient (ϕ) and loading factor (Ψ), but it does not take into account the effects of some important input parameters (individual contributions of loss, Reynolds number, Aspect Ratio, Rotor Tip Clearance (RTC)) and does not report some key design outputs (deflection angles (δ), profile weights and stresses), which have also a direct relation with the configuration position on the Smith diagram. This study employs meanline analyses incorporating traditional loss correlation models used in the turbine field to compare results with the original Smith diagram. The correlation approach allows one to obtain other important multidisciplinary information (primarily aero-mechanical) which was previously absent, which leads to some strategic design achievements. The investigation process is based on a reference two-stage turbine properly set to match specific operating points on the Smith diagram. Several three-dimensional blade geometries have been prepared and then detailed 3D CFD analyses have been performed in order to acquire confidence with respect to the meanline results. This research adds important information for turbine module design to the Smith chart and allows for a numerical revision of the diagram itself, fine tuning it with data obtained from the analyses of modern blades optimized for high stage performance. Finally numerically-based loss predictors, broadly applicable to LPTs during optimization procedures before detailed CFD analyses, are presented and discussed.