Francesco Bertini

Off-Design Performance of a Highly Loaded Low Pressure Turbine Cascade Under Steady and Unsteady Incoming Flow Conditions

The off-design performance of a highly loaded low pressure (LP) turbine cascade has been experimentally investigated, at the Aerodynamics and Turbomachinery Laboratory of Genova University, under steady and unsteady incoming flow conditions. Tests have been performed for different Reynolds numbers (Re = 70,000 and Re = 300,000), in order to cover the typical LP turbine working range. The incidence angle has been varied between i = −9 deg and +9 deg, in order to test off-design conditions characterizing the engine. For the unsteady case, upstream wake periodic perturbations have been generated by means of a tangential wheel of radial rods. The cascade and the moving bars system have been located over a common bearing in order to make them rigidly rotating. This solution allows a proper comparison of the cascade robustness at the incidence angle variation under steady and unsteady incoming flows, since all the other operating parameters have been kept the same. In order to survey the variation of the unsteady boundary conditions characterizing the off-design operation of the downstream cascade, time-mean and time-resolved wake structures have been analyzed in detail. For what concerns the cascade performance, profile aerodynamic loadings and total pressure loss coefficients at the cascade exit have been surveyed for the different incidence angles under both steady and unsteady inflows. Different total pressure loss sensitivity at the incidence angle variation has been observed for the steady and the unsteady inflow conditions. Hot-wire anemometer has been employed to obtain the time-mean pressure and suction side boundary layer velocity profiles at the blade trailing edge for the different conditions. The integral parameters at the cascade exit plane help to justify the different loss trend versus incidence angle found for the steady and the unsteady cases, explaining the different sensibility of the blade profile when this operates under realistic unsteady inflow condition.

An assessment of the laminar kinetic energy concept for the prediction of high-lift, low-Reynolds number cascade flows

The laminar kinetic energy (LKE) concept has been applied to the prediction of low-Reynolds number flows, characterized by separation-induced transition, in high-lift airfoil cascades for aeronautical low-pressure turbine applications. The LKE transport equation has been coupled with the low-Reynolds number formulation of the Wilcoxs k − ω turbulence model. The proposed methodology has been assessed against two high-lift cascade configurations, characterized by different loading distributions and suction-side diffusion rates, and tested over a wide range of Reynolds numbers. The aft-loaded T106C cascade is studied in both high- and low-speed conditions for several expansion ratios and inlet freestream turbulence values. The front-loaded T108 cascade is analysed in high-speed, low-freestream turbulence conditions. Numerical predictions with steady inflow conditions are compared to measurements carried out by the von Kármán Institute and the University of Cambridge. Results obtained with the proposed model show its ability to predict the evolution of the separated flow region, including bubble-bursting phenomenon and the formation of open separations, in high-lift, low-Reynolds number cascade flows.

Loading Distribution Effects on Separated Flow Transition of Ultra-High-Lift Turbine Blades

The suction side boundary-layer evolution in two ultra-high-lift low-pressure turbine blade cascades, characterized by the same Zweifel number but two different aerodynamic loading distributions, has been experimentally analyzed under steady and unsteady incoming flows. For the steady inflow case, a suction side boundary-layer separation has been detected for both cascades. Time-mean velocity and unresolved unsteadiness distributions have been exploited to survey the dynamics of the separated flow transition mode. The spectral analysis reveals that only the midloaded cascade is affected by a Kelvin–Helmholtz instability that induces the separated shear layer rollup, which provokes high losses. Results obtained for the unsteady case reveal that linear stability mechanisms drive the amplification of velocity fluctuations carried by wakes with dynamics similar to that characterizing the steady inflow condition. A rollup vortex has been found to be generated for both cascades as a consequence of the wake–shea...

Velocity and turbulence measurements in a separating boundary layer with and without passive flow control

Abstract The present paper reports the results of a detailed experimental study on low profile vortex generators (VGs) used to control boundary layer separation on a large-scale flat plate with prescribed adverse pressure gradients. This activity is part of a joint European research program on Aggressive Intermediate Duct Aerodynamics. The inlet turbulent boundary layer and the pressure gradient over the flat plate are representative of aggressive turbine intermediate ducts. By regulating the inclination of the wall opposite to the flat plate, different pressure gradients, typical of turbine intermediate ducts, can be obtained. To avoid separation on the movable wall, boundary layer suction is applied. Previous measurements showed the effectiveness of VGs in delaying separation and revealed their optimum configuration for the different prescribed pressure gradients. In the present work, laser Doppler velocimetry (LDV) is applied to the most significant pressure gradient case, in order to obtain a more thorough knowledge of the near-wall flow field. Velocity and turbulence profiles are determined up to the near-wall region in order to provide an in-depth analysis of turbulent boundary layer at separation conditions, with and without application of control devices. LDV allowed high spatial resolution and accurate statistical analysis of the boundary layer velocities. The results show velocity and turbulence profiles typical of separated turbulent boundary layers for the baseline case, and non-conventional unseparated boundary layer profiles when VGs are installed on the flat plate.

A CFD Study of Low Reynolds Number Flow in High Lift Cascades

A study of the separated flow in high-lift, low-Reynolds-number cascade, has been carried out using a novel three-equation, transition-sensitive, turbulence model. It is based on the coupling of an additional transport equation for the so-called laminar kinetic energy with the Wilcox k-ω model. Such an approach takes into account the increase of the non-turbulent fluctuations in the pre-transitional and transitional region. Two high-lift cascades (T106C and T108), recently tested at the von Karman Institute in the framework of the European project TATMo (Turbulence and Transition Modelling for Special Turbomachinery Applications), were analyzed. The two cascades have different loading distributions and suction side diffusion rates, and therefore also different separation bubble characteristics and loss levels. The analyzed Reynolds number values span the whole range typically encountered in aeroengines low-pressure turbines operations. Several expansion ratios for steady inflow conditions characterized by different freestream turbulence intensities were considered. A detailed comparison between measurements and computations, including bubble structural characteristics, will be presented and discussed. Results with the proposed model show its ability to predict the evolution of the separated flow region, including bubble bursting phenomena, in high-lift cascades operating in LP-turbine conditions.Copyright

Predicting High-Lift Low-Pressure Turbine Cascades Flow Using Transition-Sensitive Turbulence Closures

This paper discusses the application of different transition-sensitive turbulence closures to the prediction of low-Reynolds-number flows in high-lift cascades operating in low-pressure turbine (LPT) conditions. Different formulations of the well known γ-R˜eθt model are considered and compared to a recently developed transition model based on the laminar kinetic energy (LKE) concept. All those approaches have been coupled to the Wilcox k-ω turbulence model. The performance of the transition-sensitive closures has been assessed by analyzing three different high-lift cascades, recently tested experimentally in two European research projects (Unsteady Transition in Axial Turbomachines (UTAT) and Turbulence and Transition Modeling for Special Turbomachinery Applications (TATMo)). Such cascades (T106A, T106C, and T108) feature different loading distributions, different suction side diffusion factors, and they are characterized by suction side boundary layer separation when operated in steady inflow. Both steady and unsteady inflow conditions (induced by upstream passing wakes) have been studied. Particular attention has been devoted to the treatment of crucial boundary conditions like the freestream turbulence intensity and the turbulent length scale. A detailed comparison between measurements and computations, in terms of blade surface isentropic Mach number distributions and cascade lapse rates will be presented and discussed. Specific features of the computed wake-induced transition patterns will be discussed for selected Reynolds numbers. Finally, some guidelines concerning the computations of high-lift cascades for LPT applications using Reynolds-averaged Navier–Stokes (RANS)/unsteady RANS (URANS) approaches and transition-sensitive closures will be reported.

Predicting High-Lift LP Turbine Cascades Flows Using Transition-Sensitive Turbulence Closures

This paper discusses the application of different transition-sensitive turbulence closures to the prediction of low-Reynolds number flows in high-lift cascades operating in low-pressure turbine (LPT) conditions. Different formulations of the well known γ – Reθt model are considered, and compared to a recently developed transition model based on the laminar kinetic energy (LKE) concept. All those approaches have been coupled to the Wilcox k – ω turbulence model. The performance of the transition-sensitive closures has been assessed by analyzing three different high lift cascades, recently tested experimentally in two European research projects (UTAT and TATMo). Such cascades (T106A, T106C, and T108) feature different loading distributions, different suction side diffusion factors, and they are characterized by suction side boundary layer separation when operated in steady inflow. Both steady and unsteady inflow conditions (induced by upstream passing wakes) have been studied. A particular attention has been devoted to the treatment of crucial boundary conditions like the freestream turbulence intensity and the turbulent length scale. A detailed comparison between measurements and computations, in terms of blade surface isentropic Mach number distributions and cascade lapse rates will be presented and discussed. Specific features of the computed wake-induced transition patterns will be discussed for selected Reynolds numbers. Some guidelines concerning the computations of high-lift cascades for LPT applications using RANS/URANS approaches and transition-sensitive closures will be finally reported.© 2013 ASME

Low-Pressure Turbine Cascade Performance Calculations With Incidence Variation and Periodic Unsteady Inflow Conditions

The performance of a Low-Pressure Turbine (LPT) cascade were investigated under both steady and periodic unsteady inflow boundary conditions with different Reynolds numbers and a reduced frequency representative of real LPT working conditions. A sensitivity analysis to the variation of the inlet flow angle was performed to assess the performance during off-design operation.The numerical framework is based on a steady/unsteady Reynolds Averaged Navier-Stokes (RANS/URANS) flow solver which includes some state-of-the-art transition-sensitive turbulence closures. Boundary conditions for the time-accurate computations upstream of the cascade were derived from the experimental characterization of a moving bar system used to generate the wake periodic perturbations.The computed performance of the cascade, as a function of Reynolds number and incidence angle variation, is discussed in comparison with experimental data. Steady and unsteady boundary layer quantities are also compared with measurements in order to gain more insights into the loss generation process and to better support the numerical results. Such an assessment proves that the proposed procedure is adequate for the analysis of the off-design behavior of cascades operating in low pressure turbine conditions.Copyright

Evaluation of unsteady computational fluid dynamics models applied to the analysis of a transonic high-pressure turbine stage

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.

URANS Analysis of Wake-Induced Effects in High-Lift, Low Reynolds Number Cascade Flows

A URANS analysis of unsteady effects induced by incoming wakes in high-lift, low-Reynolds-number cascade flows has been carried out using a novel, transition-sensitive, turbulence model. It is based on the coupling of two additional transport equations, one for the so-called laminar kinetic energy (LKE) and one for a turbulence indicator function, with a low Reynolds number formulation of the Wilcox k-ω model. Two high-lift cascades (T106C and T2), recently tested at the von Karman Institute in the framework of the two European research projects UTAT (Unsteady Transition in Axial Turbomachines) and TATMo (Turbulence and Transition Modelling for Special Turbomachinery Applications), were considered for the present study. The analyzed Reynolds number values span the whole range typically encountered in aero-engines low-pressure turbines operations, and both steady and periodically unsteady inflow conditions were considered. A detailed comparison between measurements and computations, in terms of blade surface isentropic Mach number distributions and cascade lapse rates will be presented and discussed. Results with the proposed model show its ability to predict the major effects of passing wakes on the boundary layer development and loss characteristics of high-lift cascades operating in LP-turbine conditions.Copyright