Large Eddy Simulation (LES) prediction of the flow around transonic high-pressure turbine cascade vanes is an active subject of research. For such problems, the flow topology is dictated by the geometry, inflow conditions and irreversibilities. When studied experimentally, input specifications necessarily suffer from uncertainties inherent to experimental measurement facilities. Such limits are also present in numerical applications. The following paper proposes to evaluate the relative importance of uncertainty sources in determining the adequate LES flow behaviour for the T120 transonic blade experimentally tested at UniBw (Munich) during the European project AITEB II. To do so, the nominal operating point is targeted and different simulations are obtained by changing inflow specifications with and without turbulence injection. As expected, changes in the static pressure ratio between the inlet and the outlet by more or less 4%, alter significantly the flow topology and the aerodynamic losses. Impact of turbulence injection at inlet is also addressed. Investigation of dissipation fields, including the laminar and sub-grid model contributions, allows the identification of the underlying mechanisms. Although irreversibilities have a smaller impact on the flow prediction relative to the static pressure ratio (and associated uncertainties), its relevance on the flow prediction and topology is found to be of primary importance.
Efficient design of highly loaded pressure blades often leads to the generation of a separation bubble on the pressure side of highly curved blades. For this specific region, fundamental, numerical and experimental studies have indicated the importance of the turbulence present in the main stream in determining the size of the bubble before its reattachment to the blade. Despite this important finding, many complex phenomena remain and are still present and can influence the overall flow response. In this paper, explorations of high-fidelity unsteady Large Eddy Simulations of a separated flow are studied for the high pressure T120 blade from the European project AITEB II (Aerothermal Investigation on Turbine Endwalls and Blades). For this investigation, simulations are carried out at the nominal operating point with and without synthetic turbulence injection at the inlet condition to comply with the specification from the experiment. Based on these predictions, the near wall flow structure and turbulent fields are specifically investigated in an attempt to identify the key mechanisms introduced by the turbulent main stream flow. Results show that the turbulence specification at the inlet enables the recovery of the correct pressure distribution on the blade surface contrary to the laminar inlet condition if compared to the experiment. Investigations of the boundary layer profiles show a strong impact of the freestream turbulence on the shape factor from the leading edge. As a consequence, the recirculation bubble located downstream on the pressure side is impacted and reduced when turbulence is injected. Due to this change in mean flow topology, the mass flow distribution in the passage appears strongly affected. Investigations of loss fields furthermore show that the freestream turbulence dramatically increases the loss production within the computational domain.
A mesh adaptation methodology for wall-modeled turbomachinery Large Eddy Simulation (LES) is proposed, simultaneously taking into account two quantities of interest: the average kinetic energy dissipation rate and the normalized wall distance y + . This strategy is first tested on a highly loaded transonic blade with separated flow, and is compared to wall-resolved LES results, as well as experimental data. The adaptation methodology allows to predict fairly well the boundary layer transition on the suction side and the recirculation bubble of the pressure side. The method is then tested on a real turbofan stage for which it is shown that the general operating point of the computation converges toward the experimental one. Furthermore, comparison of turbulence predictions with hot-wire anemometry show good agreement as soon as a first adaptation is performed, which confirms the efficiency of the proposed adaptation method.
The use of numerical simulations to design and optimize turbine vane cooling requires precise prediction of the fluid mechanics and film cooling effectiveness. This results in the need to numerically identify and assess the various origins of the losses taking place in such systems and if possible in engine representative conditions. Large-Eddy Simulation (LES) has shown recently its ability to predict turbomachinery flows in well mastered academic cases such as compressor or turbine cascades. When it comes to industrial representative configurations, the geometrical complexities, high Reynolds and Mach numbers as well as boundary condition setup lead to an important increase of CPU cost of the simulations. To evaluate the capacity of LES to predict film cooling effectiveness as well as to investigate the loss generation mechanisms in a turbine vane in engine representative conditions, a wall-modeled LES of the FACTOR film-cooled nozzle is performed. After the comparison of integrated values to validate the operating point of the vanes, the mean flow structure is investigated. In the coolant film, a strong turbulent mixing process between coolant and hot flows is observed. As a result, the spatial distribution of time-averaged vane surface temperature is highly heterogeneous. Comparisons with the experiment show that the LES prediction fairly reproduces the spatial distribution of the adiabatic film effectiveness. The loss generation in the configuration is then investigated. To do so, two methodologies, i.e, performing balance of total pressure in the vanes wakes as mainly used in the literature and Second Law Analysis (SLA) are evaluated. Balance of total pressure without the contribution of thermal effects only highlights the losses generated by the wakes and secondary flows. To overcome this limitation, SLA is adopted by investigating loss maps. Thanks to this approach, mixing losses are shown to dominate in the coolant film while aerodynamic losses dominate in the coolant pipe region.
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