The present paper is part of a two-part publication that aims to numerically investigate the occurrence of flow induced vibrations in a steam turbine last stage rotor at low load operations. Experimental and numerical investigations at low load operations are gaining interest in the design of steam turbine blades due to the effort required to achieve carbon free objectives, which lead to higher flexibility of steam power plants. Reliable machines able to work safely at low load conditions are required for coupling with renewable energy sources and high output power plants. In this part 1 paper, the last rotor of a low-pressure steam turbine module installed in concentrated solar plants has been numerically evaluated to assess the flutter stability at different low load conditions, while the companion part 2 deals with the investigation of unsteady aerodynamic instabilities which might arise in the last stage rotor. In the present work, aerodynamic and flutter simulations were performed at different load conditions, from the design point to very low mass flows to achieve a real sensitivity of flutter behavior to the mass flow rate. By means of steady state multistage results, aerodynamic performance and flow structures were evaluated. The decreasing mass flow and blade load reveal the formation of wide separation areas both on the last rotor pressure side and in the exhaust duct, bringing the stage up to ventilating conditions. The flutter analyses were performed with an uncoupled non-linear method in order to estimate the aerodynamic damping for a selection of mode-shape families. From aerodamping results it was possible to observe the trend in the flutter response at different load conditions and it is evident how the shock wave system near the tip plays a major role in energy exchange between flow field and the vibrating profile. The results show an overall flutter stability of the blade up to very low load conditions. This means that for these type of rotor profiles and corresponding applications, flutter occurrence is not the main issue, and further vibration causes like rotating instabilities have to be investigated as demonstrated in the companion part 2 paper.

The working conditions of airfoils along modern wind turbine blades are putting new focus on the importance of properly characterizing the aerodynamic performance of different airfoil families also at high angles of attack (AoAs) beyond stall and at Reynolds numbers much lower (from few thousands to one million) than those commonly analyzed before. Several test cases are showing that even higher-order computational methods (like RANS/URANS CFD) are unable to properly capture the complex flow physics taking place past the blades, when deep stall occurs or when the AoA changes so rapidly to provoke the onset of dynamic stall. To fill this gap, the use of high-fidelity methods, like the Large Eddy Simulation (LES) is proposed, even though it implies a massive increase of the calculation cost. In order to analyze the prospects of using LES in comparison to RANS for low Reynolds, high AoAs, this work presents an in-depth study of the NACA 0021 aerodynamics at the Reynolds number of 80,000, by means of both traditional RANS approaches and high-fidelity (LES) simulations using the OpenFOAM suite. The selected airfoil has been showing in fact several issues in the correct characterization of its performance in similar conditions in many recent wind energy applications. The LES approach showed the ability to overcome the limitations of traditional RANS simulations, improving the accuracy of the results and reducing their dispersion thanks to the fact that the flow structures in the separated-flow regions are properly captured. Overall, this work underlines that accurate investigations of the aerodynamic performance of the NACA 0021 at low Reynolds require multiple sensitivity studies when RANS approaches are used, and suggests the use of LES simulations in order to increase the accuracy of estimations, especially when studying the stalledflow operating conditions of the airfoil.