Variable speed rotors represent an innovative research field for the development of new rotorcraft designs. Issues related to employing a main rotor variable speed are numerous and require an interdisciplinary approach. For this reason, a preliminary effort has been made to understand the performance implications of coupling helicopter trim and turboshaft engine simulations. Following this, two different models of a UH-60 Black Hawk helicopter and a GE T700 turboshaft engine are implemented and validated against experimental data. Then, an optimization algorithm is employed to find the optimal main rotor speed with the aim of minimizing fuel consumption. Different simulation cases are analyzed to quantify the benefits related to the optimal main rotor speed depending on flight condition, altitude, and helicopter gross weight. It is found that coupling both the helicopter and engine model is necessary to adequately determine the correct rotational speed corresponding to minimum fuel consumption. More than 10% fuel saving is shown to be feasible. The results obtained employing a variable speed main rotor are broadly discussed, and future possible applications of the methodology are suggested. Nomenclature
The research project HEAVYcOPTer, a sub task of the European R&D program Clean-Sky GRC2 [1], is devoted to the efficient design and the shape optimization of the Agusta Westland AW101 helicopter turboshaft engine intake and exhaust system, to be carried out by means of advanced multi-objective optimization algorithms coupled with CFD Navier-Stokes solvers. The present paper describes the outcomes of HEAVYcOPTer in relation to the air intakes shape optimisation activities. This paper describes the technical details of such program. The optimisation method chosen for the redesign of the engine installation involves the application of the state of the art genetic algorithm GDEA, developed at the University of Padova and successfully applied in several fluid-dynamics applications, especially in the field of turbomachinery. For the present application, the set of geometrical designs constituting the genetic algorithm population are generated by means of morphing the original CFD model surface mesh: shapes are applied to baseline surface nodes with a displacement intensity driven by the GA chosen scaling factors. Then, CFD models of new designs are automatically generated and analyzed by the flow solver, returning to the GA the evaluation of the selected objective functions required in order to evolve the population in the next step of the evolutionary process. AW101 intakes have been optimised following a multi-objective/multi-point approach, minimizing inlet total pressure loss in both hovering and forward flight conditions simultaneously; optimised solutions were also constrained so as to not exceed the total pressure distortion level at the engine aerodynamic interface plane, so as to ensure inlet/engine compatibility with respect to the compressor surge limit. This approach ensured the improvement of the engine/airframe integration efficiency for the overall rotorcraft flight envelop, reducing fuel burn and increasing the helicopter propulsive efficiency.
In spite of the remarkable advances in the field of the Computational Fluid Dynamics, algebraic models built upon empirical loss and deviation correlations are still one of the most reliable and effective tools to predict the performance of gas turbine stages with reasonable accuracy, especially when low-reaction, multi-stage architectures are considered. This paper deals with a comparison among some of the most popular loss correlations used by gas turbine manufacturers; such comparison is performed on a two-stage low-reaction turbine for which detailed experimental data are available. An overall assessment on the validity of loss correlations is carried out to help the designer/analyst using the most accurate model when both on- and off-design are to be carried out.
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