Variable-speed rotor helicopters: Performance comparison between continuously variable and fixed-ratio transmissions

Variable speed rotor studies represent a promising research field for rotorcraft performance improvement and fuel consumption reduction. The problems related to employing a main rotor variable speed are numerous and require an interdisciplinary approach. There are two main variable speed concepts, depending on the type of transmission employed: Fixed Ratio Transmission (FRT) and Continuously Variable Transmission (CVT) rotors. The impact of the two types of transmission upon overall helicopter performance is estimated when both are operating at their optimal speeds. This is done by using an optimization strategy able to find the optimal rotational speeds of main rotor and turboshaft engine for each flight condition. The process makes use of two different simulation tools: a turboshaft engine performance code and a helicopter trim simulation code for steady-state level flight. The first is a gas turbine performance simulator (TSHAFT) developed and validated at the University of Padova. The second is a simple tool used to evaluate the single blade forces and integrate them over the 360 degree-revolution of the main rotor, and thus to predict an average value of the power load required by the engine. The results show that the FRT does not present significant performance differences compared to the CVT for a wide range of advancing speeds. However, close to the two conditions of maximum interest, i.e. hover and cruise forward flight, the discrepancies between the two transmission types become relevant: in fact, engine performance is found to be penalized by FRT, stating that significant fuel reductions can be obtained only by employing the CVT concept. In conclusion, FRT is a good way to reduce fuel consumption at intermediate advancing speeds; CVT advantages become relevant only near hover and high speed cruise condition

A computational assessment of the aerodynamic performance of a tilted Darrieus wind turbine

The aerodynamic performance of a Darrieus wind turbine operating with the rotation axis tilted with respect to the free-stream wind speed is investigated in this paper. An Unsteady Reynolds Averaged Navier Stokes (URANS) Computational Fluid Dynamics (CFD) model is proposed in order to provide wind turbine manufacturers with a reliable simulation tool to forecast the power conversion characteristics of vertical axis wind turbine prototypes that operate in tilted conditions. The outputs of the model are compared against experimental performance of a non-tilted rotor corrected to the standard sea level conditions. Two different tilted configurations are studied (i.e., a tilt angle of 10 and 20), and the aerodynamic performance are presented in terms of the mechanical power production and the power coecient. A sensible decrease in the power production is observed for increasing tilt angles. Comprehensive physical interpretations of the results are provided, considering also the predictions of a methodology based on semi-empirical methods

Numerical study on the internal flow field of a reversible turbine during continuous guide vane closing

The unsteady flow field in a reversible pump-turbine is investigated during the continuous load rejection using a 3D computational fluid dynamic analysis. Numerical calculations are carried out using the detached eddy simulation (DES) turbulence model and a new approach involving automatic mesh motion. In this way, the instability of the flow field is analyzed by continuously changing the guide vane openings from the best efficiency point (BEP). Unsteady flow characteristics are described by post-processing signals for several monitoring points including mass flow, torque, head and pressure in the frequency and time-frequency domains. The formation of vortices of different scales is observed from the origin to further enlargement and stabilization; the effect of the rotating structures on the flow passage is analyzed, and the influence of unsteady flow development on the performance of the turbine is investigated. Finally, the evolution during the period of load rejection is characterized in order to determine the hydrodynamic conditions causing the vibrations in the machine

Aerodynamic Optimisation of a Morphing Leading Edge Airfoil

A morphing leading edge airfoil is optimized for aerodynamic performance with different objectives. A constant arc length parameterization employing the Class/Shape Transformation technique is built to limit the axial deformation introduced by morphing. The optimization is performed with a standard methodology based on genetic algorithms, comparing the results for three different aerodynamic models: a potential flow solver with boundary layer calculation (XFOIL), a fully turbulent RANS model (Spalart-Allmaras) and a transitional RANS model (gamma-theta). Whereas the solutions obtained with the third model are standard droop nose shapes, those found via transitional models show an uncommon deformation with an upward leading edge deflection. Development of optimization strategy is also performed by building an hybrid procedure based on a metamodel-assisted approach. Several nonlinear regression methods are investigated to compare the accuracy in fitness approximation and an Artificial Neural Network (ANN) was finally selected. Application of the improved algorithm to a probelm previously solved with a standard approach shows that the use of a surrogate model, combined with a gradient based method for local individual improvement, is able to provide a reduction of the convergence effort when approximating the highly nonlinear relationship between the constant arc length parameterization and the aerodynamic behavior predicted with gamma-theta model

Numerical Study of Support Interferences on the SOAR Separation Wind Tunnel Test

A process of support design for wind tunnel models and the evaluation of interferences effect are described in this chapter. The work was performed at the Von Karman Institute for Fluid Dynamics (VKI; Sint-Genesius-Rode, Belgium), and it was commissioned by the S3-Swiss Space System company. The work concerns the separation wind tunnel test of the Suborbital Aircraft Reusable (SOAR) vehicle from an Airbus commercial plane carrier. The supports are designed for future separation wind tunnel test of the SOAR version V10 in the VKI-S1 wind tunnel. They are designed in scale 1:180 for the test of the SOAR in the presence of the Airbus and in scale 1:80 for the SOAR alone test. Two different shapes of support (circular and elliptic) are tested in each case. First there are the supports designed, then the results of the finite element method (FEM) static structural analysis and vibrational analysis, and finally the result of the computational fluid dynamics (CFD) campaign. The flow and the force interferences caused by the support are investigated by comparing simulations with and without support. The behavior of the two shapes and of the dimensional variations are investigated at an angle of attack between 0° and 15° and at Mach 0.7

Assessment of gamma−theta transitional model for laminar-turbulent transition prediction in supercritical airfoils

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.A numerical study on the capability of the γ−θ turbulence model for predicting the laminar/turbulent transition in the boundary layer developing around a supercritical airfoil (NLR 7301) is described. The range in the Mach number explored is [0.3, 0.825], thus covering a fully transonic flow regime. For this purpose, a CFD solver (ANSYS CFX©) is used on a hybrid structured-triangular grid, where an accurate mesh setup of the wall boundary layer was performed in order to ensure (i) a value of y+ less than 6 everywhere and (ii) a number of boundary layer rows within the physical boundary layer no less than 4. Results obtained are compared to the experimental data described in the open literature and discussed in detail. Despite the various sources of uncertainty affecting the experimental data, the results regarding the transition location revealed a very good model predictive capacity for low-to-medium Mach numbers (Mach<0.6), while exhibiting a less satisfactory ability in the transonic regime (Mach>0.6). In this case, prediction of transition location on both sides of the airfoil is still accurate even if the correlation on the pressure distributions gets poorer.mp201