Rita Ponza

Nonparametric Fitting of Aerodynamic Data Using Smoothing Thin-Plate Splines

This paper introduces a nonparametric fitting method for the interpolation of aerodynamic observations over a large range of multiple angles of attack. The method is based on the employment of smoothing thin-plate spline class functions, a well-renewed mathematical tool for multivariate data mining based on the generalization of the univariate natural cubic splines, in which a roughness penalty criterion is used to produce very smooth predictive hypersurfaces. Compared with other methods, such as parametric or even conventional nonparametric methods, the use of a smoothing thin-plate spline is more effective, in that the predictive surface comes directly from the observed points, thus minimizing any intervention of the analyst aimed at introducing model parameters. This forms the basis for a very reliable fitting technique, in which model construction can be relatively easy to implement. An application of the method is carried out on a case study representative of some experimental data coming from a wind-tunnel campaign on a typical three-dimensional fuselage-shaped body, aimed at the acquisition of its aerodynamic coefficients over a rather extensive attitude range. Specifically, the application is focused on the body lift coefficient as a function of both angle of attack and sideslip angle. The data set is also interpolated using concurrent response-surface methods: namely, a linear model, a bivariate spline, a radial basis function network, a support vector regression technique, a regression kriging, and a moving-least-squares approach, alternatively known as local polynomial regression. Results of data fitting are assessed using a cross-validation approach and reveal a clear superiority of smoothing thin-plate spline over the other methods, leading to a more regular fitted surface and a more reliable prediction tool, even when some observations are omitted. This is important per se, but acquires even more significance when an aerodynamic test campaign is to be planned with the minimum number of experimental observations.

Numerical Assessment of Pneumatic Devices on the Wing/Fuselage Junction of a Tiltrotor

The present study aimed at investigating the effect of boundary-layer control techniques, in particular suction and blowing, on the aerodynamic performance of the wing/fuselage junction of a tiltrotor. To this purpose, a series of numerical analyses at Re=14×106 were performed in order to obtain the best shape and location of the suction/blowing slot in terms of aerodynamic efficiency and maximum lift gain with respect to the baseline. A preliminary cost-benefit analysis was also carried out by comparing the increased onboard installed power required by these systems with the estimated reduction in the propulsive power due to improved aerodynamic characteristics of the aircraft.

Airfoil Data Fitting Using Multivariate Smoothing Thin Plate Splines

Smoothing thin plate splines, a fitting technique based on a rigorous roughness penalty approach, have been recently investigated as a promising tool for bivariate interpolation of aerodynamic data. In this paper, this technique is implemented and extended to multivariate fitting. In particular, the method is applied for estimating the aerodynamic polars of well-known two-dimensional symmetrical and nonsymmetrical airfoils as functions of some geometric parameters describing the airfoil shape and a further variable defining the flow regime (either the Mach or the Reynolds number). Therefore, the simultaneous influence of five independent variables on three responses (lift, drag, and pitching moment coefficients) is investigated. To this purpose, a large database is generated via numerical simulations (using a validated flow solver) containing all information required to build a reliable response surface. Then, the model is built and its performance validated by performing queries on complete aerodynamic polars at various flow regime conditions of a series of airfoils not included into the database. Results show a very good matching between predicted and calculated curves, thus demonstrating the remarkable predictive capability of the implemented tool.

Comparison of Optimization Strategies for High-Lift Design

The design of high-lift systems represents a challenging task within the aerospace community, being a multidisciplinary, multi-objective and multi-point problem. The DeSiReH (Design, Simulation and Flight Reynolds Number Testing for Advanced High-Lift Solution) project, funded by European Commission under the 7th Framework Program, aimed at improving the aerodynamics of high-lift systems by developing, in a coordinated approach, both efficient numerical design strategies and measurement techniques for cryogenic conditions. Within DeSiReH, different partners used several numerical automatic optimization strategies for high-lift system design purposes. A realistic multi-objective and multi-point optimization problem was defined and solved by adopting different flow models dimensionality, meshing techniques, geometry parameterization and optimization strategies. Special attention was devoted to perform a fair comparison of the results and useful information were obtained about trends, pros and cons of the approaches used. The outcome of these activities is that an efficient design

Helicopter fuselage aerodynamic data fitting using multivariate smoothing thin plate splines

Smoothing thin plate splines, a nonparametric statistical technique for multivariate data fitting, were investigated to predict the aerodynamic performance (output variables) of a generic 3D helicopter fuselage as functions of the pitch angle and of some geometric parameters describing their shape (input variables). In order for the smoothing thin plate splines to be properly applied, a database needed to be constructed containing pairs of input–output variables. To this purpose, a sample helicopter fuselage was chosen and 14 variants were generated modifying the geometric parameters; then, the pertinent lift, drag and pitching moment coefficients were obtained via computational fluid dynamics. The smoothing thin plate splines model was built excluding from the database one fuselage at a time and was then used to determine the aerodynamic performance of the left out configuration: finally, the obtained results were compared with those coming from direct computational fluid dynamics simulations over the same fuselage. The prediction capability of the smoothing thin plate splines models has been confirmed for all the analyzed fuselage geometries.

Fast-Accurate Model for Performance Prediction of Multistage Impulse Turbine Stages

A one-dimensional model for predicting on/off design performance of impulse turbine stages was developed which accounts for the effects of secondary flows. The well known Craig and Cox loss model was implemented together with unconventional correlations for the estimation of secondary and volumetric effects (those related to leakage flows in labyrinth seals, shrouded rotors and open cavities, balance holes, as well as the volumetric effect due to rotor blockage). These effects were modeled according to off-line CFD simulations of turbine stages and, then, calibrated on experimental data of a number of real multistage impulse turbines. The result is a fast and reasonably accurate model, which can profitably be used for both design and analysis purposes.Copyright

Aerodynamic Shape Optimization of Air-Intakes of a Helicopter Turboshaft

The research project HEAVYcOPTer, a sub task of the European RD 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.Copyright

Aerodynamic Optimization of an Impulse Turbine Cascade Including Laminar/Turbulent Transition Prediction

Aerodynamic optimization of an impulse turbine rotor cascade is described in this paper. The aim of the optimization is to minimize the total pressure losses through the cascade by reshaping the turbine airfoil. For a more realistic calculation of the boundary layer and the associated losses, laminar/turbulent transition is computed by means of the commercial CFD code Fluent. The solver underwent an accurate validation before it is inserted into the optimization loop. A proper optimization procedure is developed which is based on a sequential use of two modules: first, a global search is performed using a genetic algorithm, then a local optimization is carried out by means of a Sequential Quadratic Programming (SQP) algorithm. The benefits of this approach are demonstrated, in that the total pressure coefficient of the cascade can be lowered up to 25%.Copyright

Efficient Aerodynamic Optimization of an Impulse Turbine Rotor Cascade

This paper describes an efficient aerodynamic optimization method for an impulse turbine rotor cascade. The aim of the optimization is to minimize the total pressure losses through the cascade by controlling the shape of a portion of the turbine airfoil, i.e. the pressure side and the rear part of the suction side. This is done with the intention of reducing the complexity of the problem to be solved, as well as because of the particular nature of the losses being generated. For this purpose, the commercial Computational Fluid Dynamics (CFD) code Fluent© is used where a transition model is implemented, prior an accurate validation has been carried out. A proper optimization procedure is developed which is based on a sequential use of several modules: first, a Design of Experiment (DOE) analysis is performed, then a Response Surface Methodology (RSM) is employed to build a surrogate model of the fitness function, which is in turn minimized using a Sequential Quadratic Programming (SQP) algorithm. The benefits of this approach are demonstrated, in that the total pressure coefficient of the cascade can be lowered up to 13%.Copyright

Efficiency and Stall Margin Enhancement in Transonic Compressor Rotors Using Synthetic Jets: A Numerical Investigation

Several passive and active techniques were studied and developed by compressor designers with the aim of improving the aerodynamic behavior of compressor blades by reducing, or even eliminating, flow separation. Fluidic-based methods, in particular, were investigated for a long time, including both steady and unsteady suction, blowing and oscillating jets. Recently, synthetic jets (zero mass flux) have been proposed as a promising solution to reduce low momentum fluid regions inside turbomachines. Synthetic jets, with the characteristics of zero net mass flux and non-zero momentum flux, do not require a complex system of pumps and pipes. They could be very efficient because at the suction part of the cycle the low momentum fluid is sucked into the device, whereas in the blowing part a high-momentum jet accelerates it. To the authors’ knowledge, the use of synthetic jets has never been experimented in transonic compressor rotors, where this technique could be helpful (i) to reduce the thickness and instability of blade suction side boundary layer after the interaction with the shock, and (ii) to delay the arising of the low momentum region which can take place from the shock-tip clearance vortex interaction at low flow operating conditions, a flow feature which is considered harmful to rotor stability. Therefore, synthetic jets could be helpful to improve both efficiency and stall margin in transonic compressor rotors. In this paper, an accurate and validated CFD model is used to simulate the aerodynamic behavior of a transonic compressor rotor with and without synthetic jets. Four technical solutions were evaluated, different for jet position and velocity, and one was investigated in detail.Copyright