275,909 research outputs found
Orbit Transfer Manoeuvres as a Test Benchmark for Comparison Metrics of Evolutionary Algorithms
No full textIn the present paper some metrics for evaluating the performance of evolutionary algorithms are considered. The capabilities of two different optimisation approaches are compared on three test cases, represented by the optimisation of orbital transfer trajectories. The complexity of the problem of ranking stochastic algorithms by means of quantitative indices is analyzed by means of a large sample of runs, so as to derive statistical properties of the indices in order to evaluate their usefulness in understanding the actual algorithm capabilities and their possible intrinsic limitations in providing reliable information
Comparative study on the application of evolutionary optimisation techniques to orbit transfer manoeuvres
No full textEvolutionary design of a full-envelope flight control system for an unstable fighter aircraft
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Wake-tracking and turbulence modelling in computational aerodynamics of wind turbine aerofoils
No full textThis article addresses two modelling aspects of wind turbine aerofoil aerodyna-mics based on the solution of the Reynolds-averaged Navier–Stokes (RANS) equations. One of these is the effect of an a priori method for structured grid adaptation aimed at improving the wake resolution. The presented results emphasize that the proposed adaptation strategy greatly improves the wake resolution in the far field, whereas the wake is completely diffused by the non-adapted grid with the same number and spacing patterns of grid nodes. The proposed adaptation approach can be easily included in the structured generation process of both commercial and in-house-structured mesh generators.
The other numerical aspect examined herein is the impact of particular choices for turbulence modelling on the predicted solution. This includes the comparative analysis of numerical solutions obtained by using different turbulence models, and also aims at quantifying the solution inaccuracy arising from not modelling the laminar-to-turbulent transition. It is found that the drag forces obtained by considering the flow as transitional or fully turbulent may differ by 50 per cent.
All these issues are investigated using a special-purpose hyperbolic grid generator and two multi block structured finite volume RANS codes. The numerical experiments consider the flow field past a wind turbine aerofoil for which an exhaustive campaign of steady and unsteady experimental measurements was conducted. The predictive capabilities of the CFD solvers are validated by comparing experimental data and numerical predictions for selected flow regimes. The incompressible analysis and design code XFOIL is also used to support the findings of the comparative analysis of numerical RANS-based results and experimental data
An inflationary differential evolution algorithm for space trajectory optimization
No full textIn this paper we define a discrete dynamical system that governs the evolution of a population of agents. From the dynamical system, a variant of Differential Evolution is derived. It is then demonstrated that, under some assumptions on the differential mutation strategy and on the local structure of the objective function, the proposed dynamical system has fixed points towards which it converges with probability one for an infinite number of generations. This property is used to derive an algorithm that performs better than standard Differential Evolution on some space trajectory optimization problems. The novel algorithm is then extended with a guided restart procedure that further increases the performance, reducing the probability of stagnation in deceptive local minima
Probabilistic analysis of wind turbine performance degradation due to blade erosion accounting for uncertainty of damage geometry
No full textGeometry alterations of wind turbine blades due to erosion reduce the blade aerodynamic performance, yielding turbine power and energy losses. This study proposes a novel probabilistic analysis framework combining computational fluid dynamics, probabilistic and deterministic uncertainty propagation, and high-performance computing to assess this performance degradation accounting for the unavoidable uncertainty on field records of blade erosion. This uncertainty presently prevents using erosion records for improving wind turbine maintenance planning, increasing energy yield, and thus further reducing the wind energy cost. The technology is demonstrated by quantifying the statistical moments of power and energy yield losses of an eroded utility-scale turbine at a North Sea offshore site and a southern European onshore site. The expectations of the offshore and onshore annual energy production losses are found to be 2 and 3% of the corresponding nominal values, respectively, with corresponding standard deviations of 0.1 and 0.15%. In the realistic scenario of erosion varying with high radial frequency, these low standard deviations result from partial compensation of the impact of mild and severe damages. These low standard deviations indicate that present uncertainty levels of erosion geometry records can be handled with uncertainty analysis in predictive maintenance for further reducing wind energy costs. With the frequent assumption of small or no radial variation of erosion, the standard deviation of the loss is misleadingly higher. For the first time, the study reports on the significant impact of turbulence intensity of the installation site on the turbine loss variability with the site wind characteristics
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