Progress in Aerospace Sciences 

Abstract: A major challenge to the successful full-scale development of modem aerospace systems is to address competing objectives such as improved performance, reduced costs, and enhanced safety. Accurate, high-fidelity models are typically time consuming and computationally expensive. Furthermore, informed decisions should be made with an understanding of the impact (global sensitivity) of the design variables on the different objectives. In this context, the so-called surrogate-based approach for analysis and optimization can play a very valuable role. The surrogates are constructed using data drawn from high-fidelity models, and provide fast approximations of the objectives and constraints at new design points, thereby making sensitivity and optimization studies feasible. This paper provides a comprehensive discussion of the fundamental issues that arise in surrogate-based analysis and optimization (SBAO), highlighting concepts, methods, techniques, as well as practical implications. The issues addressed include the selection of the loss function and regularization criteria for constructing the surrogates, design of experiments, surrogate selection and construction, sensitivity analysis, convergence, and optimization. The multi-objective optimal design of a liquid rocket injector is presented to highlight the state of the art and to help guide future efforts.

Abstract: The evaluation of aerospace designs is synonymous with the use of long running and computationally intensive simulations. This fuels the desire to harness the efficiency of surrogate-based methods in aerospace design optimization. Recent advances in surrogate-based design methodology bring the promise of efficient global optimization closer to reality. We review the present state of the art of constructing surrogate models and their use in optimization strategies. We make extensive use of pictorial examples and, since no method is truly universal, give guidance as to each method's strengths and weaknesses.

Abstract: The aerodynamics of horizontal axis wind turbine wakes is studied. The contents is directed towards the physics of power extraction by wind turbines and reviews both the near and the far wake region. For the near wake, the survey is restricted to uniform, steady and parallel flow conditions, thereby excluding wind shear, wind speed and rotor setting changes and yawed conditions. The emphasis is put on measurements in controlled conditions. For the far wake, the survey focusses on both single turbines and wind farm effects, and the experimental and numerical work are reviewed; the main interest is to study how the far wake decays downstream, in order to estimate the effect produced in downstream turbines. The article is further restricted to horizontal axis wind turbines and excludes all other types of turbines.

Abstract: This paper presents a review of the control of flow separation from solid surfaces by periodic excitation. The emphasis is placed on experimentation relating to hydrodynamic excitation, although acoustic methods as well as traditional boundary layer control, such as steady blowing and suction, are discussed in order to provide an appropriate historical context for recent developments. The review examines some aspects of the excited plane mixing-layer and shows how its development lays the foundation for a basic understanding of the problem. Flow attachment to, and separation from, a deflected flap is then shown to be a paradigm for isolating controlling parameters as well as understanding the basic mechanisms involved. Particular attention is paid to separation control on airfoils by considering controlling parameters such as optimum reduced frequencies and excitation levels, performance enhancement, efficiency, reduction of post-stall unsteadiness, compressibility and other important features. Additional topics covered include excitation of separation bubbles, control and exploitation of diffuser flows, three-dimensional effects, the influence of longitudinal curvature and possible applications to unmanned air vehicles. The review closes with some recent developments in the control and understanding of incompressible dynamic stall, specifically illustrating the control of dynamic stall on oscillating airfoils and identifying the crucial time-scale disparity between dynamic stall and periodic excitation.