48,744 research outputs found
Adjoint differentiation of a structural dynamics solver
No full textThe design of a satellite boom using passive vibration control by Keane [J. of Sound and Vibration, 1995, 185(3),441-453] has previously been carried out using an energy function of the design geometry aimed at minimising mechanical noise and vibrations. To minimise this cost function, a Genetic Algorithm (GA) was used, enabling modification of the initial geometry for a better design. To improve efficiency, it is proposed to couple the GA with a local search method involving the gradient of the cost function. In this paper, we detail the generation of an adjoint solver by automatic differentiation via ADIFOR. This has resulted in a gradient code that runs in 7.4 times the time of the function evaluation. This should reduce the rather time-consuming process (over 10 CPU days by using parallel processing) of the GA optimiser for this problem
Optimization using surrogate models and partially converged computational fluid dynamics simulations
No full textEfficient methods for global aerodynamic optimization using computational fluid dynamics simulations should aim to reduce both the time taken to evaluate design concepts and the number of evaluations needed for optimization. This paper investigates methods for improving such efficiency through the use of partially converged computational fluid dynamics results. These allow surrogate models to be built in a fraction of the time required for models based on converged results. The proposed optimization methodologies increase the speed of convergence to a global optimum while the computer resources expended in areas of poor designs are reduced. A strategy which combines a global approximation built using partially converged simulations with expected improvement updates of converged simulations is shown to outperform a traditional surrogate-based optimization
Second-order cone programming formulations for a class of problems in structural optimization
No full textThis paper provides efficient and easy to
implement formulations for two problems in structural
optimization as second-order cone programming
(SOCP) problems based on the minimum compliance
method and derived using the principle of complementary
energy. In truss optimization both single and
multiple loads (where we optimize the worst-case compliance)
are considered. By using a heuristic which is
based on the SOCP duality we can consider a simple
ground structure and add only the members which
improve the compliance of the structure. It is also
shown that thickness optimization is a problem similar
to truss optimization. Examples are given to illustrate
the method developed in this pape
Multi-variable geometry repair and optimization of passive vibration isolators
No full textA range of techniques are considered for the search of a high dimensional design landscape with extensive, unknown and disjointed regions of infeasibility. We present the use of a hybrid genetic-algorithm / gradient-descent search of the objective function / feasibility problem. The genetic algorithm is used to optimize the vibration isolation of a novel passive structure concept, while the gradient descent method is used to repair infeasible geometries. For a more complicated structure the gradient search fails to find feasible geometries and we resort to a non-dominated sorting multi-objective genetic algorithm which searches the vibration isolation and geometry feasibility simultaneously. Although the complicated geometry is more difficult to optimize, there is potential for significant improvements in vibration isolation
The Keane and Runkle estimator for panel-data models with serial correlation and instruments that are not strictly exogenous
No full textIn this article, we introduce the new command xtkr, which implements the Keane and Runkle (1992a, Journal of Business and Economic Statistics 10: 1– 9) approach for fitting linear panel-data models when the available instruments are predetermined but not strictly exogenous. This is a common case that includes dynamic panel-data models as a leading example. Monte Carlo simulations show that, in certain situations, this approach offers an improvement over the popular difference generalized method of moments and system generalized method of moments estimators in terms of bias and root mean squared error. An empirical application to cigarette demand also demonstrates its usefulness for applied researchers
Multidisciplinary design optimization of UAV airframes
No full textIf one considers the problem of converting an aircraft mission profile into an airframe design from an optimization theory perspective, it becomes obvious that the search problem comes with all the trimmings. The design space is large and multidimensional, there are multiple and often highly multimodal objectives and constraints, these depending not only on the design variables, but often on each other as well. Multidisciplinary Design Optimization studies can be conducted at different levels of detail, depending on the chosen trade-off between the size of the design space and the fidelity of the analysis. In this paper we discuss some of the challenges arising at the conceptual level, where simple, but versatile models and low cost analysis tools are used to guide the designer through the first, fundamental decisions of the design process. At the centre of our proposed design workflow lies a parametric geometry, residing in an off-the-shelf Computer-Aided Design (CAD) tool - this provides the models required by the multidisciplinary analyses. We also touch on some of the issues specific to the design of our chosen class of aircraft - Unmanned Air Vehicles (UAVs). To summarize: a CAD-based UAV conceptual design framework is proposed and demonstrated
The integration of advanced active and passive structural noise control methods
No full textThis paper reports on an investigation into the feasibility of using active and passive means of vibration control in aerospace structures. In particular, attention is focused on controlling vibration transmission through light weight satellite structures at medium frequencies. The initial structure under test here is a 4.5-meter long satellite boom consisting of 10 identical bays with equilateral triangular cross sections. This structure is typical of those that might be used in space telescopes, space stations or synthetic aperture radar systems. Such a structure is typically used to support sensitive instruments in precise alignments spaced tens of metres apart. While a great deal of work has been done on this problem at low frequencies, relatively little has been achieved to date at medium frequencies (here taken to be between 150 Hz and 250 Hz). Nonetheless, this is of importance to new space missions. Using the techniques described here, an overall reduction in vibration transmission of 31.0 dB is achieved in an essentially undamped structure using passive means alone. The amounts of attenuation achievable for active control with one, two and three actuators are found to be 15.1 dB, 26.1 dB and 33.5 dB, respectively. With the combined passive control (using 10% geometric deviations) and active control (using three actuators) an overall reduction of 49.5 dB is achievable in practice
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