Optimization problems that arise in engineering design are often characterized by several features that hinder the use of standard nonlinear optimization techniques. Foremost among these features is that the functions used to define the engineering optimization problem usually require the solution of differential equations, a process which is itself computationally intensive. Within a standard nonlinear optimization algorithm, the solution of these differential equations is required for each iteration of the algorithm. To mitigate such expense, an attractive alternative is to replace the computationally intensive objective with a less expensive surrogate.In conformance with engineering practice, we draw a crucial distinction between surrogate models and surrogate approximations. Surrogate models are auxiliary simulations that are less physically faithful, but also less computationally expensive, than the expensive simulation that is regarded as "truth." An instructive example is the use of an equivalent-plate analysis method in lieu of a finite element analysis, e.g. to analyze a wing-box of a high-speed civil transport. Surrogate models exist independently of the expensive simulation and can provide new information about the physical phenomenon of interest without requiring additional runs of the expensive simulation.Surrogate approximations are algebraic summaries obtained from previous runs of the expensive simulation. Examples include the low-order polynomials favored in response surface methodology (RSM) and the kriging estimates employed in the design and analysis of computer experiments (DACE). Once the approximation has been constructed, it is typically inexpensive to evaluate.When surrogates are available, be they models or approximations, the optimizer hopes to use them to facilitate the search for a solution to the engineering optimization problem. Our ultimate goal is to design robust optimization algorithms, but we would like to do so in ways that allow us to make effective use of the information that good surrogates can provide. Toward this end, we adopt the perspective that the surrogate can be used to accelerate the optimization technique by exploiting the trends that such surrogates tend to identify. We do not worry about accuracy in the surrogate until it becomes clear either that the optimization technique is in the neighborhood of a minimizer or that the surrogate is not doing a sufficiently good job of identifying trends in the objective.We consider a methodology that constructs a sequence of approximations to the objective. We concentrate on approaches such as DACE, that krige known values of the objective, but our general strategy is also amenable to other classes of approximations. We make use of pattern search techniques to handle the optimization, though other approaches are possible. We choose pattern search techniques because they can be easily amended to exploit surrogates, * Department of Computer Science, College of William & Mary, Williamsburg, VA 23187-8795 (e-mail:
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