Illustration of the Inclusion of Sound-Flow Interactions in Lighthill's Equation

vectors are only added during evaluation and do not permanently affect the design vectors xj in the current GA population. The time to produce each structure is proportional to N . For the L81 method each structuretook about5.5 days to produce (compare,1.7 h for the NP method, where N D 1) on an SGI hardware platform running at 90 MHz. After each optimization f95:300 for the best resultingdesign from each GA, denotedby xopt, was evaluated to measureaccurately the expectedperturbedperformance.The resultsare shown in Fig. 2, where the changes in the average values of f .xopt/, f95.xopt/, and the robustness r for each method are compared with the average performanceof the 10 structurespreviouslyoptimizedusing a nominal performance-onlymeasure. r is a measure of the variability of the performance deŽ ned as

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