Structural-acoustic optimization of a point-excited, submerged cylindrical shell

Methods are formulated and evaluated for minimizing the structural weight and radiated far— field pressure spectrum of coupled structural-acoustic systems. A prototype problem, a simply supported, fluid loaded cylindrical shell excited with a time-harmonic point force at its center, is solved using constrained and unconstrained search direction optimization techniques. An analytical series solution for the far-field radiated pressure spectrum of a finite length cylindrical shell is used for design evaluations, and the multi-purpose optimization program ADS is used to solve the minimization problems. A constrained minimization problem is investigated where the goal is to reduce the material volume of the cylinder subject to constraints on the radiated far— field pressure spectrum. An unconstrained minimization problem is formulated using a composite objective function composed of the cylindrical material volume and a penalty function summation of the radiated far -field pressure constraint violations. In both formulations, the design variables are restricted to the cylinder half-length to inner radius ratio, or L/a; and the cylinder thickness to inner radius ratio, or h/a. Since only two design variables are used, the design spaces defined by the objectives and/or constraints are plotted graphically, and their characteristics are investigated to determine their suitability for the application of various search direction minimization techniques. Evaluation of the problem formulations and the resulting ADS optimizations is done by considering the quality of the final solutions and the total number of function evaluations required to find each solution. In general, the unconstrained, composite objective function formulation was found to produce mathematical programs less sensitive t the methods used to minimize them.