Stiffener Shape Design to Minimize Interior Noise

Abstract

Results of a research program to develop computational methods to minimize noise transmission into aircraft fuselage interiors are discussed. A design tool to perform a constrained optimization of the acoustic environment within a vibrating structure is developed utilizing nite element methods and boundary element methods (FEM/BEM), and its application to aircraft cabin noise problems is studied. The results of a study to optimize the cross section shapes of frames and stringers of an idealized aircraftlike stiffened cylinder are reviewed. The structure is optimized for minimum noise at speci ed points in the interior, as a result of a single frequency (tonal) exterior acoustic disturbance. For the cylinder and excitation frequency studied, it has been found that spatially varying the stiffener sizes over the cylinder is more important than optimizing the shape of the cross sections. Because FEM/BEM methods are only reliable for lower frequencies, the problems studied are applicable to low-frequency tonal noise such as seen in turboprop aircraft. Nomenclature vectors of lower and upper bounds F(b)= objective function f 0 = initial value of objective function p i (b) = complex acoustic pressure at node i S = scale factor design variable v = normal velocity of shell structure, or acoustic velocity at shell wall W = structural weight W max = constrained maximum weight x i , y i = coordinate design variables at node i

Experimental Verification of Structural-Acoustic Modelling and Design Optimization

“…With respect to these results, they emphasized the particular value of structural-acoustic optimization in passive noise control. In references [13,14], they presented the results of optimal geometry and cross-sectional shapes of frames and stringers. A reduction of 8)6 dB was gained mainly by optimizing the geometry of the sti!ening structures over the cylinder.…”

Developments in structural-acoustic optimization for passive noise control

“…Examples of the application of uncoupled structure-fluid interaction in structural-acoustic optimization were reported for interior acoustics by Cunefare and Engelstad et al [82,86,94] and by Marburg and Hardtke et al [210,[213][214][215]218], and for exterior problems by Koopmann and Fahnline [102,180], by Koopmann and other co-workers [28,74,89,240,301], by Tinnsten et al [309,311,312], by Hall [138], by Fisher [106], and by Milsted et al [232]. This is likely due to its simple formulation because two smaller models are computed one after the other, instead of one large model.…”

“…Approaches to formulate an objective function to judge the desired properties can be categorized into three different groups, the first one being the sound pressure level at one or more specified points, primarily utilized for closed domains [82,86,94,181,184,210,213,215,216,218,252,253,293,299]. For general-purpose noise control in open domains, the emitted sound power accounted for the objective function in a number of papers [28,70,74,75,102,106,138,160,161,180,189,190,232,240,301,330].…”

“…A cylinder clamped at both ends accounted for a design problem similar to an aircraft fuselage in several papers by Cunefare, Engelstad et al [82,86,94]. The structure was excited by a monopole source outside at a single frequency that coincided with an eigenfrequency of the original model.…”

“…However, this was essentially dedicated to analytic sensitivity analysis. Cunefare and Engelstad et al [86,94] investigated performance of stiffening stringers and frames on an aircraft skin. Similarly, Jog [161] optimized thickness of a long plate that might also be considered as a beam.…”