The effect of aspect ratio on the inerstion loss of lined ducts

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Handbook provides tabulated information that can be used to determine the insertion loss of a variety of lined and unlined ducts. However, the tabulated cross-sections are primarily for square ducts. Not surprisingly, there is a need for information on the insertion loss of ducts having different aspect ratio cross-sections. Hence, the insertion loss for large aspect ratio cross-sections are investigated using a previously validated finite element simulation approach. A coupled structural-acoustic finite element analysis is performed, and data is compared to measurement results from the literature for a few configurations. An analysis campaign is then performed which better explains the effect of aspect ratio on duct insertion loss.

Acoustic boundary conditions at an impedance lining in inviscid shear flow

The accuracy of existing impedance boundary conditions is investigated, and new impedance boundary conditions are derived, for lined ducts with inviscid shear flow. The accuracy of the Ingard–Myers boundary condition is found to be poor. Matched asymptotic expansions are used to derive a boundary condition accurate to second order in the boundary layer thickness, which shows substantially increased accuracy for thin boundary layers when compared with both the Ingard–Myers boundary condition and its recent first-order correction. Closed-form approximate boundary conditions are also derived using a single Runge–Kutta step to solve an impedance Ricatti equation, leading to a boundary condition that performs reasonably even for thicker boundary layers. Surface modes and temporal stability are also investigated.