Estimation of vibro-acoustic boundary conditions based on an experimental acoustic two-port characterization of a flexible flow duct

Abstract

For the accurate numerical prediction of the vibro-acoustic interactions in flow ducts, correct structural boundary conditions modeling the joints of flexible duct walls are key in the low frequency region. Therefore, a more elaborated model than the standard simply-supported or clamped conditions is needed. Robin boundary conditions allow a better approximation, although they introduce additional parameters of which the concrete values are unknown. In this paper, an indirect methodology is proposed to estimate these parameters, using two-port data obtained from acoustic measurements as reference for iterative optimization. As the two-port method is independent of the acoustic boundary conditions at the duct in- and outlet, the indirect methodology is useful for early-design phases when the whole duct system is unknown. To illustrate the proposed methodology, a flexible duct segment is tested and modeled with the finite element method. The structural boundary conditions are parametrized with frequency-independent stiffness and damping coefficients for linear spring-damper combinations. Non-uniform, mesh-independent distributions along the boundary are used and the pretension imposed by the joint is included. Optimization leads to parameter values improving the match with the reference data compared to standard boundary conditions. An experimental Operational Modal Analysis validates that the optimized numerical model not only approximates well the two-port data, but also captures the underlying vibro-acoustic physics, making the updated model valuable for vibro-acoustic predictions. To demonstrate the robustness of the methodology, a parameter study is conducted with numerical reference data, which shows that the structural boundary estimation is independent of structural material properties.