Nonlinear Time Domain Boundary Conditions from Liner Response Data

Effective suppression of jet engine noise by inlet and exhaust duct liners continues to be an important part of producing environmentally acceptable aircraft. Because methods for transfer of frequency-domain impedance data to time-domain boundary conditions are still under development, Computational Aeroacoustics (CAA) has yet to be used widely either to design new liners or predict pre-installation performance of existing ones. In addition, the methods that do currently exist are not easily extended to cover finite amplitude effects, whether they are due to high sound pressure levels or liner material nonlinearities. Nielsen Engineering and Research proposes to fill this need for a general, nonlinear, time-domain liner characterization scheme by novel application of Volterra (integral) series methods. Applying this modeling technique to data from a given liner results in forced, time-domain, differential equations for liner velocity response to an imposed acoustic pressure. The differential equations can be linear or nonlinear, and are generated using either existing frequency-domain impedance data or new time-domain acoustic pulse experiments. Once in existence, the equations can be discretized in any fashion consistent with the CAA calculation where they are used.