Finite element micro-model of Helmholtz liner configurations

Turbofan engines rely on acoustic liners in both the inlet and exhaust ducts to ensure that acoustic radiation levels are kept below noise certification limits. In the quest for improved attenuation performance innovative liner configurations are currently being considered. To analyse existing Helmholtz resonator types and to aid in the evaluation of new concepts a novel three-dimensional finite element predictive technique is developed. The numerical model can represent a liner sample consisting of an array of individual cells of arbitrary shape and configuration. By enforcing local resistance and inertance conditions, porous surfaces of arbitrary impedance distribution can be treated. The new technique may be applied in many different ways. A typical approach is to analyse an array of liner cells, either in an impedance tube or in a configuration similar to a single mode device. The method solves for acoustic pressures both inside the liner structure and along the SMD duct, enabling impedance properties and attenuation characteristics to be calculated. An extensive programme of verification and validation of the method has been undertaken, and some sample comparisons are presented. These results illustrate the power of this numerical approach, and demonstrate its ability to realistically predict the detailed acoustic behaviour of liners.