Validation of an analytical model for scattering by intake liner splices

Circumferential variations in the impedance of intake liners occur in aero-engine ducts due to the presence of hard strips or splices, which can cause a significant reduction in liner performance at certain conditions. New intake liner designs are currently aiming to minimise the number and width of splices, and while CAA or CFD methods can be used to model and predict the scattering effects, these are normally impractical for direct application to parametric or liner optimisation design studies, due to the significant CPU time required to solve this 3D problem at realistic frequencies. In the early 1990's at Rolls- Royce, Cargill developed an analytical model for the scattering by liner splices based on the Kirchhoff approximation. Now that robust, accurate CAA methods are becoming available, it is possible to conduct a theoretical validation of the Cargill model, which is the subject of this paper. The CAA code used, ACTRAN, is a commercially available finite element method, which assumes an irrotational mean flow and linear propagation. The ACTRAN and Cargill results agree well for no-flow case, except for some minor discrepancies in the back-scattered modes but these are of secondary importance compared to the forward scattered. A striking feature of these results is that the forward scattered modal sound power is almost equal for all spinning modes except for one or two modes adjacent to the rotor-alone modes. Results for flow Mach numbers up to 0.4 show that agreement between ACTRAN and Cargill is acceptable except again for the field dominated by the back- scattered field. In general, for engineering purposes, Cargill's model agrees well with those obtained from the ACTRAN code, for relevant combinations of parameters. Cargill's model has also helped to improve our understanding of the scattering process and provides a rapid method of evaluating splice scattering effects.