Scattering of Acoustic Duct Modes by Axial Liner Splices

Finite difference computation of acoustic scattering by small surface inhomogeneities and discontinuities

“…[14,15]). Some of the scattered modes are not as heavily damped by the liner as the original incident duct mode because of their different modal structures.…”

“…This model has been used by numerous investigators for computing duct/acoustic mode propagation in jet engine ducts; e.g. Regan and Eaton [12], McAlpine et al [13], Tester et al [14] and Tam et al [15]. Due to a number of practical reasons, it is a standard practice to install hard wall splices to separate a liner into smaller pieces.…”

“…The duct wall boundary conditions (liner impedance, Z = 2 + i, time factor exp(ÀiXt)) are enforced by the ghost point method [26]. Recently, this method has been used successfully by Tam et al [15] in their spliced liner study. On the left and right end of the computational domain, a perfectly matched layer (PML), using the methodology of Hu [27,28], is implemented.…”

“…The PML absorbs all the outgoing waves. The incident acoustic mode, that enters the computational domain on the right side, is introduced into the computation by the split variable method as in the work of Tam et al [15]. Fig.…”

“…The difference is the energy of the scattered waves that are clearly quite substantial at the duct inlet. To check the accuracy of the wave number truncation method, a third computation using the quasi-two-dimension azimuthal mode expansion method of Tam et al [15] is carried out. This method expands the propagating acoustic waves in a large number of azimuthal modes.…”