## 2aPAb10. Sound propagation in a periodically lined duct with a fluid flow.

### Session: Tuesday Morning, May 14

### Time: 11:00

**Author: Susann Boij**

**Location: Dept. of Mech. Eng., Rm. 3-445, MIT, Cambridge, MA 02139**

**Author: Borje Nilsson**

**Location: KTH, 100 44 Stockholm, Sweden**

**Abstract:**

Resonant lining is an effective measure to reduce tonal noise in ducts. It
has been observed that a fluid flow in the duct can interact with the lining in
such a way that sound is amplified. A model for the acoustic performance of an
infinite duct with periodic lining (quarter wavelength resonators) and a nonzero
mean velocity is developed. The fluid in the duct and lining are separated by an
infinitely thin shear layer, the velocity in the duct is uniform, and the
velocity in the lining is zero. Such a layer is unstable for all frequencies, a
so-called Helmholtz' instability. The linear acoustic equations are assumed to
be valid. Based on the building block method, the acoustic modes in the periodic
structure are calculated from the duct modes and scattering effects of the sharp
edges. Numerical computations are done for a number of cases, where
higher-order, evanescent, acoustic Floquet modes are omitted in order to
simplify calculations. The direction of propagation of the computed modes is
determined by the requirement of causality.

from ASA 131st Meeting, Indianapolis, May 1996