A Note on the Interpretation of Acoustic Impedance in Confined Flows

The use of acoustic impedance to interpret the aeroacoustic behavior of flow ducts is discussed. The test case is a T-junction subjected to various combinations of grazing and bias mean flow. This geometry is not only prone to whistling but its aeroacoustic response varies with the incidence of the acoustic excitation, making it difficult to define a representative impedance. The acoustic impedance should, if correctly defined, have a real part that represents the exchange of energy between the hydrodynamic and acoustic fields and an imaginary part that can be interpreted as the inertia of the orifice. The appropriate definitions of the acoustic impedance and state variables are discussed and compared with experimental data.