The acoustic properties of an orifice under a turbulent boundary layer

The acoustic impedance of an orifice was measured in still air and in a wind tunnel where the orifice was subjected to a turbulent boundary layer. Tests were carried out at four frequencies -- 250, 500, 800, and 2,200 Hz; and at five wind tunnel velocities -- 16, 19, 23, 26, and 30 m/s. For the 250 Hz tests, both linear and nonlinear orifice behavior was investigated. It was observed that usually with flow the resistance increases over the no-flow resistance. And as the flow velocity increases the resistance increases. At the same time, the reactance with flow present is less than the no flow reactance, and decreases as the flow velocity increases. However, cases were observed where resistance decreased and reactance increased with increasing flow velocities. The results of a dimensional analysis of the data are discussed.