Measurement of the acoustic impedance of an orifice under a turbulent boundary layer

The acoustic impedance of an orifice under a turbulent boundary layer was measured. A single orifice, 0.508 cm in diameter and 0.32 cm thick, was tested in a wind tunnel where the velocity was varied from 16 to 30 m/sec. An impedance tube mounted behind the orifice was operated at frequencies from 250 to 2200 Hz with sound pressure levels from 85 to 140 dB. It was found that the resistance and reactance end correction could be nondimensionalized as R/ρ (νω)1/2 and δ/δeo, respectively, and correlated as functions of l/d, ωd2/ν, u*d/ν, and V/u*. The symbols are R resistance, ρ density, ν kinematic viscosity, ω frequency, δ end correction, δeo linear end correction without flow, l orifice thickness, d diameter, u* boundary layer friction velocity, and V acoustic velocity amplitude. Nonlinear behavior occurred when V/u* was greater than three. Amplitudes greater than three were only obtained in the 250-Hz tests, however, at this frequency the V/u*≳3 criteria was independent of u*d/ν. In the linear range R/ρ (νω)1/2 versus ωd2/ν produced U shaped curves parametrically dependent on u*d/ν. When u*d/ν was greater than 250 the resistance with flow was greater than the no flow for all frequencies. If u*d/ν was less than 250 there was a frequency range where the resistance with flow was less than the no-flow resistance. The reactance end correction as a function of these same variables gave an inverted U shape. Above u*d/ν=250, the reactance decreases with flow for all frequencies, while below 250 there is a frequency range in which the reactance increases over the no-flow reactance. Subject Classification: [43]85.20; [43]28.60.