The whistling potentiality of an orifice in a confined flow using an energetic criterion

Using a two-source method, the scattering matrices of 10 sharp-edged thin orifices are measured under different subsonic flow conditions. The data are analysed in terms of net acoustical energy balance: the potential whistling frequency range is defined as the one associated with acoustical energy production. A Strouhal number describing the maximum whistling potentiality is found to be equal to 0.2-0.35, based on the orifice thickness and the orifice jet velocity. It appears to depend on the Reynolds number and on the ratio of orifice to pipe diameters. Tests are performed to compare theoretically and experimentally the potential whistling frequency to the actual whistling frequency. They are found to coincide within the measurement uncertainty.