Whistling of an orifice in a reverberating duct at low Mach number.

An experimental investigation of the parameters controlling the whistling frequency and amplitude of an orifice in a confined turbulent flow is undertaken. A circular single hole orifice with sharp edges, a hole diameter equal to 0.015 m and a thickness equal to 0.005 m, is arranged in an air test rig with an inner diameter equal to 0.03 m. The Mach number ranges around 0.02 and the Reynolds number around 10(4). Variable reflecting boundary conditions are arranged upstream and downstream, and several flow velocities are tested. It is found that the Bode-Nyquist criterion accurately predicts the conditions of self-sustained oscillation and the value of the whistling frequency. Furthermore, it is found that the acoustic velocity in whistling regime varies from 1% to 15% of the steady flow velocity, and that it depends on the overall acoustic reflection of the surrounding pipe and on the Strouhal number.