Sound-excited flow and acoustic nonlinearity at an orifice

In this paper a potential model is developed to study the sound-excited flow and the acoustic nonlinearity at an orifice. It is assumed that the vortices shed from the orifice edge are simply pushed to move by the fluid slug coming out of the orifice. The fluid slug is either cylindrical or in the shape of an inviscid steady jet through the orifice. The strength of the shed vortices is determined by applying the Kutta condition at the orifice edge. For a sinusoidal applied sound pressure, the fluctuating orifice flow is computed, and some basic flow features such as the velocity distortion in the orifice and the asymmetry of velocity between the inflow and outflow half cycle at positions away from the orifice are described. The nonlinear acoustic resistance as well as reactance of the orifice is also obtained from the average flow velocity through the orifice. The present theory has been compared with the existing studies for the acoustic nonlinearity of an orifice. For the purpose of further validation an experiment is carried out. The theoretical predictions agree fairly well with the experimental data despite the simplifications having been made in this model. It is thereby concluded that the nonlinear acoustic behavior of an orifice depends mainly on the vortex shedding rate at the orifice edge and the convection speed of the shed vortex in the vicinity of the orifice rather than the fine details of the shed vortices.