Radiation of sound from ducts: A numerical approach

Numerical computation of sound radiation from ducts is attempted. A fourth order accurate implicit compact difference scheme was utilized for all computations. Numerical computations were performed to predict radiated acoustic field from an aeroengine inlet when a plane wave is incident on the inlet. The Helmholtz numbers(ka) chosen were 3.76 and 0.2 to represent a high frequency case and a low frequency case respectively. Comparison with experimental and analytical results showed good agreement. The effect of a uniform steady mean flow was also studied. A simplified model for scarfed inlet is then used for predicting the modified radiated field. It has been found for a scarfing angle of $64\sp\circ,$ the radiated acoustic field was significantly modified. The finiteness of the inlet length that causes inlet reflections was also studied. The results indicate that the interference between the reflected waves and the incident waves is substantial to necessitate the extra complication to be included. Computations using a non-harmonic pulse were also performed. All the computational results show that the present numerical method has the potential to compute the radiated acoustic field accurately. For the purpose of modeling the acoustic liner, a high accurate impedance boundary condition in time-domain was developed. Computed results match very accurately with analytical solution based on Fourier Transform method. It is believed that utilizing the present numerical method along with the impedance boundary condition, accurate prediction of the radiated acoustic field from aeroengine ducts can be accurately made