In-duct orifice and its effect on sound absorption

A numerical investigation of sound absorption by an in-duct orifice with and without flow was carried out using a sixth-order finite difference direct numerical simulation (DNS) scheme with explicit fourth-order time marching to solve the governing Navier–Stokes equation. The DNS scheme has previously been validated against benchmark aeroacoustic problems and good agreement was obtained. Thus, it was applied to simulate the acoustic impedance of a circular orifice with different openings and a laminar flow through the same orifice. Both discrete frequency and broadband excitations were studied. When the in-duct orifice is exposed to discrete frequency sound wave in the absence of flow, alternate vortex shedding on both sides of the orifice is observed. The strength of shed vortices is stronger at low frequencies and thus the reduction of sound energy is higher. These vortices dissipate while moving away from the orifice. Therefore, the process provides a mechanism for adsorption of incident sound. The numerical results of broadband excitation indicate that small orifice opening is a more efficient sound absorber whereas a large opening is more or less transparent to the incident wave. The absorption, reflection and transmission coefficients of the in-duct orifice are calculated by a transfer function method. It is found that the sound coefficients are strongly dependent on the orifice opening size and frequency. In the presence of a flow, only alternate vortex shedding on one side of the orifice is observed. In spite of this, the results show that sound absorption behavior is very similar to the no flow case, i.e., sound absorption is more effective with small orifice.