Sound absorption by in-duct orifice

A numerical investigation of sound absorption by an in-duct orifice is reported. The problem is calculated using a sixth-order finite difference DNS with explicit fourth-order time advancement. The DNS numerical scheme was snccessfully validated by comparing the numerical calculations with two experiments, namely laminar flow through an in-duct orifice and the acoustic impedance of a circular orifice. Both discrete frequency and broadband excitations are studied. When the in-duct orifice is exposed to discrete frequency sound wave, alternate vortex shedding on both sides of 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 processes provide 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 transfer function method. It is fonnd that the sound coefficients are strongly dependent on the orifice opening size and frequency.