Quasisteady aero-acoustic response of orifices

The low frequency response of orifices (slit, circular diaphragm, and perfo rated plate) in the presence of mean flow is well predicted by a quasistead y theory. A refinement is brought to the theory by considering a Mach numbe r dependent vena contracta coefficient. The measurements of the vena contra cta. coefficient of a slit agree well with the simple analytical expression existing in the case of the Borda tube orifice. The measured scattering ma trix coefficients do not depend strongly on the geometry of the element. If the frequency is increased the moduli remain relatively unaffected while t he arguments exhibit a complex behavior which depends on the geometry. From these considerations an anechoic termination efficient at high mass flow i s designed. (C) 2001 Acoustical Society of America.