Liner impedance modification by varying perforate orifice geometry

The present work explored the feasibility of controlling the acoustic impedance of a resonant type acoustic liner. This was accomplished by translating one perforate over another of the same porosity creating a totally new perforate that had an intermediate porosity. This type of adjustable perforate created a variable orifice perforate whose orifices were non-circular. This arrangement also formed a stepped geometry in the transverse direction, referred to here as a stepped oval orifice. The key objective of the present study was to quantify the degree of attenuation control that can be achieved by using such a concept on the buried septum in a two-degree-of-freedom acoustic liner. An additional objective was to examine the adequacy of the existing impedance models to explain the behavior of the unique orifice shapes that result from the proposed sliding perforate concept. Different orifice shapes with the same area were also examined to determine if highly non-circular orifices had a significant impact on the impedance. This was primarily an experimental study consisting of normal incidence impedance measurements and flow-duct insertion and transmission loss measurements. The flow-duct measurements were made with grazing flow velocities up to 76.2 m/s (250 ft/s). It was found that perforate translational movements on the order of the orifice diameter (1.6 mm) resulted in shifting of the primary resonance frequency of the liner approximately 14% (200 Hz) and the secondary frequency approximately 16% (800 Hz). Mass reactance values derived from impedance tube measurements of the variable orifice perforate were found to be lower than dim predicted by available theoretical models. This effect was more pronounced at lower porosities where the orifice shapes were highly non-circular. A single stepped oval orifice was also found to have lower mass reactance, compared to a single circular orifice. This could not be predicted by available theoretical models of elliptical shaped orifices. Flow-visualization of the stepped oval orifice revealed that the typical orifice jetting accompanying sound/orifice interactions was “vectored” or deflected off the orifice centerline. Other highly non-circular orifice shapes tested, such as star-shaped, resulted in higher absorption at lower incident sound pressure levels.