Experimental investigation of acoustic properties of orifices and perforated plates in the high mach number regime

The acoustics of industrial systems like pipelines, gas turbines, or rocket engines are commonly of high interest to developers as they may influence the overall system behavior. As full scale testing on those systems is generally financially and technically demanding, componentwise testing on scaled test rigs is often conducted instead. Those test rigs embed the component of interest in an environment similar to real operating conditions and allow efficient testing. In acoustics, this involves the creation of appropriate boundary conditions. A frequently investigated boundary condition is the choked nozzle boundary. Its acoustic characteristics have been thoroughly investigated at different flow condition in the past. The general finding was that a choked nozzle boundary imitates an acoustically rigid wall with a reflection coefficient magnitude of or close to unity. Transferring that to an inlet boundary condition one could intuitively assume that for a choked inlet the reflection coefficient also approaches this value. The aim of this study is to investigate the influence of mean flow on the reflection coefficient of orifices and perforated sheets acting as an inlet for a test rig. Therefore orifices and perforated sheets with different area ratios are investigated at flow states varying from Mach number zero up to choked conditions. The reflection coefficient of the elements from upstream and downstream are computed. Also the scattering matrix of the test element is computed from the recorded data. This paper shows that the reflection coefficient of a choked inlet can reach values significantly below unity.