Reflection of large amplitude acoustic pulses from an axial flow compressor

Abstract

The reliability of unsteady inlet flow computations may be seriously degraded by the lack of experimentally validated compressor-face (outflow) boundary conditions. It is common practice to require some flow variable to be constant (pressure, velocity or Mach number, etc.), but there is little if any documented evidence to support these assumptions. In addition, these assumptions do not have a basis in observation nor are they likely to actually occur during a rapid transient of a real inlet/engine system. The present paper presents measurements of acoustic reflection coefficients for an operating multistage compressor, a quantity appropriate for the characterization of the compressor face for computational purposes. The experiment used an impulse method, in which shortduration, large amplitude acoustic pulses (one msec, with a peak value of nearly 4% of the mean static pressure, respectively) were generated in an constantarea, annular inlet. The pulse and its reflection from the compressor-face were tracked by fast-response pressure transducers. Frequency-domain analysis of the data yields transfer functions that may be thought of as frequency-resolved reflection coefficients. It was demonstrated that transfer functions can be used for the accurate prediction of transients induced by arbitrary acoustic incident disturbances. None of the currently imposed compressor-face boundary conditions predict the data obtained in this study, indicating that current practices concerning outflow boundary conditions are in need of revision. Nomenclature