Sound generation from a transonic compressor stage

Abstract

Some aspects of the sound pressure wave characteristics produced by a transonic compressor stage are studied by the use of a simplified mathematical model involving flow parallel to a moving corrugated board. The critical parameters of the system are correlated with the various unknowns in the mathematical model resulting in a suitable representation of the physical system. The analysis is first performed for the rotor alone, calculating the sound pressure wave upstream and downstream from the rotor, spinning at a tip speed Mach number of 1.2. The model is then modified to account for the stator blade row, 3/4" axially displaced from the rotor. A study of the reflective characteristics of the stator blade row is performed with the result that the reflective characteristic due to straightening of the flow is much more significant than that due to the physical hardware. The complete description for the sound pressure wave in the three regions: upstream of the rotor, downstream of the stator, and between the rotor and the stator, is determined. The results of this analysis are presented in the form of graphs showing: 1) the reflection coefficients, RF and RS , as functions of tip speed, 2) the pressure wave magnitude as a function of rotor tip speed for specific values of flow speed, 3) the axial dependence of the pressure wave magnitude in the region between the rotor and stator, 4) the ratios PO/P1 , PO/P2 , and P1/P2 as functions of Mt . Experimental data obtained prior to this analysis at the M.I.T. Blowdown Facility is presented and shown to correlate quite favorably with the theoretical determination of the pressure ratios presented.