A novel noise source mechanism whereby sound is producedby time-dependent temperature nonuniformities in a flow with a meanvelocity gradient is investigated experimentally. The research ispart of a theoretical and experimental study of this noise source initiated by Professor F. E. Marble.
A steady accelerating flow was produced in a rectangular supersonicnozzle with an entrance Mach number of 0.2, an exit Machnumber of 1.38, and a throat area of 1 in.^2. A rotary valve bleedflow system and an electrical wire resistance heater upstream of thenozzle introduced mass flow and temperature fluctuations into thenozzle flow at frequencies up to 500Hz. Sound measurements weremade at six positions along the nozzle and outside the nozzle assembly,which was enclosed in an anechoic chamber 10 ft. on a side.
All data acquisition and processing was done with a computer-controlleddata acquisition system, making the experiment essentially"computerized.”By interfacing the control circuitry of the bleed flowand heater systems to the data acquisition system) a digital signal-averagingtechnique was developed which enabled the sound producedby the bleed flow and temperature fluctuations to be detected and accuratelymeasured in high-level backgrounds of turbulent flow noise.
By synchronization of the bleed flow and heater, a pure temperaturedisturbance without an associated pressure disturbancecaused by heating was produced. This pure temperature disturbancewas found to produce sound upon being convected through the nozzle.The novel noise source mechanism was thus verified.
Excellent agreement was found between the nozzle soundmeasurements and one-dimensional small disturbance theory appliedto the flow occurring in the nozzle.
External field measurements yielded jet noise levels and spectraagreeing with other reported studies, spatial variation of the soundfield due to a pure pressure fluctuation at the nozzle exit, and crosscorrelation data between the external sound field and the pressurefluctuations at the nozzle exit.
