Experimental And Theoretical Studies On Jet Acoustics

Abstract

A systematic research on aeroacoustics conducted around the world for the last few decades has revealed various inherent characteristics of the jet noise radiation. However, a lot more needs to be done for the theoretical as well as experimental predictions of various jet noise features based on actual flow details. The work reported in the present thesis is an attempt in this direction.

A critical study of existing literature on jet noise shows that none of the general wave equations lends itself easily for predictions of all the jet noise features. It is shown that while LighthilPs classical acoustic analogy approach, with some reasonable approximations, can be used to yield most of the information needed by the engineers, the convected wave equations of Phillips and Lilley are required to study the acoustic radiation in what has come to be known as "Refraction valley" or "Cone of relative silence".

The characteristics of the sound field of underexpanded cold jet impingement flows were studied by measuring the noise emanating from two convergent nozzles of throat diameter 2.5 mm and 5 mm each and a convergent - divergent nozzle of exit diameter of 6.49 mm, when the jet impinges on a flat plate kept perpendicular to the direction of the jet. The measurements were conducted upstream of the nozzle over an extensive envelope of jet operating conditions such as chamber stagnation pressure, mass flow rate through the nozzle and diameter of the nozzle.

The source strength at the jet boundary was obtained by measuring acoustic pressure amplitude close to the jet contour assuming it as locally cylindrical. Particular attention was focussed on backward projection of the sound field on to a cylindrical surface. This is the application of acoustic holography to study the sound radiation in the audio frequency region. With the help of FFT and software developed for this purpose, the theoretical predictions using data from several cylindrical surfaces were compared.

A detailed analysis of noise radiation from a cold sonic and supersonic free jet was also carried out. The experimental work involved the measurement of noise field from a 2.5 mm, 5 mm convergent and a convergent - divergent nozzle of exit diameter of 6.49 mm and area ratio 1.687 for designed Mach number of two.

The experimental setup consisted essentially of a pressure chamber made of mild steel, designed to withstand 50 bar pressure. This chamber is a cylinder with dia 0.421 m and length 0.85 m. The nozzles were made of mild steel. Compressed air approximately at room temperature is supplied to the nozzle via a control valve.

The measuring and recording instruments consists of B & K Microphones, Preamplifiers, Conditioning amplifier and a Mediator, which measure a Sound Pressure Level at a point. The nozzles were operated at pressure ratio upto 25 bar. The noise signal was processed through 12 channel data acquisition system. Acoustic pressure and SPL were" calculated using theoretical relations and software developed. Using this software Fast Fourier Transformations of raw signal was obtained from 20 Hz to 20 kHz. Also constant SPL contour graphs were obtained.

Source strength distribution at the jet boundary has been obtained by the principle of acoustic holography. Experimental values are closely matching with the results obtained by acoustic holography. The percentage error for acoustic pressure and SPL were less than 12%. The experimental results were used to obtain the source distribution in terms of gross jet parameters.