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Detection of direct and indirect noise generated by synthetic hot spots in a duct

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De Domenico, F 
Rolland, EO 


Indirect noise generated by the acceleration of entropy or vorticity perturbations through a nozzle is of interest both as an additional source of transmitted noise, but also as a source of combustion instabilities via backward propagating waves. There is a lack of experimental data, mostly due to difficulties in separating direct and indirect noise. Previous experiments attempted to isolate indirect noise by generating thermoacoustic hot spots electrically and measuring the transmitted acoustic waves, yet there is no data on the reflected or backward propagating acoustic waves. In this work, synthetic hot spots are generated by unsteady electrical heating of thin wires. These hot spots are accelerated through an orifice plate, producing a strong acoustic signature upstream of the orifice plate. Using a time separation argument, we identify the indirect noise signal, showing that its contribution to the overall noise is not negligible in either in subsonic or sonic throat conditions. However, the amplitude of direct noise is larger, making indirect noise difficult to identify if the two contributions are merged. We demonstrate the importance of appropriate pressure transducer instrumentation and accounting for the respective transfer functions in order to account for low frequency effects in the determination of pressure.



indirect noise, entropy spots, pressure transducers

Journal Title

Thermoacoustic Instabilities in Gas Turbines and Rocket Engines: Industry meets Academia May 30 – June 02, 2016 Munich, Germany

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Thermoacoustic Instabilities in Gas Turbines and Rocket Engines: Industry meets Academia

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