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research-article

Direct and indirect noise generated by entropic and compositional inhomogeneities

[+] Author and Article Information
Erwan Rolland

Department of Engineering, University of Cambridge, Cambridge, CB2 1PZ, United Kingdom
eor21@cam.ac.uk

Francesca De Domenico

Department of Engineering, University of Cambridge, Cambridge, CB2 1PZ, United Kingdom
fd314@cam.ac.uk

Simone Hochgreb

Department of Engineering, University of Cambridge, Cambridge, CB2 1PZ, United Kingdom
sh372@cam.ac.uk

1Corresponding author.

ASME doi:10.1115/1.4039050 History: Received August 16, 2017; Revised September 21, 2017

Abstract

Flow disturbances are generated inside a duct via pulsed injection of helium into a flow of air. This leads to the generation of an acoustic pulse (direct noise), as well as the production of entropic and compositional inhomogeneities which are convected with the mean flow. As these inhomogeneities are convected through a choked nozzle, they generate indirect noise. The resulting acoustic pressure fluctuations are measured experimentally using pressure transducers upstream of the nozzle. Insight obtained from theoretical models and a time-delay analysis can be used to isolate and extract the contributions of direct and indirect noise in the experimental signal. These results are directly compared to existing one-dimensional direct and indirect noise models. The experimental measurement of indirect noise is found to be in good agreement with the theoretical models for entropy noise and compositional noise for a compact one-dimensional isentropic nozzle.

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