Shock-cell noise investigation on a subsonic/supersonic coaxial jet

The work is aimed at the experimental investigation of shock-cell noise on a coaxial jet with subsonic primary stream and supersonic secondary stream. This kind of noise is nowadays an important component of the total noise emitted by aeronautic engines, particularly affecting cabin noise in cruise conditions. In this thesis, the design and commissioning of a new supersonic coaxial jet rig, at the von Karman Institute for Fluid Dynamics (BE), are discussed, with a specific focus on the design choices that have been made to obtain good flow quality, low background noise and the possibility to perform a variety of flow and acoustic measurements. The maximum achievable Mach number at the outlet of the primary (central) and secondary (annular) nozzles is equal to 2.2, with a baseline operating point being M_p=0.89, M_s=1.21. To commission the facility, several test campaigns on a supersonic single stream jet were conducted using PIV in synchronous with microphones mounted on a polar antenna. Multiple screech harmonics and subharmonics tones have been documented, showing a directivity pattern similarly to the supersonic broadband noise (BBSAN). Turbulence integral length scales have been computed using correlation functions. The average of Reynolds shear stress fields shows the presence of lobes in the jet near field which are the trace of a standing wave caused by the screech. Following the commissioning, the coaxial jet has been investigated. Multiple combinations of pressure conditions for the primary and secondary flows have been tested. Acoustic measurements have been performed in synchronous with the PIV, which has been applied for the first time in the literature on a supersonic coaxial flow. The presence of both screech and broadband noise was recorded in the majority of the tests, and a directivity pattern was recognized for the latter. For a certain pressure conditions, the screech tone naturally disappeared. Experimental evidences suggest this may be related to a complex shock interaction occurring at the end of the primary nozzle. A simple method to infer the screeching dynamics from the spatial correlation functions was proposed. The correlation suggests the presence of a pulsation (or breathing) motion of the internal jet, which is cause/effect of the screech. A second screech mode was also retrieved from acoustic data, for which, the correlation functions suggest the presence of a sinusoidal motion of the internal jet.