Entropy and compositional spots
Recent experiments have confirmed previous theories of the generation of entropic and compositional noise through choked and unchoked nozzles. These are acoustic disturbances associated with the change in gas properties as they propagate through an accelerating flow such as in a gas turbine nozzle.
Pressure trace upstream of a nozzle through which a hot (entropy) spot propagates
F De Domenico, E O Rolland, S Hochgreb, Detection of direct and indirect noise generated by synthetic hot spots in a duct, J. Sound Vib., 394:220-236,2017
E O Rolland, F De Domenico, S Hochgreb. Theory and application of reverberated direct and indirect noise. Journal of Fluid Mechanics, 819: 435-46, 2017
Pressure trace upstream of a nozzle through which a compositional helium spot
E O Rolland, F De Domenico, S Hochgreb. Direct and indirect noise generated by entropic and compositional inhomogeneities, Journal of Engineering for Gas Turbines and Power, 140(8), 082604, 2017.