Including acoustic modes in the vortex-sheet eigenbasis of a jet.

Vortex-sheet (V-S) models of jets are widely used to describe the dynamics of modes, such as the Kelvin-Helmholtz instability and guided acoustic waves. However, the absence of the free-stream acoustic modes in the V-S eigenspectrum is seldom pointed out in the literature. This family of modes is important if, for example, one is interested into the problems of sound emission or flow-acoustic interactions. In this work, it is shown how a distantly confined jet may be used as a surrogate problem for the free jet, in which the free-stream acoustic waves appear as a set of discrete modes. Comparing the modes observed in the free jet with those of the distantly confined jet, we show that other than the free-stream acoustic modes, the eigenvectors and eigenvalues converge exponentially with the wall distance. The proposed surrogate problem, thus, efficiently reproduces the dynamics of the original problem while accounting for the dynamics of the free-stream acoustic eigenmodes.

Experimental investigation of jet-induced wall pressure fluctuations over a tangential flat plate at two Reynolds numbers.

The wall pressure fluctuations induced by a subsonic circular jet on a rigid flat plate have been investigated considering two jets with different exit section diameters at the same Mach number. The analysis is aimed at completing the series of papers presented by the authors on the interaction between a subsonic jet and infinite tangential flat plate where the exit Mach number was the only parameter of the jet flow that was varied. In order to analyse other effects out of the Mach number, two configurations with different nozzle exhaust diameters were explored with the objective of isolating the Reynolds number effect keeping fixed the exit Mach number. The nozzle exhaust diameters are 12 mm and 25.4 mm and the instrumented flat plate, installed parallel to the jet flow, is moved at different radial distances from the jet axis. The pressure footprint on the plate has been measured in the stream-wise direction by means of a pair of flush-mounted pressure transducers, providing point-wise pressure signals. Wall pressure fluctuations have been characterised in terms of spectral and statistical quantities. The effect of Reynolds is evidenced and possible scaling relationships that account for the Reynolds dependence are proposed. Implications for modeling the spectral coherence have been considered by the application of the Corcos' model and the effect of the jet Reynolds number on the model coefficients is analyzed.