Closure Mechanism Investigation of Axisymmetric and Helical Screech Modes in Underexpanded Round Jets

This work provides an experimental contribution to the study of the Screech phenomenon. Two jets undergoing axisymmetric (A2) and helical (C) modes are considered, corresponding to Mj=1.13 and 1.5, respectively. Acoustic measurements using a microphone and Particle Image Velocimetry (PIV) are employed in combination with Proper Orthogonal Decomposition (POD) in order to obtain information about the velocity fluctuations fields. We investigate the Screech closure mechanism by extracting the signature of upstream travelling instabilities using the spatial Fourier transform of the complex-valued leading POD eigenfunctions. The spatial support of these instabilities is compared to the respective theoretical upstream neutral waves modes of the jet, predicted by a Vortex-Sheet model. A close agreement between the experimental and the theoretical data is obtained for the axisymmetric Screech mode A2, confirming that these upstream waves may be involved in the closure mechanism of this one, as already mentioned in recent works. In addition, the signature of these kinds of instabilities for the helical mode C shows a reasonable concordance with theoretical data, suggesting that this Screech mode may also be closed by upstream neutral modes.