Experimental study of a self-excited jet precession in a sudden expansion flow

Abstract

This study presents Stereoscopic particle image velocimetry measurements of a precessing motion resulting from a sudden expansion of an axisymmetric jet into a coaxial chamber. The sudden expansion has an expansion ratio of 4.9 and an aspect ratio of 2.03. The velocity profile in the inlet tube is measured upstream of the expansion using Laser doppler anemometry. The Reynolds number (Re) is varied from 3160 to 63,870. The time-averaged flow field exhibits an axisymmetric behavior, and the velocity profile near the expansion resembles that of a jet. The half-width and the opening angle of the jet are similar for all Reynolds numbers measured, with values of $$3.73\pm 0.04^{\circ }$$ and $$8.73\pm 0.19^{\circ }$$ , respectively. Despite its axisymmetry, the flow field contains unsteady coherent structures in nature, which are analyzed using both spectral proper orthogonal decomposition and phase averaging. These structures make the instantaneous flow field asymmetric and create a precession around the central axis of the geometry. The structure of this precession is similar for $$8300 \le \hbox {Re} \le$$ 63,870. For $$3160 \le \hbox {Re} \le 8300$$ , the precession is very weak and hard to extract from the turbulent flow fields. The Strouhal number (St) increases linearly for $$8300 \le \hbox {Re} \le$$ 23,600, after which it reaches a value of around 0.003. Precessing structures of this kind were previously only reported for swirling sudden expansion flows, fluidic precessing jet nozzles or annular free jets. However, this study shows that they also exist in confined, non-swirling sudden expansion flows, without exiting to an ambient fluid.