3-D POD Analysis of a Naturally Excited Jet

Abstract

A high-speed 3-D flow visualization system is used to capture large sets of instantaneous 3-D images of a naturally excited jet with Reynolds number of 6700. Images were acquired in both the near-field before the end of the potential core and the far-field of the jet just beyond the end of the potential core. Proper orthogonal decomposition (POD) is used to objectively characterize and classify the 3-D images and elucidate the fundamental structure of the flow. Preliminary results from this analysis are presented. As expected, the near field of the jet is dominated by the formation and growth of ring vortices, which is also reflected in the shape of the POD modes. The onset of azimuthal instabilities is also clear in the instantaneous 3-D images, which show long streamwise fingers of fluid surrounding the vortex rings. In the far-field, 3-D images continue to show the presence of unmixed jet core fluid; however, the shape of this region is highly convoluted with a different structure than in the near-field. In some images the core is seen to follow a helical type path downstream while in other images the core appears as a vortex puff type structure. Both types of structures are captured in the POD analysis along with some other unique 3-D flow structure. The results presented here are preliminary and further analysis is necessary to fully appreciate the information contained in the 3-D images. Future work will look at additional locations in the flow as well as the influence of axial excitation on the fundamental structure of the jet both in the near field and the far field. I. Introduction xcited jets are an excellent platform for the study of fluid dynamics and turbulence as they display numerous phenomena that are encountered throughout the field of fluid dynamics. Topics represented in an excited jet include instability, receptivity, vortex dynamics, transition, coherent structures and fully developed turbulence. As these features present themselves and develop with increasing distance from the jet nozzle, a jet flow field is convenient for the in-depth study of any of these topics by making measurements at the appropriate location downstream of the nozzle exit. In this paper, we present our preliminary efforts towards using a high-speed 3-D flow visualization technique and proper orthogonal decomposition to study the physics of transition to fully developed turbulent flow in an excited jet. Jets have received considerable attention over the last several decades [see Refs. 1-20 for a small sampling of the literature available on the subject]. Briefly, a jet’s flow field can be summarized as follows. The flow at the exit of the nozzle is uniform at the jet centerline with a region of shear near the wall. Upon exiting the nozzle, the shear layer with thickness, θ, is susceptible to the Kelvin-Helmholtz, or shear-layer, instability where small disturbances, typically characterized by their Strouhal number (Stθ = fθ/U), are amplified and eventually roll-up into organized and quasi-periodic sets of vortices. Further shear layer growth is dominated by the dynamics of these vortices and described by events such as vortex pairing. As the vortices grow and move towards the end of the potential core, the dominant jet instability mode becomes that of the preferred mode, or jet column mode, which is characterized by the Strouhal number based on the nozzle diameter (StD = fD/U). Near the end of the potential core and beyond, the