Flow visualization of interactions among large coherent structures in an axisymmetric jet

Abstract

Interactions between large coherent structures are visualized with both schlieren photography in two air jets and dye photography in a water jet. The density difference needed for the schlieren technique is provided by an electrically heated wire ring surrounding the jet. External forcing with either single axisymmetric, single non-symmetric, combined axisymmetric or combined non-symmetric modes was applied. It was found that forcing the jet with a pair of different spinning modes leads to azimuthal distortions of the mean flow. This observation confirms and explains existing hotwire data. Simultaneous excitation with two axisymmetric modes may produce structures of higher modes or even cause structurally undistinguishable development. Streamwise structures are observed both in the unforced jet and in the axisymmetrically forced jet. They do not seem to be caused by a G6rtler instability from the concave curvature of the conventional nozzle, since they were also found in a jet flow from a specially designed nozzle with only convex contraction surface.

Three-dimensional evolution of flow structures in transitional circular and chevron jets

“…The shedding occurs regularly for a laminar jet exit as reported in previous works. [8][9][10][11][12] The instantaneous flow organization of the circular jet is illustrated in the temporal sequence of Figure 15 showing the Kelvin-Helmholtz vortices by iso-surfaces of normalized azimuthal vorticity x h ¼ x h *D/W j . The development towards the three-dimensional regime occurs mostly downstream of the pairing region and is visualized by the normalized axial and radial vorticity components x z ¼ x z *D/W j and x r ¼ x r *D/W j , which are plotted separately.…”

“…At t ¼ 2.44, on the cross-section at the vortex ring trailing edge (Z ¼ 3.2), a total of 11 streamwise instabilities 12 with axial vorticity magnitude 0.6 < jx z j < 1.2 occupy an approximately constant radial distance of 0.6, and as shown by the velocity vectors, they are subjected to a rather uniform radial inward motion of 0.3 that is induced by the ring vortex. The difference in the number of waves between the azimuthal instability (4 primary waves) and the streamwise structure (11) suggests that these do not develop from the primary ring instabilities but are rather formed in the braid region. 12 The cross-sectional plot at Z ¼ 3.5 illustrates the 4 longitudinal pairs ("STP") located in the vicinity of the external edge of the ring, with magnitude ranging from 0.5 to 1.5.…”

“…More complex geometries such as lobed and chevron nozzles at high Reynolds numbers have been recently afforded with LES, 6,7 whereas investigations with DNS have mostly focused on the fundamental aspects of circular jets at low Reynolds number. 4 Experimental investigations on the organization of coherent flow structures require field measurement techniques such as flow visualization [8][9][10][11] and, in the last decades, particle image velocimetry (PIV). PIV measurements are typically conducted along streamwise or cross-sectional planes to investigate the unsteady behavior of the shear layer and the transition patterns.…”

“…12,26,27 Counterrotating pairs of streamwise vortices are also reported to exist in the braid region between two vortex rings. [10][11][12][13]15 Modifying the shape of the jet exit geometry has dramatic effects on the organization and stability of the shear layer with consequences on the entrainment rate, the spectral characteristics, and the related acoustic emissions. Early investigations from Paterson 28 showed that nozzles with a lobed exit generate coherent streamwise vortices emanating from the exit.…”

Scanning tomographic particle image velocimetry applied to a turbulent jet

“…For laminar inlet conditions, the prominent flow feature in the region close to the nozzle of a round jet is the roll-up of the vortex sheet into axisymmetric vortex rings due to Kelvin-Helmholtz instability. 12,40,41 Azimuthal instability of the vortex rings lead to the formation of secondary streamwise vortices which distort the main rings further to the point of breakdown. 42 For our fully turbulent inlet conditions these vortex structures will not be as dominant as for a transitional flow but we do expect the flow features in our downstream experimental domain to be highly vortical.…”